Glossary Plant Breeding

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Contents
Preface ........................................................................................................................ 4
Chapter 1. A ............................................................................................................... 5
Chapter 2. B .............................................................................................................. 23
Chapter 3. C .............................................................................................................. 33
Chapter 4. D .............................................................................................................. 57
Chapter 5. E .............................................................................................................. 72
Chapter 6. F .............................................................................................................. 82
Chapter 7. G ............................................................................................................. 89
Chapter 8. H ............................................................................................................. 105
Chapter 9. IJ ............................................................................................................. 118
Chapter 10. KL........................................................................................................... 129
Chapter 11. M ........................................................................................................... 137
Chapter 12. N ............................................................................................................ 157
Chapter 13. O ............................................................................................................ 165
Chapter 14. P ............................................................................................................ 169
Chapter 15. Q ........................................................................................................... 193
Chapter 16. R ........................................................................................................... 196
Chapter 17. S ............................................................................................................ 210
Chapter 18. T ............................................................................................................ 232
Chapter 19. UV ......................................................................................................... 247
Chapter 20. W ........................................................................................................... 255
Chapter 21. XYZ ....................................................................................................... 257
References ................................................................................................................ 259
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Preface
Plant Breeding has passed through different stages and phases of its growth and
development. Before 20 th century, it was almost exclusively an art. Accomplished
naturalists and farmers were practicing plant breeding activities, perhaps without knowing
the inheritance of characters. After rediscovery of Mendel’s laws of heredity, it became
possible to start with breeding programmes with some predictable outcome. This was
substantiated by Darwinian principles of evolution (based on struggle for existence and
survival of the fittest in natural populations). Some other facts related to inheritance of
quantitative traits as per Mendelian expectancy, and the analysis and measurement of
variation through translating the covariance between relatives into components of genetic
variance also aided to the efficiency of plant breeders in the middle of 20 th century. In the
later part, rapid advancement commenced in molecular biology, which embraced even
quantitative genetics. This paved the way for marker aided selection even for quantitative
trait loci (QTLs). Now marker assisted selection (MAS) has become an inseparable
component in most of the breeding programmes.
The present-day plant breeding encompasses almost all fields of agriculture, and is
dependent upon classical to modern techniques. This “Glossary of Plant Breeding” has
been prepared keeping in view of these facts, and is likely to cater to the needs of both
students and teachers and researchers alike. This monograph is not simply a collection of
terms of plant breeding. Wherever possible, terms have been conceptualized to provide a
better understanding, and one can hardly need the textbook of plant breeding to develop
concept on that aspect. Terms with historical importance are accompanied by the name(s)
of their discoverer(s). Most of the terms of utmost importance including those of
agricultural statistics, population genetics, ecological and evolutionary genetics have been
described and discussed. Some general terms of agriculture have also been explained in
order to meet general needs of readers. Students and researchers going in for interview
will find it especially useful. We hope and trust readers will like it, and may find at least
something, if not everything, in this book.
AK Choudhary
AG Vijayakumar
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AB–QTL Method (Tanksley and Nelson 1996). Advanced Backcross QTL Method. It is
a method for simultaneous discovery and transfer of valuable QTLs from unadapted
germplasm into elite breeding lines. It involves use of BC 2 /BC 3 (or their self or test
cross progenies) for phenotyping and genotyping using molecular markers. Such
populations are non-random resulting from intense selection in BC 1 /BC 2 and allow to
identify yield and yield component QTLs. Using this method, two wild QTL alleles
namely, Yld 1.1 and Yld 2.1 from O. rufipogon (using ‘RM5’ and ‘RG256’ markers,
respectively) have been identified and introgressed into cultivated hybrid rice. Each of
the two has been shown to confer yield advantage of more than 18% in the hybrid
rice. IthasbeenalsousedtoidentifysuchgenesinLycopersicon prempenellifolium.
One of the main demerits of this method is that BC 2 /BC 3 populations are effective for
detecting dominant and overdominant QTLs only; recessive QTLs go undetected.
Acaryotic. Referring to a cell without nucleus.
Accessory Chromosomes. Sex chromosomes and other supernumerary chromosomes
such as B-chromosomes.
AChromosome.Any member of the normal chromosome complement in contrast to the
B-chromosomes.
ALine. The male-sterile parent line to produce hybrid seed. It is the seed-producing line,
always used as the female parent. It has ‘sterile’ cytoplasm and recessive nuclear
gene(s).
A Posteriori. Derived by reasoning from observed facts (data).
APriori.Based on hypothesis or theory rather than on experiments or experience.
Abiotic Stress. Any kind of abnormal condition due to non-living components of
environment (high or low temperature and moisture, high salinity or acidity in soil,
forexample)thatadverselyaffectsgrowthand reproduction of crop plants. Breeding
for abiotic stress is considered more difficult because of: (a) complexity of conditions
causing abiotic stresses, (b) complex nature of abiotic resistance in a variety, (c)
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occurrence of one stress more often in conjunction with the other, (d) low heritability
of abiotic resistance, and (e) variable intensity of such stress under field condition.
Acclimatization. The ability of a population to recover and adapt to a new climatic
situation. The process of acclimatization is a population phenomenon. Unfit
genotypes of the population are eliminated and the frequency of the ‘fit’ increases
generation after generation. After a few generations, the performance of a population
is better once all the unadapted types get eliminated. Thus, it is the result of genetic
shift towards the adapted types. The efficiency with which it takes place depends on:
(a) genetic variance of the population, (b) mode of reproduction and (in sexually
reproducing plants) pollination, and (c) the nature and intensity of environmental
stresses. One should note that it is not a breeding method; instead, it explains the
process by which new introductions gradually become adjusted and established as a
better performing population.
Acentric Chromosome. A chromosome without the centromere.
Acrocentric Chromosome. A chromosome having the centromere located slightly nearer
to one end than the other.
Active Collections. Genetic resources maintained for immediate use by the plant
breeders. Such collections are for multiplication, regeneration, evaluation,
documentation, distribution, and use by breeders. These collections have frequent
usage and obviously need frequent multiplication. The storage period is, therefore,
short (1-2 years) or at best medium (3-4 years).
Active Resistance. A type of resistance reaction of the host plants induced in response to
the attack by a parasite. The formation of phytoalexins (antifungal compounds) in
some host plants in response to inoculation with certain pathogens is an example of
such resistance. It is known that seedlings of cotton are susceptible to many a pest
owing to the absence of gossypol ; but once infected by Verticillium, plants begin to
synthesize gossypol and seedlings become resistant. Normally, only mature plants
have gossypol and not the seedlings.
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Active Site. The part of a protein that must be maintained in a specific shape if the
protein is to be functional; for example, in an enzyme, the part to which the substrate
binds.
Active Variability. Also called free variability; gnetic variability that is freely expressed
as phenotypic differences between homozygotes and thus is exposed to selection. This
is immediate cause of response to selection, for example, difference between AA and
aa.
Adaptation. The process by which individuals (or parts of individuals), populations or
species change in form or function in such a way to better survive under given
environmental conditions. Also the result of the process.
Adaptedness. The degree of suitability (fitness) of an organism for its environment,
developed over time as a result of selection. Alleles which are predominant in most of
the cultivars and breeding stocks representing a broad range of environments make
significant contributions towards wide adaptedness and high productivity in nearly all
genetic backgrounds and in nearly all environments. Alleles with intermediate-to-high
frequencies in the varieties and breeding stocks of some (but not all) eco-geographical
regions, on the other hand, make differing contributions to adaptedness and
performance in different eco-geographical regions and in different genetic
backgrounds with year-to-year variations. This class of alleles may enter into
favourable inter-locus interactions (epistasis) in diploids as well as polyploids.
Favourable epistasis (acting to assemble and sustain favourable multi-locus
genotypes) is indeed a major mechanism responsible for adaptedness (biological
fitness) in natural populations.
Adaptability. The ability of a genotype to adapt under given environmental conditions;
the potentiality for adaptation; flexibility or capacity to change in adaptedness
(fitness).
Additive Effect. The plus effect produced by the presence of each additional gene on the
phenotype. Either of the two genes may produce the same effect, but the effects are
additive if both genes are present. For example, in barley, A or B produce medium
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awns, while together produce long awns and thus giving an F 2 ratio of 9A-B-(long
awns): 6aaB-/A-bb (medium awns): 1aabb (awnless).
Additive Gene Action. The way the linear quantitative effect (plus or minus) associated
with the substitution of one allele for the other at a given locus is produced on the
phenotype. This change occurs on the phenotypic scale of measurement regardless of
presence of genes at other loci. For example,
1. 1AABB (7), 2AaBB (5), 1aaBB (3) → replacement of a by A leads to a change of 2 units,
2. 1AABB (7), 2AABb (6), 1aabb (5) → replacement of b by B leads to a change of 1 unit.
The replacement effect is the same whether it occurs in a homozygote or heterozygote
and is also constant regardless of the phase at other locus. It is interesting to note that
the mean values in these examples are 5 and 6, respectively. So replacement of a for
A is by two units and of b for B by one unit; this is called the average effect of an
allele. Thus it can also be defined as the type of gene action in which the change is
associated with the average effect of substituting one allele for the other at a given
locus.
Additive Genetic Variance. That portion of genetic variance which describes difference
between homozygotes at any given locus. In the examples quoted under additive gene
action, the numerical values of additive variance amounted to 2.0 and 0.5 unit 2 ,
respectively which were produced as a consequence of additive effects of 2.0 and 1.0
units, respectively. Thus, one can also define it as that portion of genetic variance
which is produced by additive gene action. As additive gene action itself is dependent
upon the average effect of substitution of one allele for the other, thus it can be
appropriately defined as that portion of the genetic variance which is associated with
the average effect of substituting one allele for another at one, two, or more loci.
Adenine. One of the two purine bases that normally pairs with thymine in the DNA
double helix.
Adenosine. The nucleoside containing adenine as its base.
Adjacent-1 Segregation. A kind of segregation in a translocation heterozygote in which
each of the structurally normal chromosomes moves to the opposite poles along with
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one of the translocated ones. Meiotic products are duplicated and deficient for
different regions; therefore, gametes are non-functional.
Adjacent-2 Segregation. Another kind of segregation possible in a translocation
heterozygote. In this case also, one normal and one translocated chromosomes move
to the same pole; but in such segregation, homologous centromeres migrate to the
same pole. Gametes are also inviable for the same reason (as mentioned in the
adjacent-1). Since Mendel’s second law still applies to different paired centromeres,
the possibility that the homologous centromeres move to the same pole is almost
negligible. Thus, this is a rare event.
Adult Resistance. A type of plant resistance that is identified in mature plants and is
difficult to identify in seedlings.
Adventitious Embryony. A form of apomixis that leads to the production of seeds
without a sexual process.
Aerobic Respiration. A process requiring O 2 by which organisms generate energy from
ingested organic molecules.
Aerobic Rice. A new concept aimed at reducing water requirement in rice production
through developing a variety that can survive moderate drought. Genetically
engineered novel trehalose accumulation pathway in rice can confer drought
tolerance.
Aggressiveness. A measure of pathogenicity. It is used to describe the capacity of a
parasite to invade and grow in its host plant and to reproduce on or in it.
Aggressiveness and non-aggressiveness in the pathogen are the counterparts of
horizontal susceptibility and horizontal resistance in the host, respectively. It may
well involve enzyme dose (dosage effect) and switching on and off of enzyme action.
Agricultural Drought. See drought.
Agricultural Output. A function of the kind and intensity of input use.
Aids to Selection. Techniques used to enhance the efficiency of selection. For example,
the creation of epiphytotic condition helps determine disease reaction among
individuals of a segregating population in a disease-free environment.
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Albinism. A condition of absence or marked reduction of pigments. Affected plants are
characterized by the absence of chlorophyll in the leaves. In humans, the affected
persons have a very light skin in addition to white hair and pink or red eye colour
(because of the reflection of retinal blood vessels). A recessive albino gene controls
the condition.
Alkylating Agent. A chemical agent that can add alkyl groups (e.g., - CH 3 or C 2 H 5 )to
another molecule. Many mutagens act through alkylation. For example, EMS (ethyl
methane sulphonate) adds ethyl groups more specifically to O 6 position of guanine (in
the DNA) and causes mutation through GC to AT transition.
Allele Frequency. A measure of the commonness of an allele in a population. The
proportion in which an allele of a particular gene occurs in a population is called
allele or gene frequency.
Allele. Also called allelomorph. It is one of a pair or series of forms of a gene, which are
alternative in inheritance because they are situated at the same locus in homologous
chromosomes. It is a member of a specific gene pair. It may be identical or dissimilar.
If they are dissimilar, the use of alternative forms of a gene is appropriate. One can
express it by saying that there is a gene, phonetically called a “see” gene, with alleles
C and c. The members with respect to this gene can be CC, Cc or cc. Individuals
represented by CC or cc are said to be containing similar or identical alleles or genes.
The term alleles or genes are used interchangeably with respect to this gene pair only.
Notice that although different alleles of a gene pair can produce different effects, they
obviously both affect the same character. Thus an allele, which may be any one
among many possible variant forms of a gene, results ultimately from mutation.
Allelopathy. A general term used for the secretion of chemical messengers, which
provide a competitive advantage for one species against another in the association.
Also called antibiosis.
Allogamy. Cross-fertilisation (allomixis).
Alloplasmic Line. The nucleo-cytoplasmic combination resulting after the substitution of
the genome of a species into the cytoplasm of an alien species. The parental type
having its own cytoplasm is called an euplasmic line.
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Alloploidy. A situation wherein the individual contains genetically different sets of
chromosomes, for example, from two or more species. If two distinct species are
hybridised, and the chromosome complement of the resulting hybrid is doubled, the
result is the merging of the two species into a new amphidiploid species. This is
exemplified by N. digluta (2n=72), which can be synthesized artificially by doubling
the chromosome complement of interspecific hybrid between N. tabacum (2n=48)
and N. glutinosa (2n=24). Among various types of polyploidy, alloploidy has been the
most important in the ancestry of cultivated species. It provides a mechanism for
direct speciation. A large portion of the cultivated species, possibly as many as half
(and 70% of the grasses), is alloploids. Induced alloploidy has found an even smaller
place in practical breeding than autoploidy breeding. Principles that govern the
success remain the same in both auto- and alloploidy breeding (see polyploidy).
However, induced alloploidy can be used to (a) establish phylogeny of a polyploid
species, (b) produce new plant genotypes, (c) facilitate transfer of genes from related
species, and (d) facilitate transfer or substitution of individual chromosomes or pairs
of chromosomes.
Allopolyploid. A polyploid that contains sets of dissimilar genomes having been derived
from two or more distinct taxonomic species. As the genomes are different, the
polyploid is also called hybrid polyploid. As it combines complete chromosome sets
from two or more species, it is sometimes called amphidiploid. In general, alloploids
combine in more or less blending fashion the characteristics of the species from
which they are derived. They have generally increased vigour and have a high degree
of fertility and stability compared to their diploid parents. The high degree of fertility
and stability of the naturally occurring alloploids is because of regular bivalent
formation at meiosis. Since different genomes are highly divergent, thus pairing
occurs only between homologous chromosomes belonging to identical genomes. Thus
for practical purpose, there are only two chromosomes of each kind in the alloploids.
There are, however, many exceptions to this generalization. For example, Ph gene
present in the long arm of 5B chromosome ensures bivalent formation in bread wheat.
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Allosomes. Accessory chromosomes that differ from the rest chromosomes (autosomes)
in shape, size, and other attributes.
Allosyndesis. Pairing between chromosomes belonging to different genomes.
Allozymes. Allelic forms of enzyme loci that can be distinguished by electrophoresis.
Alternate Hypothesis. The conclusion drawn after rejection of the null hypothesis (Ho).
It is denoted by H 1 . If sample result fails to substantiate the null hypothesis, it is
concluded that something else is true; this conclusion is referred to as alternate
hypothesis. (Also see null hypothesis).
Alternate Segregation. The passage of both normal chromosomes to one pole and both
translocated chromosomes to the other pole in a translocation heterozygote
(reciprocal). As there is no net loss or gain of hereditary materials, gametes are 100%
functional.
Alternation of Generations. The alternation of gametophytic and sporophytic stages in
the life cycle of a plant.
Aluminium Toxicity. A form of abiotic stress limiting crop production in acid soils. The
problem is particularly serious in strongly acid subsurface soil horizons (pH<5.5) that
are difficult to lime. Aluminum (Al) toxicity negatively affects growth of both root
and shoot. The root growth reduction may stem from restricted absorption of water
and nutrients, and ultimately causes yield reduction in such a problem soil. Under
such a situation, use of tolerant cultivars may be a satisfactory solution to this
problem. Use of tolerant genotypes and breeding of crops for Al tolerance is a reliable
approach to enhance production on acidic soils. This requires a rapid and effective
technique to discriminate between tolerant and sensitive genotypes. There are several
screening methods for Al tolerance such as solution, sand and soil cultures, root regrowth
and hematoxylin staining techniques, and field screening. Selection of
seedlings in hydroponic assay has been used as a rapid screening method to screen for
aluminum tolerance in several crops. Hematoxylin staining and root re-growth
techniques are also frequently used techniques because they have produce consistent
results. In addition, the results obtained with solution culture screening method
correlate positively with those obtained using field screening, showing that this
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method could be representative of what happens in the field. Hydroponic assay
including hematoxylin staining has been recommended to identify Al tolerant
genotypes in several crops. The reaction of hematoxylin with Al–stressed roots has
been used by several researchers in different crop species such as wheat, soybean,
maize, peas, pigeonpea, and the like.
Amaranthus Albumin Potato (Chakraborty et al. 2000). A transgenic potato containing
agene(AmA 1 ) for a highly nutritive protein from Amaranthus hypochondriacus. The
potato so-obtained has 35-45% more protein, improved protein quality (5-8 fold
higher essential amino acids) and tuber yield up to 3-3.5 fold with no evidence of
allergens.
Amber Codon. A triplet of bases (UAG) that stops translation of genetic message from a
part of DNA. It is also called chain (polypeptide) terminating codon. The two other
stop codons are opal (UGA)andochre (UAA)ones.
Amber Suppressor. A mutant allele encoding a tRNA whose anticodon is altered in such
a way that that the tRNA fails to stop translation (that is, it inserts an amino acid) at
the site of an amber codon.
Ameiosis. The failure of or no meiosis. Meiosis is replaced by a mode of nuclear division
that does not involve a reduction of the chromosome number.
Amitosis. Nuclear division by a process other than mitosis. In this case, direct nuclear
division takes place.
Amensalism. A kind of inter-species interaction in which one is inhibited but the other
remains unaffected.
Amino Acid. A peptide; the basic building blocks of proteins (or polypeptides). It is
characterized by the presence of an amino group (-NH 2 ) in addition to a carboxyl
group (-COOH) and a side-chain (R). The side-chain (R), which is bound to the alpha
carbon, is different in each amino acid. Because of the simultaneous presence of
acidic (carboxyl) and basic (amino) groups, amino acids have both + and – charges
and are therefore amphoteric molecules or zwitterions. Glycine is the simplest amino
acid. The two sulphur-containing amino acids are cysteine and methionine, which are
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deficient in proteins of legumes. Cereal proteins have in general low lycine content in
the protein.
AMMI Model (Gauch and Zobel 1996). Additive Main Effects and Multiplicative
Interaction; a statistical model for megaenvironment analysis. It is based on subdividing
a region into several relatively homogeneous megaenvironments and
breeding and targeting adapted genotypes for each megaenvironment. AMMI analysis
of regional data can help in controlling errors and gaining accuracy. Results with this
model indicate that a small and feasible number of megaenvironments often suffice to
exploit useful interactions and increase yield by growing a specific genotype in a
specific megaenvironment. AMMI analysis is helpful in assigning a genotype to low
productivity environments (which are prone to large errors, minimize genotypic
differences, and have less repeatability of genotypic performance). Thus planting the
winning genotype in each megaenvironment maximizes the yield; and planting the
genotype that wins in each megaenvironment is helpful in assigning each area to the
appropriate genotype recommendation. Therefore, identifying megaenvironments and
targeting genotypes are interwoven and interrelated.
Amphidiploid (Clausen et al. 1945). An alloploid derived from combining entire
chromosome sets from two or more diploid species that are separated by barriers of
hybrid sterility. Thus it contains distinct genomes in duplicate, that is, two copies of
different genomes (for example, AABB or AABBDD). It is more stable meiotically
than autoploids owing to regular bivalent formation at meiotic prophase. In general,
polyploidy has the effect of broadening the potential base of variation by increasing
the number of genes that can mutate.
Amphipathic. Of a molecule having both a hydrophilic and a hydrophobic region.
Amplification. The production of many DNA copies from one master region of DNA.
Anaerobic glycolysis. Degradation of glucose into lactic acid in the absence of oxygen,
yielding two molecules of ATP.
Analysis of Variance. A statistical procedure of partitioning the total variation into
various components owing to contributing causes. The two main objectives of
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analysis of variance are: (a) it helps in sorting out variance due to different sources,
and (b) it provides a basis for the test of significance.
Anaphase. An intermediate stage of nuclear division during which chromosomes are
pulled to the poles of the cell.
Androecious. Of plants having only male flowers.
Androecium. The male reproductive organ of a plant; the stamens of a flower taken
collectively.
Androgenesis (Guha and Maheshwari 1966). In vitro genesis of a plant from anthers or
microspores. The resulting individual is a haploid. The method was first used in
Datura innoxia for the production of haploid individuals.
Aneuploid. An individual whose somatic chromosome number (2n) is not an exact
multiple of the basic number (x). Aneuploids arise when normal gametes (n) are
fertilised by abnormal ones (n+1, n-1, etc.) and vice versa. Abnormal gametes may
result due to non-disjunction during meiosis. Asynaptic genes may cause formation of
univalents that eventually results in the formation of n+1 orn-1, gametes. Somatic
aneuploids occur as a result of mitotic non-disjunction or random chromosome loss
during cell multiplication. Monoploids and triploids are another source of aneuploids.
Aneuploids are usually less vigorous and physiologically less efficient than their
diploid counterparts. This is because: (a) the balance of chromosomes that is
necessary for a finely tuned cellular homeostasis, carefully put together during
evolution, is grossly disturbed, and (b) any deleterious recessive on the single
remaining chromosome becomes hemizygous and may be expressed directly
(monosomics, for example). Aneuploids (such as nullisomics and monosomics) are
ordinarily viable in polyploids where homeologous chromosomes appear to offset
missing chromosome materials. Although monosomics have been found in diploid
like maize, but there has been still a debate on the diploid-like nature of this crop.
Aneuploids such as trisomics are usually not found in diploid species because the
unbalance caused by even a single extra chromosome is lethal. However, in a diploid
species like rice, where viable trisomics are found, trisomy usually exerts a profound
effect on morphology of the individuals. But in a “polyploid” species like bread
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wheat, the degree of tolerance to repetitions of a particular chromosome(s) is so great
that even tetrasomics may be nearly indistinguishable from normal types. In general,
aneuploids are sterile (partly or even highly) and genetically unstable. Owing to
sterility, genetic instability and reduced physiological efficiency, aneuploids have
found little if any direct use as marketing varieties. However, some have been used in
plant breeding for: (a) locating genes on a particular chromosome, (b) transfer of
particular chromosomes with desirable genes from one variety to another, or even
from one species to another, (c) mapping of genes on a chromosome, and (d)
facilitating production of hybrid varieties in barley (balanced tertiary trisomics during
1960s).
Aneuploidy. A situation wherein individuals or cells have chromosome number, not
exact multiple of the basic number (2n ± afewchromosomes).
Angstrom. A unit of length (usually wave length of radiation energy such as X-ray).
Numerically 1Å = 10 -10 m = 0.1nm (nanometer) = 0.0001m (micrometer).
Anisoploid. A mixture of diploid, triploid, and tetraploid plants obtained with seed
harvested from a mixture of diploid and tetraploid plants.
Anlage. A German word that appeared in Mendel’s original paper – Experiments in plant
hybridisation – for the hereditary factor (now called the gene).
Annealing. Spontaneous alignment of two single DNA strands to form a double helix.
This property has been used to unravel a number of cellular mysteries.
Annidation. See Ludwig theorem.
Anther. The pollen-bearing portion of the stamen.
Anthesis. The process of dehiscence of the anthers or the period of pollen distribution.
Antibiosis. Antagonistic association in which one organism has an injurious effect on
normal growth and development of another. It is a true mechanism of insect
resistance. It includes all those factors or characters of a resistant plant that cause
adverse effects on the insect life cycle when the insect uses it for food. Antibiosis is a
specific resistance; however, it is not always monogenic. The main effects of
antibiosis are to retard growth and decrease the rate of reproduction of individual
insects. Four possible physiological explanations were suggested by Painter (1951) to
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account for all those adverse effects on insects that stem from feeding on different
resistant varieties: (a) the deleterious effects of specific chemicals including toxins,
(b) food materials present but for some reason not available to the host, (c) the lack of
specific food materials in the parts of plants eaten by the insects, and (d) the presence
of repellents that prohibit the use of host by the insects leading eventually the insects
to starve and die. However, in this context, two points (b & d) may not account for the
adverse effects associated with antibiosis. Whenever, it is used as a mechanism of
resistance (as in alfalfa against certain aphid species), intense selection pressure is
exerted upon the insect population. Consequently, new biotypes of insects develop
very rapidly especially if the insect is primary or obligate feeder on one crop.
Antibody. A protein (immunoglobulin) molecule, produced by the immune system that
recognizes a particular substance (antigen) and binds to it.
Anticodon. A nucleotide triplet in a tRNA molecule that aligns specifically with a
particular codon in mRNA under the influence of the ribosome, and thus facilitates
insertion of an appropriate amino acid into a growing polypeptide chain.
Antigen. A molecule (usually a foreign one) that is recognized by antibody
(immunoglobulin) molecules. Generally, multiple antibody molecules can recognize a
given antigen.
Antimutagen. Any agent reducing the rate of spontaneous and/or induced mutation.
Antimutagens may act either as protective agents or promote the repair of
premutational lesions (e.g. photo-reactivation). Mutagens and antimutagens are
counteracting factors.
Antisense RNA. The RNA that is synthesised using antisense strand of a DNA segment
as the template. Its base sequences are complementary to the mRNA, which is
produced naturally using sense strand of DNA as the template.
Antisense RNA Technology. A technology that utilises antisense RNA to silence
undesirable genes, and thus causes post-transcriptional down regulation of the gene
activity. Antisense RNA complexes with mRNA, thus making it (mRNA) unavailable
for translation. Using this technology, ripening of the fruit has been delayed by down-
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regulation of the fruit-ripening genes. In other words, it can extend the shelf life of
immediate biological products.
Antixenosis (Kogan and Ortman 1978). Plant characters that cause differential
oviposition and habitation by the insects on the host plants. Antixenosis means that
the plant is avoided because it is an undesirable host.
AP Sites. Apurinic or apyrimidinic sites resulting from the loss of a purine or pyrimidine
residue from the DNA.
Apogamy. Development of an individual from a gametophytic cell (such as synergids or
antipodal cells) other than the egg cell without fertilisation. Such individuals are
haploids.
Apomixis. A naturally occurring way of asexual reproduction through seeds. In this
method, reproductive organs or related structures take part but seeds are produced
without fertilization so that resulting seed is vegetative in origin. Such individuals
contain a genome derived entirely from the female parent (gynogenetic clone). It
produces progeny that are clones of mother, thus allowing the fixation of favourable
genetic combination. It is inherited as one or two dominant Mendelian traits located
on a recombinationless chromosome segment. It is typically facultative. Apomicts are
mostly polyploids or segmental polyploids, suggesting that ploidy levels could be a
key component of the apomixis. Increased ploidy levels cause failures of normal
meiosis; plants have to resort to this means of reproduction in order to maintain its
perpetuality. Further genomic asynchrony in the hybrids between two related species
having different reproductive conduct and timing may also be a possible cause of
apomixis. Both dominant and recessive control of apomixis has been observed. It has
a great potential vis-à-vis fixation of hybrid vigour and is expected to be one of the
intense areas of future research; however, transfer of apomict gene to cultivated
species such as maize, bajra, wheat, and the like has not been successful thus far. The
possibility that apomixis and sexuality are not alternatives has made it difficult to
induce apomixis in sexual diploids. The phenomenon of apomixis (apospory) has
been recorded as the notorious one since it deprived Mendel of being recognized
during his lifetime in the past.
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Apostatic Selection. Selection for rare genotypes.
Approach Crossing. A technique of artificial crossing that involves bagging of
emasculated inflorescence with the inflorescence of pollen parent. The shaking of the
bag daily helps disseminate the pollen to the stigma of emasculated flower. It usually
results in a higher percentage of seed setting because fresh pollen is shed over a long
period of several days. It has been successfully used in wheat and can also be used in
small grain and forage grasses.
Archesporium. A cell or group of cells formed by mitosis of the micro- and megaspore
mother cells.
Arrow. The inflorescence of sugar cane. The branch length of the panicle (inflorescence)
reduces towards the apex giving it a distinct arrow shape.
Artificial Neural Network (ANN). A network of nodes or inputs (like neurons) that are
processed in a non-linear fashion to provide a meaningful output. It may be used to
predict total food-grain production in a specific year on the basis of certain inputs or
variables such as fertilizer consumption, rainfall and other weather parameters.
Ascospore. A sexual spore from certain fungus species in which spores are found in a sac
(fruiting body) called an ascus.
Asexual Reproduction. A mode of reproduction that excludes involvement or union of
gametes or sex cells, that is, reproduction by somatic cells, tissues, or organs. The
ensuing individuals resemble almost exactly their parent. Asexually propagated
species in commercial practice reproduce by vegetative means; genetic segregation
and recombination do not occur in the successive clonal generations. These species
are highly heterozygous due to gradual accumulation of naturally occurring mutations.
They have higher levels of sterility due to constant disuse of sex. The clones of such
species, previously known to be resistant to pathogens (viruses and mycoplasma) and
pests, soon become susceptible due to either mutation of resistant genes or subsequent
changes in biotype composition of these pests and pathogens. This occurs because
genetic filter through meiosis is precluded in successive vegetative generations. This
factor, at the same time, also provides a clue why such species evolve in nature more
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slowly compared to sexually reproducing ones as genetic variation through Mendelian
segregation and recombination is not generated.
Assortment. Matching or grouping of a chromosome with other non-homologous
chromosomes during cell division.
Asynapsis. Failure of pairing of homologous chromosomes during meiosis I.
Atavism. The sudden appearance of an ancestral character in the progeny. It may be due
to a mutation in an otherwise “inactive” gene or to a change in the position of that
gene (owing to inversion or translocation) or even deletion of the regulatory gene.
ATP. Adenosine triphosphate. It is usually used for energy storage in the living systems.
It is synthesized mainly in mitochondria and chloroplasts. The energy from the
breakdown of ATP drives many important reactions in the cell. There is no obvious
reason why ATP is better suited for energy storage than GTP, CTP or UTP. Among
the organic bases, the purine molecule, adenine is simply a pentamer of HCN and is
most readily synthesised. It may be that the relative simplicity of the synthesis of
adenine makes ATP better suited for energy storage than any other triphosphate of N-
bases.
AugmentedDesign(Federer 1956). An experimental design used to evaluate either
larger number of test genotypes or genotypes with small quantity of seeds.
Assessment of larger no. of genotypes is not possible with the most frequently used
design (RCBD) due to problems of soil heterogeneity. Similarly, lower quantity of
seeds of test genotypes poses problems in their replication. Augmented design has
been suggested to obviate these problems. The three proposed augmented designs are:
(1) Augmented Completely Randomized Design, (2) Augmented Randomized
Complete Block Design, and (3) Augmented Latin Square Design. The first of these is
most frequently used when seeds of test genotypes are small in quantity, whereas the
second is most fit under condition of larger number of test genotypes. It is worthmentioning
that what RCBD does by replicating the whole entries, augmented design
accomplishes the same by replicating only the checks and thus saving the space.
Autogamy. Self-fertilization.
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Autonomous Controlling Element. A genetic element that seems to have both regulator
and receptor functions combined in a single unit. It enters a gene and causes unstable
mutation.
Autonomous Phenotype. A genetic trait in multicellular organisms in which only
genotypically mutant cells exhibit the mutant phenotype. Conversely, a
nonautonomous trait is one in which genotypically mutant cells cause other cells,
regardless of their genotype, to display a mutant phenotype.
Autopolyploid. An individual containing multiple copies of the same sets of
chromosomes (AAA, AAAA, AAAAA, etc). Autopolyploids have usually large stomatal
size, cell size, increased vigour, large flowers and fruits, large seed size and other
plant parts compared to their diploid counterparts. However, these diagnostics are far
from universal; many autoploids may be weak and lacking in vigour. This may be due
to the existence of an optimum level of polyploidy. In most groups, this level seems to
be achieved at fairly low levels of ploidy; but in some groups, octaploids or even
higher polyploids are still vigorous types. Autopolyploids are invariably reduced in
fertility and produce fewer seeds than their diploid counterparts due to several meiotic
complexities.
Autosyndesis. Pairing between chromosomes that belong to identical genomes. (Also see
allosyndesis).
Autotetraploid. A cell having four similar sets of chromosomes (AAAA) oranorganism
composed of such cells. Autotetraploids seem to be agriculturally more important
than any other autoploids. Because four is an even number, they can have a regular
meiosis, but this may not always be the case. In general, meiotic process is much
more complex than of diploids, which involve partition of four chromatids, one to
each of the four gametes. In an autotetraploid, five genotypes are possible at each
locus compared to only three in diploids (at two loci, the no. of possible genotypes for
tetraploids and diploids are 5 2 and 3 2 , respectively; would you like to formulate the
no. of possible genotypes in hexaploids at twoloci?). Theexpectedsegregationand
assortment in tetrasomic inheritance is much complex. Factors affecting gametic
output in tetraploids are: (a) the regularity with which tetravalents (quadrivalents) are
21

formed, and (b) the randomness of disjunction from quadrivalents, which depends
primarily on the distance between the kinetochore and the locus in question. Though
several types of pairing possibilities exit in autotetraploids, but the bivalent and
quadrivalent formation tend to be more frequent. Examples of naturally-occurring
tetraploids are potatoes, coffee, perennial rye grass, alfalfa, sweet potatoes, pea nuts,
and the like.
Autoradiogram. A pattern of dark spots in a developed photographic film or emulsion,
in the technique of autoradiography.
Autoradiography. A technique in which radioactive structures take their own
photograph when placed next to a film or photographic emulsion and thus helping in
locating the radioactive compound within the cell.
Autosomes. Chromosomes aside from sex chromosomes.
Autotrophic. A term referring to cells which are able to synthesize their own
macromolecules and obtain their energy from simple nutrient molecules such
ammonia, carbon dioxide, etc.
Average Effect (of a gene). Mean deviation from the population mean of those
individuals, which received that gene from one parent, whereas the other gene of the
pair under consideration came at random from the population studied.
Average Effect (of a gene substitution). The difference between average effects of the
two genes involved in the substitution.
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Backcross. The crossing of an F 1 hybrid or heterozygote with one of its parents.
B 1 , B 2 , …. Symbols used to designate the first, second, etc. backcross generations. The
second and following backcrosses are made by crossing of individuals of B 1 , B 2 , etc.
generations with the same parent.
Backcross Breeding (Harlan and Pope 1922). A system of breeding whereby recurrent
backcrosses are made to one of the parents of the hybrid, accompanied by selection
for a desirable character(s).
B-Line. The fertile counterpart of A-line. It is also called maintainer of A-line. It has
‘fertile’ cytoplasm; however, it does not have fertility restorer nuclear gene(s). The
fertile cytoplasm enables it to produce fertile pollen and thus it is used to maintain A-
line.
Back Mutation. A heritable change in a mutant gene resulting in reversion of the wildtype
phenotype or lost function by so-called forward mutation. A true back mutation,
which is a rare event, restores the original nucleotide sequence of the mutant gene.
Backcross Pedigree. A system of breeding whereby one or a few backcrosses (2/3/4) are
made to one of the parents of the hybrid followed by pedigree selection. It provides
for possibility of obtaining superior transgressive segregants at the cost of losing
genetic control over breeding population.
Bacteriophage. A virus that uses bacteria as its host.
Balance. A condition that refers to adjustment of genetic components in proportions that
give satisfactory development. It applies to individuals as well as populations.
Balanced Polymorphism. Stable genetic polymorphism maintained by natural selection
under the condition of selective superiority of the heterozygote over either
homozygotes (overdominance). Suppose a population that contains three genotypes at
a locus, that is, AA, Aa, and aa. If the fitness of the heterozygote is greater than
homozygotes, the population eventually attains equilibrium with a balance proportion
of both homozygotes and heterozygotes. The equilibrium frequency of allele A =
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[(W Aa –W aa )/(W Aa –W AA )+(W Aa –W aa )], whereas, W represents the fitness of
corresponding genotypes. This frequency also equals to [(S aa )/(S AA +S aa )], whereas,
the letter S represents selection coefficient against the corresponding homozygotes. At
the equilibrium, if the population is slightly perturbed, it will again return to the
equlibrium.
Balanced Tertiary Trisomic (Ramage 1965). A tertiary trisomic set up in such a way
that the dominant allele at a locus is carried on a translocated extra chromosome and
the counterpart recessive alleles on the two normal chromosomes. The system using
genetic male sterility was proposed during 1960s for producing hybrids in barley.
With the development of CMS system, it has lost its significance.
Balancer. A chromosome with multiple inversions, used to retain favourable allele
combinations in the uninverted homologue.
Balancing Selection. Selection favouring heterozygotes. This kind of selection, for the
most part, maintains more or less balanced polymorphism and thus sustains stable
equilibrium in the populations. It maintains diversity within the population. It may
also show some divergence between populations provided environment is different
enough between them. The opposite process will, of course, produce unstable
equilibria. Such selection will cause homozygosity and divergence between
populations.
Barr Body. A densely staining mass that represents an inactivated X-chromosome.
Barriers (to Wide Hybridisation). The sum total of all hindrances encountered while
transferring genes from wild relatives to the cultivated species. Pre-fertilization
barriers include all factors that hinder effective fertilization. These include failure of
pollen to germinate, slow pollen tube growth, inability of pollen tube to reach down
the style, arresting pollen tube in the style, ovary and ovule, and the like. These can be
overcome through manipulation of the chromosome number (of the parent species),
bridge cross technique, shortening style, use of recognition mentor pollen, growth
hormones, in vitro/vivo fertilization, protoplast fusion, etc. Post-fertilization barriers
hinder or retard development of the zygote and normal development of seed including
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reproductive abnormalities in hybrids (hybrid inviability and sterility) and latergeneration
progenies (hybrid break-down).
Base Analogue. A chemical whose molecular structure mimics that of a DNA base.
Owing to mimicry, the analogue may act as a mutagen through mispairing.
Base Collections. Accessions that serve as a broad germplasm base for possible future
use by plant breeders. The base collections are maintained for long periods. This calls
for taking special pains so as to avoid any genetic change likely to be brought in
through mutation and other mechanisms.
Basic Number. The number of chromosomes in any one genome. It is also called the
monoploid number. It is usually designated by x. Allard (1960) has defined it as the
number of chromosomes in ancestral diploid ancestor of a polyploid.
Basmati Rice. Also called aromatic or scented rice. It emits typical aroma or scent (bas=
aroma) in fields, at harvest, in storage, during milling, cooking and eating. It has
special grain quality, for example, intermediate amylose content (21-25%), medium
gelatinization temperature (65-70°C), soft gel consistency, high grain elongation on
cooking, and a typical aroma (as in Basmati 370). The aroma is due to a volatile
compound, known as 2-acetyl-1-pyrroline. They are tall indica types with poor yield.
Attempts have been made to improve it, and now semi-dwarf types are available..
BC 1 , BC 2 ,… . Symbols used to designate the first, second, etc. backcross generations.
B-chromosomes. Small plant chromosomes of variable number between individuals of a
species with no known direct phenotypic role. These are also called supernumerary or
extra chromosomes. Randolf (1928,’46) reported their presence in maize and Longley
named them B-chromosomes. These are largely heterochromatic, and there is fairly
high distribution of B-chromosomes in maize. Certain strains of Indian corn do not
bear these chromosomes, while in others they may go up to 25 or even more.
Bead Theory. The disproved hypothesis that genes are arranged on the chromosome like
beads on a necklace, indivisible into smaller units of mutation and recombination.
Bias. A consistent and false departure of a statistic from its proper value.
Bimodal Distribution. A statistical distribution having two modes.
Bimodal Curve. A curve with two-mode distribution of a variable.
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Biochemical Genetics. A branch of genetics dealing with the chemical nature of genes
and their mode of action in development and function.
Biochemical Markers. Biochemicals such as proteins or enzymes used to identify a
particular biological structure or function. They are limited in number, and specific to
tissues and developmental-stages. However, they can be used reliably in the selection
of superior genotypes. These are inexpensive, and can be generated on a large scale.
Biofertilizers. A misnomer that includes a variety of biologicalagentssuchasbluegreen
algae, azolla, rhizobium, azospirillum, and a host of green manure and green leaf
manure crops including the stem nodulating Sesbania rostrata.
Biofortification. Genetic modification and bioprocessing for the improvement of
nutritional quality of food. It is beneficial in developing countries compared to the
developed ones, which have easy access to alternative means for meeting
requirements of mineral nutrients and vitamins. Recent developments of golden rice,
high iron rice, high protein potatoes are but a few examples of genetic fortification.
Bioinformatics. A truly inter-disciplinary subject that combines biology, mathematics
and computer softwares. It involves the use of information technology in studying
life sciences: a combination of IT and biotechnology. All the information (results) of
molecular biology are based on experiments on the theory of trial and error. Also, it
requires costly chemicals, time and wastage of labour. This wastage can be avoided
with bioinformatics because simulation of protein sequences using mathematical
models in computers and studying them helps us save a lot of time, and provides
quicker and accurate results.
Biological Nitrogen Fixation. The fixation of atmospheric nitrogen in form of protein
by microorganisms such as bacteria. Such bacteria contain nif genes, expression of
which lead to the production of an enzyme, called nitrogenase. This enzyme converts
free nitrogen into ammonia, then to amino acid and ultimately to protein.
Biological Yield. The total yield of a plant material.
Biometrical Genetics. The statistical description of similarity and differences between
groups of organisms for the characters showing continuous variation. It uses
statistical parameters such as means, variances, covariances, and the like. It is the
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fusion of two approaches, that is, genetical and biometrical to the understanding of
continuous variation. The former gives us principles on which the analysis must be
based; the latter shows the way in which to handle continuous variation.
Biometry. Also called biometrics. The branch of science that deals with statistical
analysis of biological observations.
Biparental Mating. A form of mating between randomly selected two individuals
usually in segregating generations (such as F 2 , F 3 , etc). In such generations, each
individual is different from the other, and may be heterozygous for different gene
pairs; thus, mating between them is expected to break undesirable linkage and
provides favourable gene combinations. In clonally propagated crops (such as potato
and sugar cane), F 1 itself is a segregating generation as each parental individual is
highly heterozygous. Thus, crossing between any two individuals either in parental
generationorinF 1 generation corresponds to biparental mating.
Biparental Mating Design. A mating design that is based on the evaluation of biparental
progenies. Male and female of a particular biparental mating is never used for the
second biparental mating. Thus (½)n progeny family is produced. In this design,
genetic variance is overestimated relative to environmental component of variation. It
is the simplest method, and can be utilized in both self- and cross-pollinated crops. In
addition, a large number of BIPs can be included and compared. The main
disadvantage of the method relates to its incapacity to provide for detection of
epistasis.
Biopesticide. A pesticide of biological origin. Among biopesticides, BT protein, which is
produced by a soil borne bacterium, Bacillus thuringiensis, is the most important. In
India, its use was banned till 1991 because of the fear that spores of BT will affect
silkworm. To circumvent this problem, non-sporulating strains of BT, which are toxic
to specific insects, have been developed at NRC for Plant Biotechnology.
Biopecticides have received special attention as: (a) these are environment friendly,
(b) these are not carcinogenic, and (c) they operate with a high degree of insect
specificity.
Biophages. Organisms that consume other living things.
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Biotechnology. A technology involving living organisms and molecules as means of
production while using a variety of techniques such as tissue culture, cloning, cell
fusion, embryo transfer, rDNA technology, and the like. It is seen as the most
powerful technology having the potential to pave the way for evergreen revolution.
Biotype. A group of individuals having same genetic constitution. The group may be
homozygous or heterozygous. The term is used more frequently by entomologists, and
is analogous to the physiological race of a pathogen. Biotypes of insect pests are less
frequent compared to physiological races of pathogens because: (a) insects’
physiology are complex, (b) pest resistance in host plants is often related to the hostfinding
behaviour of insects, (c) most of the insects have more than one hosts
(polyphagous), and (d) asexual mode of reproduction is not so well-developed as in
pathogenic fungi. The frequency with which new biotypes of insect pests is produced
is directly related to the selection pressure exerted by the resistant cultivar and mode
of reproduction of insect pests. All the above four phenomena tend to reduce selection
pressure by the insects themselves. The techniquesusedtocharacterizedifferent
pathotypes are also applicable to distinguish different biotypes.
Bisexual. Of organisms having both male and female reproductive organs producing
respective gametes; of flowers having both stamens and pistils inside them.
Bivalent. A pairing configuration during the first meiotic division that consists of two
completely or partially homologous chromosomes. The number of bivalent per
meiocyte equals half of the somatic chromosome number of diploid species.
Blending Inheritance. A discredited model of inheritance suggesting that the
characteristics of an individual results from mixing of the essences
(pangenes/gemmules) from its parents.
Blocking. The process of putting together experimental units as similar as possible in the
same group (referred to as block). The number of block is equal to the number of
replication. Thus, all the treatments are included into each block separately and
independently. Blocking facilitates measurement of variation among blocks and its
removal from experimental error.
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BOAA. β-oxalyl amino α-alanine. It has also been designated ODAP (β-oxalyl diamino
propionic acid). The neurotoxin is found in the seeds of grass pea. Its continued
consumption is associated with a disease, called lathyrism (paralysis of lower limb).
There are certain cultivars (e.g., Ratan, Prateek, etc), which have negligible BOAA
(ODAP) content.
Boll. The fruiting body of cotton. It consists of lint and seeds besides outer coverings.
Bolting. Production of seed stocks the first season in a biennial crop, as in sugar beet.
Breeder Seed. The seed or vegetative propagating material produced by or under direct
control of the plant breeder or agency sponsoring a variety. It is used to produce
foundation seed.
Breeders’ Right. Breeders’/researchers’ interests. It implies exclusive authority of plant
breeders over what comes out of their experimentation (plant variety, for example).
And, protection for plant breeders’ rights is called varietal protection. If plant
breeders’ rights are safeguarded, farmers’ interests will ultimately be marred.
However, the Government of India is trying to protect the rights of both.
Breeding. The controlled evolution of plants and animals by humans with the goal of
producing populations that have superior agricultural and economic characteristics.
Breeding Nursery. A place for rearing of breeding materials under a specific condition.
Breeding Size. The number of individuals in a population that are actually involved in
reproduction in a given generation.
Breeding System. A system comprising all variables excluding mutation that affect the
genetic relationship of uniting gametes in sexual reproduction. These variables are
classified into two groups: (a) those, which affect the ability of particular parents to
mate or gametes to fuse, and (b) those, which affect the probability of mating of
parents or of fusing of gametes. The first group of variables constitute the mating
system. The breeding system controls the extent of outbreeding through various
forms: exclusive outbreeding, predominant selfing, and a mixture of selfing and
crossing.
Breeding Value. Performance of a parent adjudged by the mean performance of its
progenies following some definite systems of mating. It is a more authentic index of
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an individual’s true genetic potential than its per se performance. It equals the sum of
the average effects of the genes contained in its genotype. The b.v. is closely related to
parent-offspring correlation and is nothing but general combining ability effect of
each parent in cross combinations. Since segregating generations are not identical to
single crosses (F 1 ’s), b.v. is obtained from hybrid progenies only. However, it
amounts to half the deviations of its progeny mean from the population mean when
the genotype mates at random with other members of the population.
Bridging Cross (Sears 1956). A cross made to transfer alleles between two sexually
isolated species by first transferring them to an intermediate species that is sexually
compatible with both. It is also called Sears’ technique. He wanted to transfer
resistance to leaf rust from Aegilops umbellulata (CC) to common wheat (AABBDD),
but found it difficult due to crossability barrier between these two species. He first
madeacrossbetweenA. umbellulata and T. dicoccoides (AABB). The chromosome of
the F 1 was doubled. Thereafter, the doubled hybrid was crossed to T. aestivum and the
resulting F 1 was backcrossed twice to T. aestivum. Hegotaleaf-rustresistantplant
that carried an extra chromosome. Then irradiated pollen from this plant was used to
pollinate T. aestivum. He got a plant with leaf rust resistance as well as normal
chromosome complement. Apparently, the gene(s) for resistance was translocated to
one of the chromosomes (near kinetochore) of T. aestivum through a process called
unidirectional insertional translocation. When bred to homozygosity, the resistant
plants were indistinguishable form parental ones except for slightly later maturity. In
the entire crossing programme, T. dicoccoides acted as the bridge species.
Bridging-Host Hypothesis (Ward 1902). A discouraging hypothesis towards breeding
for disease resistance. It assumed that breeding for resistance is futile because the
pathogen can produce new races in steps with the breeding of new varieties, and thus
resistance would decline in the new varieties as fast as they are produced. The
hypothesis made two-fold impacts: (1) it discouraged many from attempting to breed
for resistance, and (2) it necessitated further investigations to search for the
mechanisms by which new pathogenic types would appear from the previous ones.
Broad Genetic Base. Having high genetic variability as in an open-pollinated variety.
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Bt Genes. Genes (from the Gram Positive soil bacterium Bacillus thuringiensis)
encoding insecticidal crystal proteins (ICPs). ICPs are commonly designated “Cry”
proteins, and the genes encoding them are better known as “Cry” genes. Over fifty
such genes (Cry1Ac, Cry1C, Cry1B, Cry1D, and the like) have been identified and
sequenced. These are effective against caterpillars of Lepidopteran insects. The gene
has been incorporated in several crops including cotton. The gene product is a
crystalline protein, which remain in such plants in active form. When the insect
utilises such plants as its food, the crystalline protein causes paralysis of mid-gut, and
the insect ultimately dies.
Bt Crops. Genetically engineered crops known to carry stably integrated and expressed
Bt genes (Cry genes). Maize, rice, cotton and several other crops have been
genetically modified with Bt genes. These genes lead them to produce delta endotoxin
protein, which upon hydrolysis in alkaline environment of the mid-gut releases delta
endotoxins. These toxins through several intermediate reactions cause paralysis of the
digestivesystemsandleadtothedeathoftheinsect. Acottonvariety, BollGuard,
which is a product from a multinational company, is known to carry such a gene (Cry
1Ac). Apprehensions have been raised that the widespread cultivation of such crops
may cause intense selection pressure on the insects, and Bt genes are likely to be fixed
in the insect populations. Upon fixation, such crops will become ineffective as insectcontrol
agents. The second apprehension relates to exposure of human populations
continuously to such Bt-proteins.
Bud Sport. A somatic mutation expressed in a bud or branch of plants. Also called bud
mutation.
Bud Pollination. Placement of mature pollen on immature stigmas. This may allow
slow-growing incompatible pollen tube to reach ovary in time so that fertilization may
occur. The rationale is that stigma accumulates inhibitory biochemical substances
only if it gets matured. As temperature also plays an imortant role in the accumulation
of such inhibitory biochemicals, pollinating at low temerature may also accomplish
thesamepurpose.
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Bulbosum Technique (Kasha and Kao 1970). A technique to induce haploids by
crossing a species with Hordeum bulbosum. Haploids of cultivated barley have been
produced through this technique. When diploid cultivated barley (H. vulgare) is
crossed with a diploid wild relative (H. bulbosum), fertilization occurs; but during the
ensuing somatic cell divisions, the chromosomes of the wild relative (H. bulbosum)
are preferentially eliminated from the zygote resulting in a haploid embryo. (The
haploidization process seems to be caused by a genetic incompatibility between
chromosomes of different species). Although originally proposed for barley,
pollination of bread wheat with H. bulbosum also results in haploid plantlet
production.
Bulk Population Breeding. The growing of genetically diverse population of
autogamous crops in a bulk plot for several generations with or without mass
selection followed by individual plant selection. During the period of bulking, natural
selection usually determines the course of differentiation, and hence it is also called
evolutionary method of plant breeding.
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C 306. A tall wheat variety developed through a complex cross in 1965. This variety still
continues to be dominant wheat for drought (soil moisture stress) and low-fertility
conditions in some parts of the plains of India.
C 4 Pathway. A pathway of CO 2 fixation in many angiosperms in which the first stable
product of photosynthesis is C 4 compounds such as aspartate, malate, and
oxaloacetate (instead of 3-phosphoglycerate of the Calvin cycle). In mesophyll cells,
the CO 2 is assimilated by carboxylation of PEP. C 4 acids so-produced are then
transferred to the bundle sheath cells, probably by diffusion. Within these cells, the
CO 2 released by decarboxylation enters the PCR cycle, giving origin to 3-
phosphoglycerate. It is interesting to note that the C 4 pathway and PCR cycle are
integrated in different cells of the same plant. From evolutionary point of view, this
system of CO 2 fixation is most efficient, as photorespiratory losses (a characteristic of
C 3 plants) do not occur in this case. The initial enzyme in C 4 pathway is PEP
carboxylase, which is insensitive to O 2 . The bundle sheath cells get only CO 2 through
decaboxylation of C 4 acids, and the enzyme ribulose-1,5-diphosphate carboxylase
produces only phosphoglyceric acid (in absence of O 2 ). There is thus no
photorespiration.
Callus. An undifferentiated mass of cells, originating from an anther, microspore, plant
tissue ex plant, or other cellular sources, when cultured in vitro.
CAM. A recently discovered photosynthetic mode especially adapted to deserts called
Crassulacean Acid Metabolism. In this mode, several desert succulent plants
including the cacti keep their stomata closed during the hot daytime and open them in
the cool of the night. CO 2 absorbed through leaf openings is stored in organic acids
and not fixed until the next day. This delayed photosynthesis greatly reduces water
loss during the day, thereby enhancing the succulent plant’s ability to maintain water
balance and water storage.
Canalisation. A developmental buffering system reducing potential variation due to
genotype or environment.
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Canalised Character. A character whose phenotype is kept within narrow bounds
despite disturbing forces. Development is such that all the different genotypes have
the same constant phenotype over the range of environments that is usual for the
species. The genetic differences are revealed if the organisms are put in stress
environment or a severe mutation stresses the developmental system.
Candidate Gene. A sequenced gene of previously unknown function that, because of its
chromosomal position or some other property, becomes a candidate for a particular
function such as disease determination.
Carbohydrate. A macromolecule composed of carbon, hydrogen, and oxygen.
Carbohydrates are the main source of energy and are also important components of
cell wall and intercellular materials. Monosaccharides are simple sugars having the
general formula C n (H 2 O) n . Ribose, deoxyribose, glucose, fructose, galactose,
mannose, etc. are monosaccharides. Disaccharides are sugars formed by the
condensation of two hexose monomers with the loss of one molecule of water. They
are, therefore, C 12 H 22 O 11 . Sucrose and lactose fall in this category. Polysaccharides
result from the condensation of many hexose monomers, with a corresponding loss of
water molecules. Their formula is (C 6 H 10 O 5 ) n . The most important polysaccharides in
living organisms are starch (plants) and glycogen (liver and muscles), and cellulose,
the most important structural element of the plant cell wall.
Carbon Source. A nutrient (such as sugar) that provides carbon to organisms needed for
the synthesis of organic molecules.
Carboxydismutase. Also called ribulose-1,5-diphosphate carboxylase; the primary
photosynthetic enzyme that represents about 50% of the stromal proteins. It leads to
the integration of CO 2 and H 2 O with ribulose-1,5-diphosphate to produce two
molecules of 3-phosphoglycerate (the first stable product of the PCR cycle). The
enzyme is highly sensitive to O 2 , and it also brings about oxygenation of ribulose-1,5-
diphosphate to produce one molecule of 3-phophoglycerate and one molecule of
glycolate. The latter then enters photorespiratory cycle and ultimately oxidised to C0 2
and H 2 O. This loss is the characteristic of PCR cycle (C 3 plants). The enzyme
comprises 16 sub-units (L 8 S 8 ): 8 large sub-units of high molecular weight (L 8 ), and 8
34

small sub-units of smaller molecular weight (S 8 ). The large sub-unit is coded by the
genes (rbc-L) present in chloroplast DNA, while the small one is produced by nuclear
genes (rbc-S). The rbc-L is the highly conserved gene; only little variation has been
observed across species.
Carcinogen. A substance that causes cancer.
Carrier. An individual who possesses a mutant allele but does not express it in the
phenotype owing to the presence of its dominant counterpart. Thus the genotype Aa is
acarrierofa if there is complete dominance of A over a.
Carrying Capacity. The ability of an ecosystem, community, organism or any part
thereof to maintain and sustain further growth. The amount of biomass that can be
supported under inputs (accumulated organic matters) and outputs (respired organic
matters) balance is called maximum carrying capacity. No further increase in size
(growth) is possible if the system has reached this theoretical limit. Increasing
evidences suggest that the optimum carrying capacity (where growth rate is highest)
sustainable over long periods in the face of environmental uncertainties is lower by as
much as 50% than the theoretical limit of maximum carrying capacity.
Caryogamy. The fusion of nuclei of two opposite sex cells following immediately after
plasmogamy in higher plants.
Catabolite Repression. The inactivation of an operon caused by the presence of large
amounts of the metabolic end products of that operon.
Catalyst. A substance that accelerates chemical reactions. However, the catalyst itself is
not modified in the process, so that it can be used again and again.
Cation. A positively charged ion such as Na + , K + , etc.
cDNA Library. A library composed of cDNAs (complementary DNAs), not necessarily
representing all mRNAs.
Cell (Hooke 1665). The smallest morphologic and physiologic unit of a living system
characterized by growth, individual metabolism, independent energy cycle, and
reproduction.
Cell Biology. The study of structure, function, development, reproduction, and life
history of cells.
35

Cell Cycle. The life cycle of the individual cell encompassing interphase and nuclear
division phase.
Cell Division. The process by which two cells are formed from one. It is the reproduction
of cells by karyokinesis and cytokinesis. The process of cell division should be
appropriately called cell multiplication.
Cell Lineage. A pedigree of cells related through asexual division.
Cell Sap. The soluble phase of the cell. It may consist of at least three general fractions:
(a) micromolecular (water, inorganic ions, and dissolved gases), (b) mesomolecular
(all metabolic intermediates such as lipids, sugars, nucleotides, etc), and (c) free
macromolecules (mostly proteins, and RNAs).
Cell Theory (Schleiden and Schwann 1838/1839). A statement that all living organisms
are composed of cells and cell products. The modern cell theory is a compilation of
several discoveries regarding cells, that is: (a) cells are the smallest unit of life, (b)
cells are the morphological and physiological units of life, (c) the properties of a
given organism depend on those of its individual cells, and (d) cells originate only
from pre-existing cells, and continuity is maintained through the genetic material.
Cell Wall. The rigid exoskeleton structure enclosing and protecting the contents of most
plant and bacterial cells.
Centimorgan (cM). A map unit; so-called after Thomas Hunt Morgan.
Central Dogma. The hypothesis that biological information flows from DNA to RNA to
protein. Except some exception (RNA viruses), the rule is generally valid.
Centres of Origin (Vavilov 1926). Geographical regions represented by the greatest
diversity of plant species. Vavilov considered these ares as geographical regions of
origin, especially if wild races of relevant species were also present in those regions.
He identified 8 such centres. Later he concluded that this system did not hold true,
and he developed a system of ecological groups based on specific traits. However,
this concept has been substantially modified by Harlan (1992), who suggested to
abandon the concept of centres altogether, and advocated to refer “ecological regions
rather than to centres”.
36

Centric Fusion. A reciprocal translocation in which the large arms of two acrocentric
chromosomes unite to form a metacentric chromosome. It necessarily reduces the
chromosome number by one through eliminating the small translocation product,
which is mostly heterochromatic. However, the number of large chromosome arms
remains unchanged. Also see Robertsonian translocation.
Centromere. A localized region in each chromosome to which the spindle fibres appear
to be attached and which seem to determine movement of chromosomes during cell
division. (Also called kinetochore by cell biologists).
Centripetal Selection. Also called stabilizing selection. Selection acting against extreme
phenotypes, that is, selection favouring an intermediate “stable” value (phenotype).
This happens more often in natural populations. Genotypes having intermediate value
for most survival and reproductive traits are favoured owing to greater biological
fitness. If the population has a number of different intermediate genotypes, it will
cause preservation of genetic variability.
Certified Seed. Seed produced from the foundation or certified seed under the regulation
of a legally constituted agency. This class of seed is used for commercial crop
production. It is produced on a large scale in a way so as to meet the standard set by
the certifying agency.
Chaos Theory. A small change to a complex system can result in a large and unpredicted
outcome. For example, the introduction of semi-dwarfing gene in the hexaploid wheat
has resulted in short stature, increased photo-receptivity and harvest index, high yield,
and the like.
Chargaff’s Rule. A rule that is followed in all DNA molecules. Total amount of purine
bases (A+G) isalways equal to the total amount of pyrimidine bases (C+T). Further,
the amount of A always equals the amount of T, and C always equals G. However,
(A+T) /(C+G) ratio varies across species; but within the species, it remains relatively
constant. This rule seemed to be one of the milestones towards deciphering double
helical model of DNA by Watson and Crick (1953).
Character. A specific property of an individual resulting from the interaction of a
gene(s) with the environment. A gene cannot cause a character to develop unless
37

proper environment is present; conversely, no amount of manipulation of an
environment will produce a character if necessary gene(s) is absent. It is the interplay
of these two agents that determine the development of the character. Geneticists use
this term as a synonym for characteristic or trait.
Character Difference. A contrasting difference between two organisms (or two pure
lines) with respect to one particular character. For example, plant height is a
character, and the two lines, one with tall and other with dwarf statures represent
character difference.
CHAs. Chemical Hybridising Agents. These are chemical agents used to selectively kill
pollens of a genotype likely to be used as female parent in a hybrid-breeding
programme (ethrel, DPX 3378, MG1 & MG2, for example). Ethrel causes female
sterility in Indian mustard when applied at the rate of 2000-3000 ppm. MG1 & MG2
are carcinogenic and have residual effect too. Oxanilates @ 1500 ppm can also be
applied for the same purpose during stamen-pistil primordia stage. However, no
chemicals are perfectly selective. In wheat, chemicals such as CH 9701, CH 9702, CH
9708, CH 9831, and CH 9832 have exhibited acceptable levels of male-sterility. The
most suitable stage for the spray of CHAs has been found as 10-15 mm of spike
length.
Chasmogamy. One of the outbreeding devices as in rice. In this case, opening of flower
buds follows after bursting of the anther. Thus, some extent of outcrossing (4-5%) is
almost always ensured.
Check Cross. The crossing of an unknown genotype with a phenotypically similar
individual of known genotype. The F 2 segregation serves to establish whether the
same phenotypes stem from the action of identical or dissimilar alleles of the same
locus or from the action of non-allelic genes.
Chi-Square ( 2 )Test. A statistical test used to determine the probability of obtaining
observed proportions by chance, under a specific hypothesis.
Chiasma. A cross-shaped structure formed following crossing over between any two
non-sister chromatids of a pair of homologous chromosomes during meiosis. It is
visible manifestation of crossing over.
38

Chimera. A plant (rarely an animal) composed of genetically distinct tissues owing to
mutation, somatic segregation or grafting. According to their structure, chimera may
be sectorial (different tissues growing side by side and occupying distinct sectors of
varying size), periclinal (one tissue placed centrally and the second grows around it as
the covering layer), and mericlinal (actually an interrupted periclinal chimera).
Chimeric Genes. Genes generated through rearrangement and shuffling of other genes
having their own coding and regulatory domains.
Chlorophyll. An asymmetrical molecule with porphyrin hydrophilic head composed of
four pyrrole rings forming a complex with an Mg atom. The molecule has also a long
hydrophobic phytol chain attached to one of the rings. There are several types of
chlorophyll (a, b, c, d, and e). Types a and b are found in chloroplasts of higher
plants.
Chloroplast. A chlorophyll-containing organelle (plastid) in plants, serving as the site of
photosynthesis. Besides chlorophyll, it also contains carotenoid pigments. It seems to
arise from pre-existing chloroplasts. It has its own DNA, and is considered semiautonomous.
Chondroid. A cell organelle in bacteria, which is functionally equivalent to the
mitochondrion of higher organisms.
Chromatid. One of two thread-like structures formed by the longitudinal division of a
chromosome during prophase, and known as daughter chromosome during anaphase.
Chromatid Conversion. A type of gene conversion that is inferred from the existence of
identical sister-spore pairs in a fungal octad that shows a non-Mendelian allele ratio.
Chromatid Interference. A situation wherein the occurrence of a crossover between any
two non-sister chromatids affects the probability of those chromatids being involved
in other crossovers in the same meiosis.
Chromatid Segregation. See double reduction.
Chromatophore. Any particle isolated from photosynthetic bacteria, which contains
photosynthetic pigments. Chromatophores are scattered throughout the cytoplasm of
such bacteria.
Chromogene. Any gene localised in chromosomes as opposed to cytogene.
39

Chromomere. A small beadlike structure visible on a chromosome during prophase of
meiosis and mitosis.
Chromoplast. A plastid containing coloured (yellow or orange) pigments with reduced
chlorophyll content. Yellow or orange chromoplasts occur in petals, fruits, and roots
of certain higher plants. The red colour of ripe tomato fruits is due to the presence of
chromoplasts having the red pigment, lycopene, which is a member of carotenoid
family. Chromoplasts containing pigments such as phycoerythrin and phycocyanin are
found in red and blue green algae, respectively.
Chromosomal. Pertaining to structure, constituents, and function of chromosomes.
Chromosomal Inheritance. Inheritance of a genetic information contained in the
chromosomes (Mendelian inheritance) as opposed to extrachromosomal inheritance.
Chromosome. Structural units of the nucleus that carry genes in a linear order.
Chromosomes undergo drastic morphological changes during the life cycles of
organisms. Most commonly, chromosome numbers range from 6 to 25 homologous
pairs in diploid condition. At the extreme, one pair has been found in certain strains of
the horse nematode Parascaris equorum, and 630 pairs in the fern – Ophioglossum
reticulatum.
Chromosome Aberration. Any type of change in the chromosome structure or number;
also called chromosome mutation. It is the occurrence of cytologically visible changes
in the genome. The changes may include chromosome parts, whole chromosomes, or
whole chromosome sets. The change in chromosome parts is called structural
aberrations or chromosome rearrangements. The changes in whole chromosome sets
are referred to as numerical aberrations or euploidy.
Chromosome Addition. The addition of a chromosome (pair) to the normal complement
of a variety. If the extra chromosome is from another species, it is called alien
chromosome addition. O’Mara (1940) developed disomic addition lines by attempting
acrossbetweencommonwheat(AABBDD) andrye(RR) followed by chromosome
doubling and selecting a disomic addition line of wheat with a pair of added
chromosomes from rye (2n= 42+I r I r ) in later generations.
40

Chromosome Complement. The group of chromosomes derived from a particular
gametic or zygotic nucleus. It may be composed of one (monoploid nucleus), two
(diploid nucleus), or more (polyploid nucleus) chromosome sets.
Chromosome Disjunction. The separation of paired chromosomes during anaphase of
first meiotic division.
Chromosome Loss. Failure of a chromosome to become part of the daughter nucleus at
cell division. Slow replication of DNA of the chromosome (compared to the rest
ones), its slow movement towards the pole, and similar other reasons may cause
chromosome loss during the cell multiplication.
Chromosome Map. The graphic representation of a chromosome in which the genes
belonging to a particular linkage group are plotted according to their relative
distances. In genetic maps, recombination frequency expressed in map units is used as
a measure of distance between linked genes. However, in cytological maps, genes are
located on the basis of cytological findings obtained with the aid of chromosome
mutations such as deletions, inversions, or translocations.
Chromosome Rearrangement. See chromosome aberration.
Chromosome Substitution. The process by which a chromosome of one variety is
replaced or substituted into the genome of another variety. If a non-homologous
chromosome is substituted into the genome of another species, the process is referred
to as alien chromosome substitution. Nullisomics and monosomics can be used to
facilitate such substitutions.
Chromosome Theory of Inheritance (Sutton and Boveri, 1902). The unifying theory
stating that chromosomes are containing structures of genetic factors. This theory
accounts for inheritance pattern of Mendelian traits.
Chromosomal Hybrid Sterility. Sterility associated with the failure of chromosome
pairing and other meiotic misadventures indicating lack of structural homology
between chromosomes of two parental species. All these upset formation of
functional gametes.
cis Dominance. The ability of a gene to affect genes next to it on the same chromosome.
41

cis-trans Test. A test to determine whether two mutant sites of a gene are in the same
functional unit or gene.
Cistron. Originally defined as a functional genetic unit within which two mutations
cannot complement. Now, it is equated with the term gene as the region of DNA that
encodes a single polypeptide, tRNA or rRNA.
CK60A (Stephens and Holland 1954). A cytoplasmic-genetic male sterile line
developed through placing Kaffir (pure line) genome in the cytoplasmic background
of Milo (pure line) by repeated backcrosses in sorghum. It provided the initial choice
of male-sterile line for hybrid breeding programme after its introduction from USA.
This cytoplasm is designated as A 1 . Other versions of male-sterile cytoplasm are also
available (A 2 and A 3 , for example).
Clade. A group of related biological taxa that includes all descendants of an often remote
common ancestor.
Cladistics. A system of biological taxonomy that reconstructs phylogenies in terms of
successive sequences of branching ancestor-descendant lineages.
Cladogenesis. Evolutionary change characterized by tree-like (dendritic) branching,
illustrating phylogenetic relationships.
Clean Crop. An approach of establishing a multiline variety in which all the components
lines were resistant to all the prevalent races of the pathogen(s). The aim of this
approach was to keep the crop as free of disease as possible, and at the same time to
reduce the catastrophic disease losses following shifts in the racial composition of the
pathogen population.
Cleavage. Rapid cycles of DNA synthesis followed by cell division in which the
cytoplasm is partitioned without growth, occurring very early in embryonic
development.
Cleistogamy. Fertilisation within closed flowers. It is a mechanism in which flower buds
do not open at all. Therefore, it enforces self-pollination and imposing restriction to
outbreeding. However, the precision with which it works is subject to modification by
both genetic and environmental forces. True cleistogamy is exemplified in the basal
inflorescences of California oatgrass (Danthonia californica), lettuce, etc. It is
42

perhaps favoured in a dry flowering season (Mediterranean climates in spring)
because such flowers remain closed during much of the pollination period.
Cleistogamy inhibits desiccation of anthers and stigmas, thus promoting full seed set
by self-pollination. Flowers remain open only for a brief period, providing
opportunities for occasional outcrossing. In some grain legumes such as pigeonpea,
cleistogamy has been found under simple genetic control (monogenic recessive trait).
Clone. A group of individuals descended vegetatively (asexually) from a common
ancestor. Organisms so derived are highly uniform and homogeneous, and genetically
identical to their ancestor. A number of agricultural crops are clonally propagated.
Nearly all clonally propagated species are perennial outcrossers in nature, and all such
plants are highly heterozygous and conspicuously intolerant of inbreeding. They often
experience inbreeding depression following inbreeding (selfing). Only highly superior
clones that carry favourably interacting alleles of many different loci, much like the
genotypes of elite F 1 hybrids of corn, succeed in agriculture.
Cloning. Asexual production of a line of cells or organisms or segments of DNA
genetically identical to the original one.
C-meiosis. Also called colchicine meiosis. It is a modified meiosis that occurs following
the action of colchicine and other spindle poisons. If spindle formation is completely
suppressed during the first or the second or both meiotic divisions, meiotic products
with more than one chromosome set result. As a rule, spindle inhibition during the
first or second meiotic division yields two diploid meiotic products. If the spindle
cannot function in both meiotic divisions, the result is a tetraploid meiotic product.
Similarly in C-mitosis, complete spindle suppression leads to the formation of
restitution nuclei with doubled chromosome numbers.
Coadaptation. The selection processes that tend to accumulate harmoniously interacting
(coadapted) individuals or (coadapted) alleles into the gene pool of populations. Also
the selection processes that tend to favour individuals that have mutually beneficial
phenotypic associations with each other in populations.
43

Coarse Cereals. A group of cereals consisting of maize, sorghum, pearl millet, ragi,
barley and a host of small millets including proso and foxtail. These are nutritionally
rich cereals.
Code Dictionary. A listing of 64 possible codons and their translational meanings such
as corresponding amino acid(s), chain termination, and the like.
Codominance. The situation in which a heterozygote shows the phenotypic effects of
both alleles equally.
Codon. A triplet of bases of mRNA that encodes a single amino acid or specifies
commencement or termination of translation of a genetic message.
Coefficient of Coincidence. The ratio of the observed number of double recombinants to
the expected number. It is a measure of the intensity of interference. Its value ranges
from 0 (absolute interference, that is, no double recombinants) to more than 1
(negative interference).
Coefficient of Selection. See selection coefficient.
Coenospecies. Two or more taxonomic species with the common evolutionary origin.
Only limited gene exchange can occur between coenospecies. Usually sterile hybrids
are produced by hybridisation.
Coenzyme. Usually a non-protein compound of low molecular weight required for the
action of some enzymes. For example, dehydrogenases require a nicotinamideadenine
dinucleotide (NAD + ) to function. In many coenzymes, as in NAD + , the
essential components are vitamins, particularly those of the B group.
Cofactor. A metal or a prosthetic group (in case of conjugated proteins) required by the
enzymes for their activity.
Coincidence. See coefficient of coincidence.
Colcemid. A synthetic equivalent of colchicine. It is used to induce polyploids usually in
animals.
Colchicine (Blakeslee and Avery 1937). An alkaloid drug used to induce polyploidy in
plant species. However, at a lower concentration, it also acts as a mutagen. It is
extracted from seeds or corms of the autumn crocus (Cochicum autumnale L.), a
member of family Liliaceae. It acts as spindle poison and thus arrests formation of
44

spindle fibres during cell division. In the absence of spindle fibres, separation of
daughter chromosomes and their migration to the opposite poles does not occur. Thus
it leads to a cell with doubled chromosome complements. For induction of polyploid,
it is applied to meristematic regions of the plant by wetting with an aqueous solution,
by spraying on in an emulsion or by rubbing on in a lanolin paste. It is effective when
applied to germinating seeds, or to young seedlings, roots, or to growing points such
as shoots or buds. The concentration, duration and temperature of the treatment all
affect the outcome. After application, the chemical arrests spindle formation in the
dividing cell and prevent separation of the daughter chromosomes and their migration
to the opposite poles. Thus the cell becomes polyploid. Following induction,
polyploid tissue may be identified on account of its larger cell size and more
chloroplasts in the guard cells. Some other chemicals that can induce polyploidy are
colcimid (used more frequently in case of animal cells), acenapthene, chloral hydrate,
ethyl-mercury-chloride, sulphanilamide, chloroform, ethyl alcohol, and the like.
Colinearity. The exact point-by-point relationship between the order of amino acids
along the polypeptide chain and the order of corresponding codons along the
polynucleotide chain of the nucleic acid.
Colony. A visible clone of cells.
cDNA. Also called complementary DNA. Synthetic DNA transcribed from a specific
RNA through the action of an enzyme called reverse transcriptase.
cRNA. Also called complementary RNA. Synthetic RNA produced by transcription from
a specific DNA single-stranded template.
Complementary. Of genes that interact to produce a distinct qualitative effect compared
to their individual effects separately.
Complementary Action. Gene action such that two nonallelic genes may be required to
produce a single effect, that is, each gene complements to the action of the other to
make the final product (function). Wherever it operates, a 9:7 ratio is obtained [9 (A-
B-): 7 (A-bb, aaB- & aabb)] in F 2 generation. If the whole population is advanced to
the F 3 generation, the ratio gets changed to 25: 39. (Can you work out?)
45

Complementation. The production of a wild-type phenotype when two different
mutations are combined in a diploid or a heterokaryon.
Complementation Test. See cis-trans test.
Complex Loci. A cluster of two or more closely linked and functionally related genes
constituting a pseudoallelic series. It may contain one operon.
Complex Trait. A trait whose inheritance can be explained by the interaction of several
genesplustheenvironment.
Complon. A complementation unit.
Conjugated Proteins. Proteins that are attached to a non-protein moiety, called
prosthetic group. To such a group belong the glycoproteins, lipoproteins,
hemoglobins, etc.
Contingency Test. A statistical test for independence of the results of a given experiment
observed under different experimental conditions. For example, if one has advanced
thesameF 1 and grown the F 2 in two environments, this test facilitates in deciding
whether there is an interaction between the results of a cross and its experimental
conditions by calculating contingency chi-square.
Continuous Variation. Variation showing an unbroken range of phenotypic values.
Controlled Environment Agriculture. Also called Protected Agriculture; a high-level
technology to gain the maximum while maintaining quality of environment through
manipulation of genetic potential of crops to the maximum under controlled
conditions.
Controlling Element. A mobile genetic element capable of producing an unstable
mutation through insertion in a target gene. Two types exist: the regulator and the
receptor elements.
Cosegregation. Parallel inheritance of two genes due to their close linkage on a
chromosome.
Cosmid. A cloning vector that can replicate autonomously like a plasmid and can be
packed into a phage.
Co 205. The first successful interspecific hybrid of sugar cane released as a cultivar in
India. It was developed by crossing a clone - Vellai - of S. officinarum with the wild
46

species, S. spontaneum. One of the seedling of this interspecific cross provided Co
205 that became commercially acceptable in sub-tropical India, and later replaced the
then existing varieties belonging to the species S. barberi.
Coefficient of Variation. A measure of relative dispersion for the purpose of comparing
two or more statistical series. The series may differ in respect of standard deviation,
means or both or may differ in units. However, cv, which is a percentage, and
therefore, unitless can be used to compare variability present in them. Numerically, it
is standard deviation expressed as the percentage of mean [cv = (σ/μ) x 100]. It
remains unaltered by a change in scale; but it is altered by a change of origin, which
affects the mean but not the standard deviation. The cv, as used in breeding
experiments, expresses experimental error as the percentage of mean. Numerically, cv
=[(σ 2 e) ½ / μ] x 100. It indicates the degree of precision with which the treatments
are compared, and is a good index of the reliability of the experiment. Thus higher the
cv value, the lower is the reliability of the experiment. The cv varies greatly with the
type of experiment, the crop being grown, the crop season, and the character
measured.
Cohorts. Progenies from different crosses in the same generation included for
simultaneous evaluation in the test nursery.
Combining Ability. The ability to produce superior hybrids when crossed with other
appropriate inbreds.
Commensalism. A kind of interaction in biotic community in which one population
(commensal) benefits but the second (host) is unaffected.
Common Phenolics. Phenolic compounds found in plants. These are present in small
concentration in both resistant and susceptible plants; but their synthesis or
accumulations appear to be accelerated following infection. These are toxic to the
pathogens, and their production and accumulations proceed at a faster rate after
infection in a resistant variety than in the susceptible one.
Community. In ecological sense, all the populations occupying a given area.
Competition Effect. Interdependence of adjacent plants because of their common need
for limited available resources such as sunshine, soil nutrients, moisture, CO 2 , O 2 , and
47

the like. Non-planted borders, varietal and fertilizer competition, and missing hills
types of competition effects are taken into account. Removing border plants from plot
measurements, grouping homogeneous treatments and stand correction are a few
techniques of controlling competition effects from the agricultural experiments.
Competition. The endeavour of two or more organisms (genotypes) of the same or of
different species to gain access to a factor(s) or a thing that is in short supply. In plant
breeding, such competition is usually for survival and reproduction of individuals in
the population.
Competitive Ability. The ability of one species or a genotype to succeed in a mixture. It
depends on the ability to (a) produce a greater proportion of offspring to the next
generation at the expense of the others, (b) reduce the number of its counterparts
without being changed, and (c) avoid the others without necessarily having an effect
on it.
Complete Block Designs. Statistical designs for conducting agricultural experiments in
which each block (replication) includes a complete set of treatments. CRD, RCBD
and LSD are the examples of complete block designs. These designs are suited for
experiments with a small number of experiments.
Completely Randomised Design. The experimental design in which the treatments are
assigned completely at random so that each experimental unit has the same chance of
receiving any one treatment. For CRD, any difference among experimental units
receiving the same treatment is considered as experimental error; hence, it is
appropriate only for experiments with homogeneous experimental units such as
laboratory experiments, where environmental effects are relatively easy to control. For
field experiments, where there is generally large variation among experimental plots
owing to soil fertility gradient (for example), the CRD is rarely used.
Complex Loci. A cluster of two or more tightly linked but functionally related genes
constituting a pseudoallelic system. These genes may be regulated in a coordinated
manner.
Component Compensation (Adams, 1967). The increase in one individual component
of a complex trait accompanied by a corresponding decrease in the other. For
48

instance, the increase in the number of grains/spike is often accompanied by the
decrease in seed weight so that total grain yield may remain unaffected. This occurs
owing to a physiological balance between source and sink. It has been shown that
component compensation (and negative correlation) arises in response to competition
between developmentally flexible components. In the presence of a stress, component
compensation is nearly complete; in its absence, component compensation (which, in
turn, indicates partial independence of genetic component traits of a complex
character) is partial, albeit not zero.
Composite Breeding Approach. A breeding strategy that involves a number of specific
procedures and modifications to meet short-term and long-term breeding objectives.
Composite-Error Effect. The effects of uncontrolled factors during experimental
measurements of genotypic values. It includes effect of plot compared to others in the
same replication, error due to sampling among plants of the same family, errors of
measurement, and the like.
Composites. A mixture of genotypes from several sources, maintained by normal
pollination in isolation. It is technically advance generation seed mixture from
intervarietal or intravarietal crosses. Dhawan (1963) outlined breeding methodology
for developing commercial varieties in maize named as composites. Initially the usual
procedure was to make intervarietal crosses and evaluate them for inbreeding
depression during F 2 , F 3 ,…., etc. generations. The final population is constituted from
crosses showing least inbreeding depression. These days composites are developed
even through making full-sibs within a variety or population. The highest yielding
composites are obtained from S 1 progenies followed by full-sib progenies.
Confidence Limits. The limits within which any given proportion of the sample means is
expected to lie under the condition of normal distribution of the means of samples.
For example, if x is the sample mean, the limits x ± 2σ/√n would contain the
population mean (μ) on an average in 95 out of 100 cases. In other words, we may
expect the inequality {x-2σ/√n ≤μ ≤ x +2σ/√n} to hold good on the average in 95%
of the samples. The two limits (on either sides) is called the confidence limits. (Also
see level of significance).
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Confounding. A technique of reducing the size of replication over a number of blocks at
the cost of loosing some impractical information on some effects. The device of
confounding consists of sub-dividing the replicates into two or more equal sub-groups
of equal size following certain rules. This leads to loss of some information on certain
higher order interaction and allocation of the treatment combinations of any group to
any block at random. If the same interaction is confounded in all replications, it is
called total confounding. In the case of partial confounding, an interaction is
confounded in one replication, but not in others or the other combination is
confounded in other replication.
Convergent Cross. A cross attempted to accumulate in one genotype the desired genes
scattered in two or more parents.
A x B C x D E x F G x H
↓ ↓ ↓ ↓
F 1 x F 1 F 1 x F 1
↓
↓
F 1 x F 1
↓
F 1
This multiple convergent cross (F 1 ), according to Harlan et al. (1940), facilitates
continued recombination through intercrossing of F 1 ’s.
Convergent Improvement. A method of improving two inbred lines simultaneously
through backcrossing the single cross independently to both of its inbred parents, that
is, (A x B) x A and (A x B) x B.
Core Collection. A subset of accessions from the entire collection representing most of
available diversity of the species. The core subset thus formed can be evaluated
extensively to derive information that could be used as guiding criterion for more
efficient utilization of the entire collection. The reduced collection size will also
facilitate in reducing expenses required to manage the gene bank. First, a
representative core collection (10%) is developed from the entire collection, using
information on origin, geographical distribution, and characterization and evaluation
50

data. The core collection is then assessed for various qualitative and quantitative
characters, and ultimately, a subset of 10% accessions from the core subset (that is,
1% of the entire collection) that captures most of the useful variation in the crop is
formed. At both stages, standard clustering procedures are used to separate groups of
similar accessions, and various statistical tests are used to assess representatives of the
core and mini core collections. Mini-core collections of chickpea, groundnut, and a
few other crops are available at ICRISAT.
Correlated Response. The change in mean value of a trait brought about through direct
selection for an otherwise associated character. For example, if x and y are associated
with each other, one can predict correlated response in y while selection is directly
practiced for x. Thus CRy =i x .h x .h y .r g .σp y , where, i x is a constant value depending
upon selection intensity for x, h x &h y are square-roots of the heritability of concerned
traits, r g is the genetic correlation between the two traits, and σp y is the phenotypic
standard deviation of y.
Correlation Coefficient. A statistic that measures the precision of relationship between
two variables. It is also called simple correlation coefficient, simple linear correlation
coefficient or product moment correlation coefficient. It is usually designated by r,
which is a ratio of the covariance between the two variables and product of their
standard deviations [r = σxy / σxσy]. It is alternatively defined as geometric mean of
the two regression coefficients. The value of r lies between +1 to –1. The extreme
value indicates the perfect association between the two variables. If the two variables
change in the same direction, the correlation is positive. However, when a + change in
one character is accompanied by a – change in the other, there is negative association.
An r value of zero indicates either (a) absence of association whatsoever between the
two variables, or (b) presence of non-linear association. Sometimes, the numerical
value of genetic correlation coefficient may exceed the extreme limit (± 1). As it is
not directly measured, larger experimental error could inflate the numerical estimate.
The r is a pure number, and is thus independent of units of measurement of the two
variables and a change of origin and scale. The calculated value of r is compared with
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the table value at (n –2) degree of freedom and 5% (usually) probability level, where
n is the number of paired observations from which computed r value is obtained.
Correlation. The association between two variables based on the amount of variability in
one character that can be explained by a linear function of the other. The association
may be bi-directional, that is, both in positive and negative direction. Positive and
negative correlations arise in reponse to coupling and repulsion phase linkages,
respectively. Correlation may also stem from pleiotropy called developmental
correlation. The same gene is controlling two traits (and thereby differences) through
influencing the same developmental or metabolic pathway. Linkage disequilibrium
may also cause transient genetic correlation. No correlation indicates that the two
genes in question are either on different chromosomes or if on the same, both are far
apart (>400 cM) to permit 50% recombination. It is emphasized here that the nature
of correlation can often be altered by selection and hybridisation. Some times, it is
observed that two variables are found associated for no valid reason, that is, the two
are correlated purely by chance; this is called spurious correlation (the correlation
between population and agricultural growth after independence, for instance).
Covariance. A statistical measure of the interrelation between variables. The variation
occurring simultaneously in two variables or characters is called covariance.
Numerically, it is the mean of the product of the deviation of two variables from their
individual means. It is an important statistical parameter as it enters the computation
of several analyses such as correlation and regression coefficients, D 2 -statistic,
stability analysis, and the like. Further, covariance between relatives is translated into
genetic variance or its components in biometrical analyses.
cpDNA. Chloroplast DNA. DNA found in the chloroplast. It is more complex than
mtDNA with respect to size, genes encoded, and in the structural organization.
Crisscross Inheritance. Transmission of a gene from male parent to female child to male
grandchild (X-linked inheritance, for example).
Critical Difference. Also called least significant difference. It is a value beyond which
difference between means of two treatments is significant. After performing analysis
of variance, it is calculated as: CD = [2 x σ 2 e/r ] 1/2 x t, whereas, σ 2 e = error
52

variance, r = number of replication, and t = table value of t at error degree of freedom
and 5% probability level.
Crop Simulation Model. A model used to study the interactive response of various
growth factors on crop yields. It is perceived as a black box in which we feed a
minimum number of data set relating to crop, soil, weather and other variables. The
model will then utilize this input data set in calculating various growth processes
using established quantitative relationships and give us the required information
regarding the daily growth and development of the crop, etc. and the simulated final
yield of the crop. In this endeavour, of course, computer plays an important role. In
the past, statistical regression type technique to forecast crop yield has been used on
the basis of only a limited number of independent variables. However, in reality, 100
or even more variables are involved. On the other hand, simulation models are based
on more variables and more physiological processes and could be universally
applicable. Such models have already been developed for wheat, rice, maize, and the
like.
Cross. A process of bringing together genetic materials from different individuals
(genotypes) to provide for genetic recombination. Also the result of the process.
Cross Classification. A system of arrangement of various treatments in relation to one
another. If all the levels of one factor are common to each level of the other, it is
called cross classification (for example, testing v varieties in r replications).
Cross Inoculation Group. Host specificity of a Rhizobium strain. A particular
Rhizobium species has a particular legume host(s). For example, Rhizobium phaseoli
infects mung bean, whereas Rhizobium meliloti infects Melilotus alba.
Crossing Over. The exchange of corresponding segments between any two non-sister
chromatids of a pair of homologous chromosomes during meiotic prophase. Its
genetic consequence is the recombination of linked genes, that is, crossing over is the
cause and recombination is the effect.
Crossover Modifier. Any gene or chromosomal structural change increasing or
decreasing the crossing-over frequency. Reduction in crossing-over frequency is
observed more frequently than its enhancement. With rare exceptions, all structural
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heterozygotes have lower crossing-over value for regions included in or immediately
adjacent to the structural change. The decrease is often offset in those segments
farther removed from the aberration.
Cross-sterility. The failure of fertilisation because of genetic or cytoplasmic
incompatibility in crosses between pairs of individuals despite formation of normal
and functional gametes.
Crossover Suppressor. An inversion (usually complex) that makes pairing and crossingover
impossible.
Cross Pollination. Pollination effected through transfer of pollens from one plant to the
floral stigma of the other with a different genetic constitution. Also called
outcrossing.
Cryptic Structural Hybridity. A kind of chromosomal sterility due to heterozygosity for
chromosomal differences too small to affect meiotic pairing appreciably. Such
structurally heterozygous individuals (cryptic structural hybrids) are not identifiable
on the basis of pairing configurations of chromosomes at meiosis.
Cryptic Species. Phenotypically very similar species, which do not hybridise under
normal conditions.
Cultivar. An international equivalent of cultivated variety. It is a distinctive
agriculturally derived cultivated variety of plants produced through selective
breeding. (Also see variety).
Culture. Tissue or cells multiplying by asexual division, grown for experimentation.
Cumulative Selection (Rickey, 1945). A method of obtaining superior inbred lines from
promising crosses between better performing families (based on their top cross
performance) derived from original population following any scheme of inbreeding.
In essence, it represents a philosophy for accumulation of desirable genes in each
cycle of inbreeding and thus increasing the chance of obtaining superior inbred lines.
Curie. The basic unit of the quantity of a radioactive substance. It can be defined as the
amount of material in which 3.7 x 10 10 atoms disintegrate each second [or 2.2 x 10 12
disintegration per minute (dpm)].
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Cuticle. A lipophilic layer on the outer wall of epidermal cells; a waxlike covering of the
exterior surface. It protects outer surface of the plants from desiccation in the air.
C-value. Characteristic DNA content of the cells of a given species.
Cybrids. Cytoplasmic hybrids having cytoplasms from two unrelated types but nucleus
from only one. Cybrids are produced through somatic cell hybridisation.
Cyclical Selection. Selection favouring different phenotypes alternatively because of
shift in environmental condition between generations/seasons. Such shifts may result
in selection in one direction, and selection in the opposite direction for the next. It
also maintains genetic differences in a population, since different traits will be
advantageous at different times.
Cytochromes. A class of proteins, found in mitochondrial membranes, whose main
function is oxidative phosphorylation of ADP to form ATP.
Cytogenetics. The cytological approach to genetics, mainly consisting of microscopic
studies of chromosomes. The chromosome theory of inheritance led birth to this
branch of genetics. In essence, it is a correlated study of cytology and genetics.
Cytokinesis. The process of segmentation of the cytoplasmic portion of a cell along with
the segregation of daughter nuclei to separate cells (as distinct from karyokinesis).
During this process, the cytoplasmic components are distributed to the daughter cells.
Cytoplasm. The protoplasm of a cell excluding nucleus and cell wall. It is the material
between the nuclear and cell membranes. It includes fluid, organelles and various
membranes.
Cytoplasmic Genetic Male Sterility. Male sterility governed by a particular
combination of cytoplasmic factor and nuclear gene(s). Plants carrying this kind of
male sterility is designated as A line. Maintenance of such lines is very easy when
plants with fertile cytoplasm (with recessive nuclear genes) called B-line is used as
pollinators. In hybrid seed production, A line is crossed with a special line (R line)
which restores pollen-producing ability in the F 1 hybrid.
Cytoplasmic Inheritance. See extranuclear/cytoplastic inheritance.
Cytoplasmic Male Sterility. A kind of male sterility caused by cytoplasmic factors
carried on mitochondria (mt DNA). The cytoplasm is designated as ‘sterile’.
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However, such male sterile plants may produce seed if suitable pollinators (line
carrying ‘fertile’ cytoplasm) are available. The maintenance of male-sterile line is
very easy, since the progeny of such sterile individuals are all sterile. This happens
because cytoplasm of the progeny is derived almost exclusively from female (male
sterile) plants. It is unsuitable for production of hybrids in seed-propagated crops
because F 1 ’s are all sterile.
Cytoplasmic Matrix. That portion of the cell cytoplasm not contained within the
endomembrane system.
Cytoplastic Inheritance. The transmission of hereditary traits through the cytoplasm as
opposed from transmission by genes carried on chromosomes. The genetic
determinants of such characters are contained on cytoplasmic organelles like
mitochondria and plastids. This type of inheritance is detected by differing
contribution of male and female parents in reciprocal crosses, and thus is opposed to
Mendelian inheritance. Also called cytoplasmic, extranuclear or maternal
inheritance.
Cytosterility. Cytoplasmic male-sterility with maternal inheritance.
Cytosol. The fluid part of the cytoplasm, outside the organelles.
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D 2 -Statistic (Mahalanobis 1928). A statistic used to determine the degree of divergence
among populations/ breeding lines/ genotypes in terms of generalized group distance.
Numerically, it is the sum of squares of differences between any two-populations/
genotypes for each of the uncorrelated variables/traits. (Correlated variables are
transformed to uncorrelated ones by pivotal condensation method). It is useful in
Plant Breeding as it facilitates in making choice of divergent parents for hybridisation
purposes.
Dalton. A unit of molecular weight. One Dalton equals one-twelfth the mass of C 12 .
Darwinian Fitness. The relative probability of survival and reproduction for a genotype;
also called biological fitness or fitness.
Darwinism. Evolutionary concept (descent with modification) as propounded by Charles
Darwin. The four main premises are: (a) life is constantly changing, and all forms of
organisms are transformed over time, (b) all organisms evolved from a common
ancestor, (c) species multiply by splitting into descendent sibling species when
isolated in geographically or ecologically divergent space, and (d) changes occur
gradually over time. Evolution under Darwinian and Mendelian principles is
essentially a two phase process: (i) production of variation, and (ii) biological fitness
of variants in the struggle for existence.
Daughter Cells. Two identical cells formed by asexual division of a cell.
Daughter Chromosomes. The sister chromatids at the anaphase of mitosis or anaphase-II
of meiosis.
Dee-geo-woo-gen. A semi-dwarf variety of rice discovered inTaiwan. Itwassemi-dwarf,
stiff strawed, erect leafed and high yielding. The semi-dwarf trait appears to be
monogenic recessive (sd 1 ). The gene reduces plant height; however, it does not reduce
panicle length.
Deep Water Rice. Rice grown in varying depths of water (1.0-5.0 metres). Such rice
varieties are usually photosensitive. Two independent recessive genes (duplicate
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recessive) govern their elongation ability with rising levels of water. Gene(s) for
semi-dwarfness can be incorporated, since it is non-allelic to those for floating habit.
Deficiency. Terminal loss of a segment from a chromosome. Also see deletion.
Degenerate Code. A genetic code in which two or more codons encode the same amino
acid.
Degree of Freedom. The number of independent comparisons that can be made in a set
of data. Suppose, there are three genotypes namely, A, B, and C; wehavenow
freedom to compare each one with the rest two ones, that is, A with B and C or B with
A and C, and so on. Thus we have lost our freedom by one degree (i.e., d.f. = n-1).
Dehiscence. Splitting open of a fruiting structure or anther.
Deleterious. Of mutations that impair the traits considered desirable by the breeder in the
cultivated species.
Deletion. The loss of a segment from a chromosome or from both members of a
homologous pair. The term is applied usually to the intercalary loss from a
chromosome. A deletion may occur spontaneously or be produced artificially by
highly energetic mutagens (X-ray, γ-ray). The loss of terminal segment causes
terminal deletion (deficiency); this is a rare phenomenon, however. Two breaks in a
chromosome can produce an intercalary or interstitial deletion. Deletion can be
detected genetically by the lack of revertability, pseudodominance and recessive
lethality and cytologically by the presence of a deletion loop (in a deletion
heterozygote). Deletion homozygotes are lethal since there is loss of biological
functions. It causes semisterility in deletion heterozygotes.
Deme. A local interbreeding group; a panmictic unit.
Denaturation. The loss of the native configuration of a macromolecule resulting from
various causes (heat treatment, extreme pH changes, chemical treatments, etc.) and
usually accompanied by loss of biological activity of the molecule in question. It may
be the separation of the two strands of a DNA double helix or the severe disruption of
the structure of any complex molecule without breaking the major bonds of its chains.
Design of Experiment. Various forms of plot arrangements (treatments) to suit the
requirements of a particular problem. The underlying principle of all the experimental
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designs is the same; that is, they seek to provide by means of randomisation and
replication an unbiased comparison of treatments against their standard errors, and
aim at reducing these errors with the help of replication and local control. Thus,
experiments are designed with the object to measure experimental error for deciding
the basis of difference between two treatments. A larger treatment difference
compared to the experimental error implies varietal difference. Estimate of error,
control of error and proper interpretation of results are the three essential components
of any design of experiment.
Desynapsis. The falling apart during diplotene of meiosis-I of chromosomes, which
paired normally at zygotene. (Differentiate it with asynapsis, which refers to the total
failure of pairing between homologues during meiosis-I). It may be a genetically
governed mechanism and one or a few recessive genes may control it. Such gene(s)
may impair chiasma formation leading ultimately to formation of univalents.
Desynaptic. Of genes that affect the maintenance of chromosome pairing between
synapsed chromosomes.
Detassel. The removal of immature tassels to facilitate controlled crossing for producing
hybrid maize.
Determinate. Descriptive of an inflorescence in which the terminal bud opens first, thus
arresting the growth of a plant or prolongation of the floral axis. The trait is ordinarily
recessive, and is usually governed by a single gene. Plants with such growth habit are
early maturing compared to their counterparts as exemplified by peas, beans,
fenugreek, etc.
Detrimental. Of mutation lowering the viability of affected individuals.
Development. The process whereby a single cell becomes a differentiated organism. It is
a process of regulated growth and differentiation that results from the interaction of
genome with its immediate environment (cytoplasm and internal cellular
environments) and external environment. It can be considered a programmed
sequence of phenotypic changes that is normally irreversible. The sum total of these
modifications constitutes the life cycle of an organism.
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Developmental Genetics. A branch of genetics dealing with the manner in which genes
control or modulate the process of development.
Developmental Homeostasis. The capacity of the developmental pathways of individuals
to produce a normal phenotype in spite of developmental or environmental
disturbances. Also called individual buffering. It has been shown that heterozygous
individuals, such as F1 hybrids, are more stable than their homozygous parents. The
stability of heterozygous individuals seems to be related to their ability to perform
under stress conditions compared to homozygous parents.
Developmental Pathway. The chain of molecular events that take a set of equivalent
cells and produce the assignment of different fates among those cells.
Deviation. Departure of an observation from its expected value.
Dhurrin. A cyanogenetic glucoside contained in the green leaves of sorghum. Upon its
hydrolysis in the presence of an enzyme, it releases hydrocyanic or prussic acid in the
rumen of the animals. At higher concentration, it can cause fatal poisoning to
ruminants that graze it. The potential HCN problem may be reduced by grazing
management practices.
Diallel. A set of crosses possible among a series of genotypes. Thus crossing n lines in
all possible combinations yields n 2 progeny families (including cross of each line to
itself). The set of crosses is called full diallel. If the selfs are omitted (on account of
male-sterility or self-incompatibility), only n(n-1) crosses are possible. Furthermore,
in absence of maternal effects, there is no need to include reciprocal crosses.
Therefore, practically only ½ n(n-1) crosses are required to complete half of the
diallel table. The evaluation of such crosses is called single cross/diallel test or diallel
analysis; it provides information for both GCA of a line, and SCA of particular pairs
of clones or lines. In addition, nature and magnitude of several genetic parameters can
also be assessed.
Diallel Selective Mating (Jensen 1970). A set of diallel selective mating procedures to
serve as a supplement to conventional breeding systems for autogamous crops. The
system uses multiple parent input into a central gene pool (population) which, through
selective mating of individuals, is advanced through successive generations. Mass and
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recurrent selection procedures are employed throughout on both the plant and seed
portions of each plant generation. The diallel and selective mating aspects of this
system infuse simultaneously multiple genotypes into a few central populations.
According to Jensen (1970), it involves a planning phase and an implementation
phase. The latter has four stages: a basic parent series of crosses (1) which set up the
F 1
diallel series of crosses (2) which when composited form P 2 ; spaced P 2 and
subsequent P 3 , etc. provide the populations within which selective mating (3) is
practiced on mass and recurrent selection principles. The final stage (4) is standard
line selection from the various F 5 composite populations. Specifically, the proposed
arrangements provide for broad use of germplasm, simultaneous input of parents,
creation of persistent gene pools, breaking of linkage blocks, freeing of genetic
variability, and general fostering of genetic recombination.
Dicentric Chromosome. A chromosome with two centromeres.
Dichogamous. Of flowers whose anthers and pistil mature at different times.
Dichotomous Classification. The classification applying to an event that can occur in
one of the two possible ways.
Diclinous. Having male and female germ cells in separate flowers either on the same
plant (monoecious) or on different plants (dioecious).
Differentiation. The changes in cell shape and physiology associated with the production
of the final cell types of a particular organ or tissue. It is the origin of differences
during embryonic development between spatial parts of an originally homogeneous
whole. In higher plants and animals, it is one of the spectacular aspects of
development.
Digenomic Species. Species containing two distinct genomes. Such species are
invariably amphidiploids. These are also called secondary species because they are
convergent products of primary species. In oilseed brassicas, B. juncea (AABB), B.
napus (AACC), and B. carinata (BBCC) are digenomic species. It is interesting to
note that these secondary species are basically self-pollinated compared to their
outbreeding diploid constituents. This is probably because genomic divergence makes
it unnecessary for continuous reshuffling of genes through cross-pollination.
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Dihaploid. Ahaploid(n=2x) of tetraploid. It contains two basic sets of chromosomes.
For example, the haploid of S. tuberosum, which is a tetraploid species (2n = 4x),
would be called a dihaploid (n=2x).
Dihybrid. Heterozygous with respect to two loci.
DIMBOA. A biochemical (2,4-Dihydroxy-7-methoxy-1,4 benzoxazin-3-one). Its
presence in the leaves of inbred lines of maize has been shown to confer resistance to
first brood larvae of stem borer. It has also been found as a primary chemical factor in
the non-preference mechanism of resistant dent maize. As the feeding habits of the
second brood larvae are different, the above correlation does not hold true in this case.
Dimorphism. The occurrence of two forms (genotypes) within a population; the
occurrence of two types of flowers on one plant.
Dioecy. The state of being dioecious. It is a condition in which male and female flowers
occur on different plants of the same species (e.g., papaya, asparagus, spinach, hemp,
hops, etc). This mating system is genetically controlled, and prevents self-pollination
and thus enforces cross-pollination. Although more frequent in animals, it is only
sporadic in higher plants, probably because it is wasteful of gametes in non-mobile
organisms.
Dioecious. A mating system wherein staminate and pistillate flowers are borne on
different individuals of the same species (papaya, for example).
Diploid. An organism with two chromosomes of each kind. Such individuals are
designated by 2n.
Diplospory.
Diplotene. The stage of meiosis which follows pachytene and during which chromatids
move apart in pairs but still united in the region of chiasmata.
Directed Mutagenesis. Altering some specific part of a cloned gene and reintroducing
the modified gene back into the organism.
Directional Selection. Selection favouring an extreme phenotype in a given direction. It
changes the frequency of an allele in a constant direction, either towards or away from
fixation for that allele. It pushes a population towards homozygosity (fixation of the
allele). If practiced in the opposite direction, it may eliminate the allele from the
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population. Much of the selection practiced by plant breeders is of this kind, since
they select for extremes of yield, productivity, resistance to diseases, etc. It may well
equally be important in evolution when the environment of a population is changing
and only extreme phenotypes happen to be adapted for new conditions.
Dirty Crop. An approach in which none of the component lines of a multiline variety is
resistant to all the prevalent races of the pathogen. Therefore, some sort of susceptible
reaction for a few races of the pathogen in such a multiline variety always occurs; this
makes the entire crop “dirty”. This approach of forming a multiline variety has a
significant potential advantage over the clean crop approach. Since moderately
susceptible lines are also considered, the breeder is in the advantageous position as he
can exercise selection for characters such as yield, maturity, height, and the like. It
will also indefinitely extend the useful life of strong resistance genes.
Discontinuous Variation. Variation having distinct phenotypic classes for a particular
character e.g., tall vs. dwarf, resistant vs. susceptible).
Disease Incidence. The proportion of plant units infected, that is, percentage of diseased
plants or ears.
Disease Severity. The proportion of the total area of plant tissue affected by a disease.
For pathogens that spread according to the compound interest law, disease severity is
the cumulative result of infection frequency (proportion of spores that result in
sporulating lesions), latent period (time from infection to spore production), spore
production (mass of spores produced per lesion or per unit area of tissue per unit
time), and infection period (period of sporulation).
Disease. A physiological disorder or structural abnormality that is deleterious to plants,
plant parts and its products, reducing its economic value. It can be considered a
malfunctioning process caused by infectious agents resulting in sufferings of plants.
The optimum conditions for a disease to occur are a combination of three factors: a
susceptible host, an infective pathogen and a favourable environment. A change in
any of the three factors will cause a corresponding change in the expression of disease
by the host plant.
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Disjunction. The separation of daughter chromosomes at anaphase. (Also see
chromosome disjunction).
Disomic. Individuals with two chromosome sets, each chromosome in one set having a
homologue in the other; a monoploid cell with one chromosome in duplicate (n+1).
Dispermy. The entrance of two spermatozoa into an egg cell.
Dispersion. The pattern of distribution of observations in a set of data. The measures of
dispersion provide information about the nature of series. This is additional
information, which measures of central tendency (mean, median or mode) cannot
provide.
Disruptive Selection (Thoday 1972). Simultaneous selection for both extremes (within
the same generation). Considerable genetic differences may be maintained between
groups in such populations in spite of crossbreeding between individuals from
different environments.
Distal. Of a part of a chromosome arm, which is farther from the centromere than another
(proximal) part.
Distribution Function. A graph of some precise quantitative measure of a character
against its frequency of occurrence.
DNA. Deoxyribose Nucleic Acid; a biochemical molecule of two chains of
polynucleotides twisted into a double helix and joined by hydrogen bonds between the
complementary bases of the two chains. The base sequence of the DNA molecule is
specific to each individual and it determines individual hereditary characteristics. It is
the primary genetic material except for a few plant viruses.
DNA Chips. Also called microarray. A flat surface about the size of a postage stamp
with 10,000 to 100,000 distinct spots, each containing a different immobilized
nucleotide sequences. Gene chip technology permits scientists to analyse thousands of
genes at once.
DNA Clone. A clone of a specific section of DNA. Such a segment is inserted into a
vector molecule (such as a plasmid or a phage chromosome), and then replicated to
form many copies.
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DNA Fingerprint. The autoradiographic banding pattern of DNA. It is produced when
DNA is digested with a restriction enzyme that cuts outside a family of VNTRs, and a
Southern blot of the electrophoretic gel is probed with a VNTR-specific probe. Unlike
true fingerprints, these patterns are not unique to each individual.
DNA Fingerprinting. Also called DNA typing. It is a procedure in which unknown DNA
samples are matched with their putative sources through the use of polymorphic DNA
markers.
DNA Markers. The specific spots (nucleotide sequences) on DNA molecules in coding
as well as non-coding regions. These specific spots show sequences polymorphism in
different individuals. (Also see molecular markers).
DNA Polymerase. An enzyme that can synthesize new DNA strands from a DNA
template. Several classes such as DNA polymerase I, II, III, etc. exist.
DNase. Also called deoxyribonuclease. An enzyme that degrades DNA to nucleotides.
Doak Method (Doak, 1954). A method of hybrid seed production in cotton. It involves
manual emasculation of flower-buds of female parent a day before anthesis and
dusting pollen grains from desired male parent on to the stigma of emasculated buds
the next day. Parental varieties are grown in contiguous fields in a ratio of four ha
under female to one ha of male parent.
Domesticates. Species/individuals under domestication. Initially they differ little from
their wild ancestors in amenability to agricultural circumstances, and selection leads
to the directional change as desired by human beings. Cultivars are also domesticates
in one sense. However, some consider domesticates as the altered type that can no
longer survive without human intervention.
Domestication. Bringing a wild species under human management. It is the oldest
method of plant breeding. It is a method of plant improvement owing to its capacity to
provide for types better than previously available ones. The present application of
domestication is to bring one or a few genes from wild relatives to our cultivated
species.
Dominance. A phenomenon in which the heterozygote falls towards one of the
homozygotes on the phenotypic scale of measurement. If the heterozygote resembles
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one of the homozygotes, the phenomenon is called complete dominance. It is
exemplified by the heterozygotes for all the seven characters studied by Mendel [e.g.,
AA (3), Aa (3) & aa (1)]. It is a case of an intra-locus interaction. In this case (also
called simple dominance), dominance is considered a functional or physiological
effect.
Dominance Gene Action. A type of intralocus interaction in which the heterozygote
approaches towards one of the homozygotes. The change is such that the substitution
of A for a in the genotype aa isnotthesameasinAa, that is, the effect of substitution
is non-linear; the effect so-produced on the phenotypic scale of measurement is called
dominance effect. Selection for dominance effect is less effective. It is so because if
all genes act in the fashion as exemplified above, the hereditary variance will be of
two types, that is, additive and dominance variances. However, selection will be
effective for additive component only.
1AABB (7), 2AABb (6), 1AAbb (4)
1AABB (7), 2AaBB (7), 1aaBB (3)
Dominance Hypothesis (Davenport 1908, Bruce 1910). A suggested explanation to
account for heterosis or hybrid vigour. It assumes that heterosis results from
accumulation of favourable dominant alleles at individual loci in the F 1 hybrid. Said
in another way, heterosis is the consequence of sheltering of deleterious effects of
recessive alleles by their adaptively superior counterparts (dominant alleles) at
respective loci. Although widely accepted hythesis to explain hybrid vigour for
quantitative traits, it fails to account for heterosis for monogenic (oligogenic)
characters.
Dominance Variance. A component of genetic variance, which results from an
interaction between different alleles at a given locus. Selection is ineffective for this
kind of variance.
Dominant. Of genetically controlled characters expressed in F 1 heterozygotes derived
from a cross between two pure-breeding strains differing in respect to some
characters. allele
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Dominant Allele. The expressed allele in a heterozygote; the suppressed allele is called
recessive. Strictly speaking, an allele is neither dominant nor recessive; rather its
effect may be dominant or recessive. However, in practical usage, it has come to be
called as such.
Dominant Phenotype. The phenotype of a genotype containing the dominant allele;
Also, the parental phenotype that is expressedinaheterozygote.
Donor DNA. Any DNA to be used in cloning.
Dosage Compensation. The process in organisms using a chromosomal sex
determination mechanism that allows standard structural genes on the sex
chromosome to be expressed at the same levels in females and males, regardless of
the number of sex chromosomes. In mammals, dosage compensation operates by
maintaining only a single active X-chromosome in each cell.
Dosage Effect. The quantitative effect on the phenotypic expression of a character
produced by varying number of particular alleles of a gene(s).
Duplicate Genes. Two identical allele pairs in one diploid individual. Either pair
performs the same function.
Domino Effect. The effect (or the theory) whereby one event sets off a series of similar
events; the fall of one domino standing on end causing the whole row to fall in turn.
In genetics and plant breeding, if a genotype becomes susceptible to a race of a given
pathogen, the neighbouring genotypes may naturally tend to follow the same path.
Donor Parent. The parent from which one or a few genes (and consequently the
characters) are transferred to the recurrent parent in a back cross breeding programme.
The character(s) in question must be in highly intense form for some intensity is
usually lost in a different genetic background of recurrent parent owing to influence of
a different set of modifiers.
Dormancy. The enability of viable seeds to germinate under conditions conducive to
germination. From evolutionary point of view, it is an adaptive advantage for natural
survival under unfavourable condition, and a mechanism to ensure distribution in
time and space, eventually leading to perpetuation of the species. If dormancy
develops due to conditions within the seed when it matures on the mother plant, it is
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termed primary dormancy. However, if a germinable seed becomes dormant under
specific unfavourable conditions during storage (excessive drying) or germination
(high temperature), it is referred to as secondary dormancy. It is regulated by external
coverings (testa and pericarp) and tissues surrounding embryo. Sometimes,
underdeveloped embryo may be the real cause of dormancy. All these physiological
variables may be governed and/or influenced by genetic factors. Dormancy may be
released by heat and light treatments, treatments by growth hormones (e.g., GA 3
@100-1000 ppm with or without kinetin @ 50 ppm) or with chemicals such as KNO 3
(0.2%), thiourea (0.5 - 3%).
Dose Rate (Radiation). The amount of radiation received per unit time. The time over
which a given dose is received is a very important consideration. Large, single doses
delivered at short time intervals (minutes/hours) are known as acute doses, in contrast
to chronic doses that might be experienced continuously over the whole life cycle or
over a large period.
Double Cross (Jones 1918). The F 1 between two single crosses [(A x B) x (Cx D)]. The
number of possible double crosses using n inbreds would be [ n.(n-1).(n-2).(n-3) / 8 ].
Double cross hybrids have now been replaced by single cross hybrids in maize.
Double Helix (Watson and Crick 1953). The structure of DNA with two interlocking
helices joined by hydrogen bonds between paired bases.
Doubled Haploid. An otherwise diploid plant produced by doubling the chromosome
number of the haploid (2n =2x, 4x, 6x, etc).
Drift. The changes in the gene and genotypic frequencies in small populations due to
random processes.
Drought Avoidance. A mechanism of drought resistance. It is also called drought
tolerance with high tissue water potential. It consists of mechanisms to reduce water
loss from the aerial portion of plants, and of mechanisms to maintain water uptake.
The maintenance of water uptake under drought condition is related to several
properties concerning roots of plants such as root size and efficiency, root density,
size of xylem vessel, and the like. Genotypic differences have been noted for all these
attributes. The reduction of water loss from aerial portions of the plant includes
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mechanisms such as stomatal regulation, morphology and orientation of leaves, water
retention capacity of leaves, leaf rolling and wilting (which curtails transpiration rate
by about 46-83% in grasses), cuticular waxiness (2-50% reduction in transpiration),
and leaf reflectance (up to 50% reduction in light absorption). These morphological
traits are easy to study. Some of them have very simple inheritance pattern. For
instance, leaf rolling which has been noticed in drought resistant germplasms of
maize, sorghum, and rice appears to be a monogenic recessive trait.
Drought Resistance. A heritable property that enables a plant to withstand moisture
deficit situation. According to Levitt (1972), drought resistance is the sum total of
drought avoidance and drought tolerance. In essence, a plant can resist drought
condition either through reduced water loss from aerial portions, increased water
uptake from deep layers of the soil or giving more yield at low water potentials. There
seems to be controversy as to which component of drought resistance, that is,
avoidance or tolerance, is most important in crop plants. In reality, a mixture of both
is required. Even the best drought avoiding species requires tolerance, since some
reduction in plant water potential is unavoidable during severe stress. Indeed, there is
evidence to suggest that drought avoidance is operative during the vegetative phase,
while the tolerance comes into play during the reproductive phase in crops such as
cereals.
Drought Tolerance. The ability of a plant to withstand low tissue water potentials.
Mechanisms affecting drought tolerance, as against avoidance, of water stress are
even less clear; thus, it becomes difficult to propose specific selection criterion for
screening of drought tolerance genotypes. Under condition of drought, germination,
seedling vigour, net photosynthesis, and ultimately seed yield/plant appear to be
affected adversely. Therefore, most studies of drought tolerance have focussed on
these aspects. There has been renewed interest in selecting genotypes having capacity
for osmotic adjustment. Attempts have also been made for screening of drought
tolerant genotypes through search for readily measured plant constituents such as
proline, non-structural carbohydrates (NSC), and the like. However, these selection
criteria vary with the crop species. Probably, the most difficult problem breeders are
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facing in breeding for drought tolerance is how to evaluate the trait. Drought tolerance
has a low heritability caused by high genotype-environment interaction. Even when
consistently selected, it is difficult to demonstrate drought tolerance consistently. This
probably reflects the fact that it has a complex inheritance and is governed by
polygenes.
Drought. A condition of moisture deficit. It refers to a situation wherein available soil
moisture is not sufficient to meet the demands of potential evapo-transpiration. The
kind of drought that is of our interest is the agricultural drought. It occurs where soil
moisture and rainfall are inadequate during the growing season to support healthy
crop growth to maturity and cause extreme crop stress and wilt. It should carefully be
distinguished with physiological drought, which refers to non-availability of water to
the plant due to unfavourable physiological conditions (such as soil salinity, low
temperature, and the like). Water is present in the real sense, but plants are unable to
use it.
Duplex. A polyploid containing recessive alleles in all chromosomes except two with
respect to a particular locus or a polyploid having two dominant alleles at a given
genetic locus, e.g., AAa, AAaa, AAaaaa, etc.
Duplicate Base Collections. Duplicates of the base collections that are housed in
geographically different locations for security purposes. The objectives and methods
of storage are essentially the same as for the base collections.
Duplicate Genes. Two gene pairs that produce identical effects, whether alone or
together. Non-floating habit of rice is governed by duplicate genes. (Floating habit is
present only when the two gene pairs are recessive). The two gene pairs are located in
different chromosomes. Duplicate genes are of frequent occurrence, and are probably
due to secondary polyploidy. A heterozygote for duplicate genes under the situation of
complete dominance gives a 15:1 F 2 ratio. If, however, dominant genes act in
additive/cumulative manner, the F 2 segregation gets modified to a 9:6:1 ratio.
Duplication. The presence of a segment twice in a chromosome. Adjacent duplication
may occur in tandem sequence with respect to each other – abcbcd – or in reverse
order – abccbd. Obviously, the pairing patterns obtained in these two cases are
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different. In general, duplications are hard to detect and are rare. However, they are
important from evolutionary point of view because they supply additional genetic
material potentially capable of assuming new functions.
DUS. A triple criteria of distinctness (D), uniformity (U) and stability (S), on the basis of
which a new genotype is considered for release in the new plant variety protection
regime. It will ensure precise descriptions on comparative basis of varieties in
cultivation. Said in another way, we should have accurate identification keys for
different varieties in a crop.
Dyad. A pair of sister chromatids joined at the centromere, as in the first division of
meiosis.
Dysploidy. A condition referring to differing basic chromosome numbers within
populations or species. It is frequently encountered among seed plants where a series
of basic number usually differing by one chromosome is displayed by individuals
under investigation (e.g., x = 5, 6, 7, 8, 9, etc).
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Early Generation Testing. Also called early testing; the evaluation of breeding materials
for specific characteristics during early segregating generations (F 2 , F 3 , and F 4 )based
on certain parameters. Early testing in maize refers to testing of inbred lines during I1,
I2, and I3 (S1/S2/S3) generations. The objective is to enhance the possibility of finding
superior inbred line by eliminating poor materials during early generations. It also
helps save extra expenditure. When materials are visually selected, it is a good
exercise for isolating families of high combining ability. It has been criticised on the
ground that the performance of inbred lines may change with the degree of
homozygosity attained by those inbreds. It is also not desirable when high combining
lines are agronomically poor, and are likely to be eliminated during inbreeding.
Ear-to-Row Selection. A form of selection in which desired ears are harvested
individually and subsequently grown an ear-to-a row, and the cycle is repeated from
the aggregate of seeds of selected rows. It differs from mass selection in basing the
selection on the performance of progeny.
Ecdysone. A molting hormone in insects.
Ecological Combining Ability. The ability of a genotype to perform synergistically in
association with other genotypes in mixed populations. Such genotypes are good
neighbours, and at the same time they are good competitors as well. In such mixed
populations, natural selection may be operating for superior neighbourhood; however,
in competition, it also appears to perpetuate many surviving genotypes with superior
pure-line performance.
Ecology (Haeckel 1869). The study of pattern of relations between organisms and their
environment. It has been derived from Greek words: oikos and logos, meaning
household and study, respectively. Thus the study of the environmental house
includes all the organisms in it and all of the functional processes that make the house
habitable. Literally, then, ecology is the study of life at home with emphasis on
totality.
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Ecological Niche. The place occupied by a species in the community structure which it is
part of.
Ecological Regions. Specific areas where crop plants originated. Six such ecological
regions have been documented by Harlan (1992); these are: (1) Mediterranean, (2)
Savanna, (3) Tropical low lands/coastals, (4) Tropical highlands, (5) Tropical forests,
and (6) Temperate woodlands. In these regions, wheat and barley, corn and rice, apple
and grapes, potato, sugarcane and coconut, and apple and oranges, respectively are
supposed to be originated.
Ecospecies. A group of ecotypes capable of interbreeding without any adverse effect on
the fitness of offspring.
Ecosphere. Also called biosphere; the largest and most nearly self-sufficient biological
system. It includes all the earth’s living organisms interacting with the physical
environment as a whole to maintain a steady-state system intermediate in the flow of
energy between the input of the sun and the thermal sink of space. Steady state
implies a self-adjusting equilibrium or balanced condition relatively immune to at
least small-scale disturbances.
Ecosystem (Tansley 1935). Also called ecological system; the interaction between or
functioning together of the community and the non-living environment.
Ecotypes. The locally adapted populations having optima and limits of tolerance
(adjusted to local conditions) of species with wide geographical ranges. It may also be
defined as a local race with genotypes adapted to a particular restricted habitat as a
result of natural selection. Ecotypes may be the result of factor compensation.
Compensation along gradients of temperature, light or other factors may involve
genetic races (with or without morphological manifestations) or physiological
acclimation without a genetic change(s).
Effective Breeding Population. The number of breeding individuals of a population,
contributing to the gene pool of the next generation.
Egg. The female gamete or germ cell. It gives rise to the zygote after getting fertilised
with a sperm.
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Egg Mother Cell. A megasporocyte that gives rise to an egg cell after megasporogenesis
in higher plants.
Electron Microscope. An instrument that uses a focussed beam of electrons to produce
an enlarged image of an object.
Electrophoresis. A technique for separating the components of a mixture of molecules
(proteins, DNAs or RNAa) through directional movement of charged particles in a gel
placed in an electrical field.
Elimination Coefficient. The frequency with which individuals carrying a specific gene
with deleterious effect(s) gets genetically eliminated from the population. For
example, suppose a certain gene has elimination coefficient of 5%; it implies that out
of every 20 individuals, one will die before the gene concerned is transmitted to the
offspring.
Emasculation. Removal of immature anthers from a bisexual flower with little injury to
the ovary. It is one of the techniques to control pollination, and is essentially a first
step to artificial hybridization. Some techniques used for emasculation include
exposure of flowers to heat, cold or chemicals such as alcohol. These techniques are
based on the fact that pollen is usually more sensitive to unfavourable environmental
conditions than the stigma. Suction has successfully been used to emasculate smallflowered
legumes. The timing of emasculation is also important. If the operation is
delayed, there is every possibility of anther bursting during emasculation. On the
other hand, emasculation at too early stage (while the floral bud is immature and
tender) will result in unnecessary mutilation of the pistil.
Embryo. The rudimentary plant in a seed. The embryo arises from the zygote (2n). It can
also be defined as the young sporophyte arising from the union of female and male
sex cells.
Embryogenesis. The origin, development, and the evolution of the embryo; also called
embryogeny.
Embryo Sac. Typically, an eight nucleate female gametophyte; the mature
megagametophyte in higher plants. It arises from one of the megaspore by successive
mitotic divisions.
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EMS. Ethyl Methane Sulphonate. It is a chemical mutagen that causes GC to AT
transition. Although it can add ethyl groups to any unsaturated positions of nitrogen
bases of the DNA, but mutagenicity is best described by the addition of alkyl group
especially at O 6 position of guanine.
Endocytosis. The processes of phagocytosis, pinocytosis, and micro-pinocytosis. It leads
to bulk ingestion of solid or fluid materials by the cell. Exocytosis is the reverse
process, by which membrane-lined products are released at the plasma membrane.
Endogamy. See inbreeding.
Endomitosis. A form of somatic polyploidization, fairly common in differentiated
tissues. It takes place within an intact nuclear envelope, and duplication of
chromosomes (increase in DNA content) occurs without nuclear division in usual
sense (endopolyploidy).
Endonuclease. An enzyme that cleaves the phosphodiester bond within a nucleotide
chain of a DNA molecule.
Endopolyploidy. An increase in the number of chromosome sets caused by replication
without cell division.
Endosperm. A tissue specialised for nourishing the developing embryo of higher plants.
It is typically a triploid tissue (3x), formed from the triple fusion of a sperm nucleus
with the two polar nuclei of the embryo sac. But it may also be diploid, tetraploid, or
even pentaploid depending on the species in question as it may also originate from the
fusion of one female nucleus with one or more others or with a male nucleus or both.
Enforced Outbreeding. Deliberate avoidance of mating between relatives.
Enhancer. A cis-regulatory sequence that can elevate levels of transcription from an
adjacent promoter. Many tissue specific-enhancers can determine spatial patterns of
gene expression in higher eukaryotes.
Enthalpy. The total energy contained in chemical compounds of the cell. The energy
exists as chemical energy primarily locked in high-energy bonds. The cell uses only
part of the total energy at any given point of time.
Entropy. A measure of randomness or disorder. It also measures unavailable energy
resulting from transformations. The law of entropy (the second law of
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thermodynamics) states that every thing, whether living or non-living, tends to
disorganise in order to attain the state of maximum stability (high entropy). To
maintain a high state of internal order (a condition of low entropy), living bodies
require energy. Although entropy in technical sense relates to energy, the word is also
used in a broader sense to refer to degradation of materials. Entropy is not all
negative. As the quantity of available energy declines, the quality of the remainder
may be enhanced greatly.
Enucleate Cell. A cell having no nucleus.
Environmental Variance. That portion of phenotypic variance caused by differences in
environmental factors to which the individuals of a population were exposed.
Enzyme. A biochemical (usually protein) that functions as a catalyst. Most enzymes
cannot function alone; they are referred to as apoenzymes. The non-protein part that
makes them functional is called cofactor (a metal or a prosthetic group in case of
conjugated proteins) or coenzyme (small molecules). The combination of protein and
non-protein parts (functional state) is called holoenzyme. There may be a group of
enzymes, which differ slightly in their molecular structure, and sometimes in their
regulation, are called isoenzymes (isozymes). Not all enzymes are proteins. Some
rRNA and tRNA have been found to have catalytic activities.
Environment. The sum total of factors/ conditions that affect survival and reproduction
of an organism(s); also, the complex of physical and biotic factors within which an
individual organism or a population exists.
Epigenetic Inheritance. Processes by which heritable modifications in gene function
occur but are not due to changes in the base sequence of the DNA of the organism
(e.g., paramutation, X-chromosome inactivation, parental imprinting, and the like).
Epiphytotic. An unarrested spread of a plant disease. Under conditions of favourable
environment and monocropping of a susceptible variety over a wide geographical
area, it has been often observed that the disease occurs in devastating form and soon
occupies the entire area even after the efforts have been made for its control. It is thus
said that the disease has appeared in an epiphytotic form (e.g., brown spot disease of
rice in W.B. during 1940s).
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Epistasis. The masking effect of one gene over a non-allelic gene; the expressed gene is
called epistatic, and the suppressed one is known as hypostatic. The term has acquired
a more general meaning in quantitative and population genetics where it is used
synonymously to include any kind of gene interaction. It should be carefully
contrasted with dominance (which refers to the masking effect of one allele over the
other at the same locus) that is necessarily associated with heterozygosity, and is only
momentary or short-lived in autogamous species. With epistasis, one can hope for a
worthwhile type to appear owing to step-by-step completion of a biochemical
pathway.
EpistaticGeneAction.Akindofinterloci interaction. The final outcome (phenotype
governed by two or more genes) gets changed according to the presence or absence of
a particular allele of the genes in question.
Epistatic Variance. That portion of genetic variance, which results from an interaction of
genes at different loci. It is of three types: (a) additive x additive, (b) dominance x
dominance, and (c) additive x dominance. If a trait is governed by two gene pairs, all
the three types of variances, viz., additive, dominance, and epistatic will be present.
So far as epistatic or interaction variance is concerned, only additive x additive
component can be exploited through selective breeding.
Equational Division. A nuclear division that maintains the same ploidy level of the cell.
Equatorial Plane. The plane midway between the two daughter nuclei of a dividing cell.
Equatorial Plate. The metaphase plate.
Equilibrium Population. A population in a state of genetic equilibrium; also see Hardy-
Weinberg equilibrium.
Error Variance. Also called environmental variance.
Erucic acid. A fatty acid containing long hydrocarbon (C 22 ) chain. Its proportion in
rapeseed - mustard oil is very high which is undesirable nutritionally. A reduction in
erucic acid through breeding results in proportional increase in oleic and linoleic acids
(low carbon containing unsaturated fatty acids with two double bonds). The presence
of erucic acid is controlled by two alleles at the same locus. However, its expressivity
appears to be enhanced by two additive genes in rapeseed.
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Escape (Drought). A mechanism of pseudo-resistance to drought, often achieved by
rapid phenological development or by developmental plasticity. The rapid
phenological development is closely related to the earliness of maturity in the crop
plants. This helps the plants complete their life cycle before the very occurrence of
moisture deficit in the soil. It is perhaps most dramatic in the ephemeral plants of the
desert regions which complete life cycles in as few as four to six weeks.
Developmental plasticity is considerable in determinate annuals such as wheat. Wheat
commonly produces more tillers than are required; the number, which survives to
produce spikes, is reduced by water deficits between floral initiation and spike
emergence. Excessive tillering would of course be wasteful of soil moisture in the
moisture-restricted environments. Screening and selection for drought escape through
maturity or developmental plasticity is relatively simple compared to screening and
selection for some other drought resistance characteristics. However, in areas where
the length of the growing season is clearly defined by cold temperatures, there is little
opportunity for manipulation of maturity.
Escape. A mechanism of functional resistance in which a plant with certain
characteristics escapes attacks of pests. For example, an early maturing variety may
escape the attack of a pest whose population build-up occurs late in the growing
season.
E. coli. The Escherichia coli; easily cultured prokaryotic organism. The bacterium is thus
far most studied and best understood.
EST. Expressed Sequence Tag; a sequence-tagged site derived from a cDNA clone and
used to position and identify genes in genomic analysis.
Estimate. The numerical value of the estimator.
Estimator. Any statistic used for estimating the parameter (e.g., mean & variance).
Ethidium Bromide. A molecule that can intercalate into DNA double helices when the
helix is under torsional stress.
Euchromatin. A chromosomal region that stains normally. It is thought to contain
normal functioning genes.
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Eugenics. Controlled human breeding in an attempt to improve future generations, based
on notions of desirable and undesirable genotypes. During early part of twentieth
century, it became quite fashionable to propose that persons having genetic disorders
should be prevented from having any offspring (a programme called negative
eugenics). Most deleterious alleles are rare, and mostly occur in heterozygotes.
Assuming frequency of 0.01 of a deleterious allele, it takes about 100 generations to
reduce its frequency to half, that is, 0.005 {q n = 1/(n+1/q 0 )}, if all the homozygotes
were prevented from producing offspring. Clearly, it was a futile exercise.
Eukaryote. A cell having true nucleus. The cell has cytoplasm separated by a distinct
nuclear membrane.
Euploidy. A situation wherein an organism has any number (one, two or more) of
complete chromosome sets; such an individual is called euploid. It covers situations
such as monoploidy, diploidy, and polyploidy.
Euthenics. Improvement in conditions for people to live in.
Ever Green Revolution. Achievement of sustainable productivity advances, rooted in
the principles of ecology, economics, social and gender equity, and employment
generation. It is a concept that applies to improvement in crop productivity in
perpetuity without ecological harm.
Evolution. Gradual change usually with a directional component. Biological evolution is
best defined as the gradual change in the diversity and adaptation of populations of
organisms. It involves change in gene frequencies, which results from (a) selection
pressure from the environment and interacting species, (b) recurrent mutation, (c)
genetic drift, and (d) migration. The process of evolution has two dimensions:
Phyletic evolution and speciation. Phyletic evolution is the gradual changes that occur
with time in a single lineage of descent; as a rule, these changes result in greater
adaptation to the environment and often reflect environmental changes. Speciation
occurs when a lineage of descent splits into two or more new lineages and is the
process that accounts for the greater diversity of the living world. Biological evolution
is quite distinct from cultural evolution that is a unique human process and is a rapid
process compared to the biological evolution. Human beings may evolve culturally
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through transmission from generation to generation of learned information including
moral and immoral values (inheritance of acquired characters).
Evolution Pressure. The combined action of mutation, migration, hybridisation, and
selection in changing the genetic structure of a population, and thereby contributing to
evolution.
Evolutionary Breeding (Suneson 1949). A breeding procedure in which the variety is
developed from an unselected progeny of a cross, or multiple crosses, that have
undergone evolutionary changes. Only natural selection operates in such populations.
A variety of barley ‘Harland’ was produced by this procedure.
Excision Repair. The repair of a DNA lesion by removal of the faulty DNA segment and
its replacement with a wild-type segment.
Exogamy. Cross-fertilisation or outbreeding leading to an increased incidence of
heterozygosity in the population.
Exon. Sequence of base pairs in a gene that participates in the coding of amino acids of a
polypeptide. It is non-intron portion of the coding sequence of a gene. Exons
constitute mRNA, and are translated into protein.
Exonuclease. An enzyme that cleaves nucleotides one at a time from an end of a
polynucleotide chain.
Experimental Error. The difference among experimental plots treated alike. For
instance, if the same variety is planted to two different plots, there may be difference
in yield between these two plots. The difference may be owing to differences in soilfertility,
soil-moisture, water holding capacity, and various other biotic factors. This
sort of difference is called experimental error. This error is the primary basis for
deciding whether an observed difference between two treatments is real or just due to
chance. Clearly, every experiment must be designed to have a measure of the
experimental error. However, it is emphasized that experimental error cannot be
eliminated totally; since two plots will not perform exactly the same no matter how
carefully these are treated alike.
Explant. An excised fragment of a tissue or organ used to initiate an in vitro culture.
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Expressivity. The extent of manifestation of a genetic character. It is the degree to which
a particular genotype is expressed in the phenotype. For instance, some leaves of lima
beans show partial chlorophyll content, while others may show only at the margins,
still others develop it only at the tip of the leaves, thus exhibiting variable expressivity
for this trait. Therefore, a gene may have variable penetrance and variable
expressivity.
Extranuclear. Of structures or processes, which are located outside the nucleus in
contrast to intranuclear structures or processes.
Extranuclear Genes. Genes located outside the cell nucleus, that is, on cytoplasmic
organelles (mitochondria, chloroplast, etc). Inheritance of such genes is called
extranuclear inheritance. The inheritance pattern is quite different from those of the
nuclear genes (Mendelian genes). Reciprocal differences (as observed for sex-linked
genes and genes conditioning sporophytic incompatibility) are encountered in the
extranuclear inheritance. Interactions among loci of nuclear and the chloroplast
genomes have been reported in various wild species. Therefore, intergenomic
interactions should also be considered as the rational approach while conducting
breeding operations.
Extranuclear Mutation. A heritable change occurring in the extranuclear genes. It is
also referred to as cytoplasmic mutation because it is carried in the cytoplasmic
organelles like chloroplast and mitochondria.
Eye Ball Index. The visual judgement of plant breeders based on their experience and
knowledge of plant traits. This constitutes the art of plant breeding. It is used for
evaluation and selection of plants, plant progenies, or even families in early
segregating generations (F 2 /F 3 /F 4 ).
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F
F 1 . The first generation of a cross between genetically unrelated parents. It is also called
first filial generation.
F 2 . The second filial generation obtained by self-fertilization or crossings inter se of F 1
individuals.
F 3 . The third filial generation progeny obtained by self-fertilizing F 2 individuals.
Factor. A genetic determinant of a character. It is synonymously used for gene by
classical geneticists; same as gene.
Factorial. Multiplication of the number by all integers down to 1. It is designated by the
symbol ‘!’. For example, 5! = 5.4.3.2.1 = 120. It should be noted that 0! (zero
factorial) equals 1, and any number raised to the zero power also equals 1 [e.g., (½) 0 =
1].
Factorial Experiment. An experiment in which the treatments consist of all possible
combinations of given levels of two or more factors. The term complete factorial
experiment is sometimes used when the treatments include all possible combinations
of the selected levels of the variable factors. In contrast, the term incomplete factorial
experiment is used when only a fraction of all the combinations is tested. Note that
the term factorial describes a specific way in which the treatments are formed and
does not, in any way, refer to the experimental design used. For instance, if a factorial
experiment is performed in a randomized complete block design, then the correct
description would be ‘a factorial experiment in a randomized complete block design’.
A factorial experiment provides valuable information on interaction, and is more
informative than a single factor experiment. However, practical considerations like
large size, complexity and cost limit the use of factorial experiments.
Familial Trait. A trait shared by members of a family for whatever reasons (not
necessarily the genetic ones). It should be carefully differentiated from a heritable trait
for which similarity arises from shared genotypes.
Family. A group of individuals directly related by descent from a common ancestor.
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Family-Line Breeding. Also called mother-line breeding; a method of breeding utilized
to develop Cercospora resistant varieties in sugar beet. It is similar to half-sib
selection procedure or an ear-to-row method of breeding corn. In this method,
superior mother beets are identified from progeny performance. Remnants seeds from
mother beets with superior progeny performance may be bulked to begin a seed
increase. Alternatively, roots from superior progenies may be harvested and bulked to
start a seed increase. The seed increase may be utilized as a new variety or it may be
used as the source population to start the next selection cycle.
Family Selection. A breeding technique of selecting a pair on the basis of the average
performance of their progeny.
Farmers’ Right. A legal right that entitles farmers to save, use, sow, resow, exchange,
share or sell their farm produce including seed of a variety protected under IPRs.
However, farmers are not entitled to sell branded seed of the variety protected under
the law. Farmers’ exemption in terms of farmers’ right has been done owing to the
past, present and future contributions of farmers in conserving, improving and making
available plant genetic resources (PGRs), particularly those in the centres of
origin/diversity. FAO Commission recognized PBRs and Farmers’ rights in 1989 to
support developing countries to build their capabilities and required infrastructures
especially gene banks for conserving available genetic resources.
Fatty Acids. Organic acids having large hydrocarbon chains with the general formula
CH 3 -(CH 2 ) n -COOH. Fatty acids have always an even number of carbons because they
are synthesized by joining two-carbon acetyl units, e.g., palmitic acid (16 carbons),
stearic acid (18 carbons). Sometimes, hydrocarbons have double bonds (-C=C-), and
in such cases, the fatty acid is said to be unsaturated (e.g., oleic acid). Unsaturated
fatty acids are desirable because the double bonds present in them increase the
flexibility of the hydrocarbon chain, and thereby fluidity of biological membranes.
Fatuoid. A mutant commonly occurring in cultivated oats, and which resembles wild oats
(Avena fatua).
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Fecundity. The reproductive potential as measured by the quantity of gametes,
particularly eggs produced. In higher plants, this potential may be measured in terms
of number of pods/plant, seeds/pod, seeds/plant, etc. Also see fertility.
Feedback. A system in which a deviation from a certain value induces control factors,
which counteract (negative feedback) or increase (positive feedback) the deviation.
Feedback Inhibition. The inhibition of an initial enzyme in a biosynthetic pathway by
the end product of that pathway. This is a control mechanism in which the end
product of a metabolic pathway acts as an inhibitor of the first enzyme of the
metabolic chain. Thus, when enough product is synthesized, the entire chain can be
shut off, and useless accumulation of metabolites is avoided.
Female Sterility. A situation of occurrence of non-functional female gametes in plants
(as in cassava). It is less commonly encountered in nature compared to male sterility
probably owing to the nurturing effects of cytoplasm.
Fertile Crescent. Also called Levant; areas comprising Jordan, Israel, Lebanon, Syria,
Turkey, Iraq and Iran. It is so called because most of the important cultivated species
originated in these regions.
Fertility. Ability to produce biologically fit offspring.
Fertility-Restoring Genes. Nuclear genes that act to restore fertility in plants with malesterile
cytoplasm.
Fertilization. The fusion of nuclei of male and female gametes.
Filial Generation. Any generation following the parental one (P). The progeny
generation from a cross between two contrasting parents is called first filial
generation, and is designated by F 1 . The selfing or intercrossing of F 1 individuals
leads to the F 2 , or second filial generation. Progeny of F 2 individuals derived through
selfing makes the F 3 , or third filial generation. Method of propagation is normally
specified in the description of filial generation.
Fingerprint. The characteristic spot pattern produced by electrophoresis of the
polypeptide fragments obtained through denaturation of a particular protein with a
proteolytic enzyme.
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FIS. International Seed Trade Federation. It was formed in 1924 with well defined
objectives such as: (a) to formulate and deliberate opinion of all concerned in the seed
trade, (b) to carry out continued action for the improvement of seed trade conditions
between various countries, and to find solutions to problem that affects international
seed trade, and (c) to facilitate settlement of international disputes in seed trade. In
fact, FIS is the prime international organization of the seedsmen of the world. India is
also a full member of FIS since 1968.
Fisher’s Theorem. A theorem pertaining to natural selection. “The greater the genetic
variability upon which selection for fitness may act, the greater will be the expected
improvement in fitness”. In mathematical terms, increase in fitness of a population
should be proportional to the genetic variability or genetic differences in fitness
present in the population.
FISH. Fluorescence In Situ Hybridisation. It is in situ hybridisation using a probe
coupled to a fluorescent molecule. This is a technique to characterize ‘alien’ genes
(introduced) in our crop plants. It can also be used in the study of nuclear architecture
and chromatin packaging, and the fundamental principles of biology such as DNA
replication, RNA processing, gene amplification, gene integration and chromatin
elimination.
First Division. The first of the two meiotic divisions. Also called reductional division.
Fitness. The relative probability of survival and rate of reproduction of a phenotype or
genotype. It is therefore the ability of alleles, multigenic combinations of alleles,
individuals, or populations to survive and transmit their genotype to following
generations. A population is said to have maximum fitness if its members have
optimum phenotypes for most ‘fitness’ traits. Although we loosely speak of fitness of
an individual, the concept really applies to classes of individuals, and is statement
about the average survival and reproduction of the individuals in that class. It is
distinct from physical fitness, which may be related to the survival. But a better
surviving individual cannot be called fit, if it is sterile. Fitness is a consequence of the
relation between the phenotype of the organism and the environment in which it lives,
so the same genotype will have different fitnesses in different environments. No
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genotype is unconditionally superior to all others in all environments. Fitness of a
population derived from an F 1 between two or more parents is often low in early
generations (F 2 to F 6 ) owing to the production of many unbalanced genotypes.
Elimination of inferior genotypes, however, leads to rapid improvement in mean
fitness. Thus gain in fitness (adaptedness) is achieved through loss in adaptability as
homozygosity increases (or heterozygosity decreases). At later stage (after F 6 ),
genotypes that cooperate in giving the population a coordinated and cohesive
structure will remain in the population. Such genotypes although slightly different
genetically may be uniform for characteristics such as height, flowering time, maturity
period, etc. Also called Darwinian fitness or biological fitness.
Fixed Allele. An allele for which all members of the population under study are
homozygous, so that no other alleles for this locus exist in the population.
Fixity of Species (Linnaeus 1707-1798). Organisms of one species giving rise to
organisms of the same type.
Flag Leaf. The last leaf of a cereal plant. It is also referred to as boenting leaf in sugar
cane.
Flexibility. The ability of an individual, genotype or population to vary and adapt to
changing conditions so as to survive. It stems from the cryptic or potential variability.
Owing to such variability, an individual may remain functional in a whole series of
variable environmental conditions. This phenotypic flexibility is also referred to as
plasticity by plant physiologists. It is of two types: (a) developmental, and (b)
behavioural. The former is the ability of an individual to adapt reversibly to local
condition. However, the latter refers to faculty of an individual to adapt itself
reversibly to local or time associated factors. For a mobile individual, it is the ability
to seek out that environment to which it is best adapted.
Floret. A small flower from an inflorescence (as in a grass panicle).
Fluctuation Test. A test used in microbes to establish the random nature of mutation or
to measure mutation rates.
Fluff. True botanical seeds of sugar cane. Seeds also contain long silken hairs present at
the base of the spikelets; the term, fuzz also applies to seeds of sugar cane.
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Foreign DNA. DNA from another organism.
Formylmethionine (fMet). A specialized amino acid that is the very first one
incorporated into the polypeptide chain in the synthesis of proteins.
Forward Mutation. A mutation that converts a wild-type allele into a mutant allele. The
term is purely descriptive.
Fossil. A remanent or an impression of an organism preserved from a past geologic age.
Foundation Seed. Seed stock produced from the breeder seed by or under the direct
control of an agricultural experiment station. It is the source of certified seed.
Founder Effect. An acute genetic drift that occurs when a small group breaks off from a
larger population to found a new colony. This drifts results from a single generation
of sampling, followed by several generations during which the population remains
small (with no alteration from mutation and migration). Founder effect may be
responsible for absence of an allele in such a population.
Frame-shift Mutation. The insertion or deletion of a nucleotide pair/pairs, causing a
disruption of the translational reading frame.
Freeze Preservation. Conditioning and preservation of plant cells, tissues, or organs at
extremely low temperatures, usually in liquid nitrogen.
Freezing Injury. The damage to plants caused by formation of ice-crystals within plant
tissues. Crops such as red gram, field peas, etc encounter freezing injury during winter
season. Genetic differences for resistance/tolerance to this injury have been noted in
these crops. (Also see winter injury).
Frego-Bract. A bract type mutant in cotton in which the bracts curl outward, exposing
flower buds and bolls.
Frequency Dependent Fitness. Fitness differences whose intensity change with changes
in the relative frequency of genotypes in the population.
Frequency Dependent Selection. Selection that depends on frequency-dependent fitness.
Frequency Histogram. A graph representing the frequencies of various arbitrarily
bounded classes.
Frequency Independent Fitness. Fitness that is not dependent on interactions with other
individuals of the same species.
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Frequency Independent Selection. Selection in which the fitnesses of genotypes are
independent of their relative frequency in the population.
Fruiting Body. In fungi, the organ in which meiosis takes place and sexual spores are
produced.
F-Test. Also called Fisher’s test after the name of a great statistician Ronald A. Fisher. It
is a statistical test to indicate whether differences in mean performance of a number of
treatments are significant. However, it does not identify the specific pair or groups of
treatments that differed. To answer the question, one has to apply another statistic
called critical difference (least significant difference). From the standpoint of
comparison of mean performance of treatments, it is considered more powerful than t
or z tests. It has wider application in that it also provides an overall test of several
differences whereas the t-test provides a test of single difference.
Function. Any relationship between one variable entity and another.
Functional Complementation. The use of a cloned fragment of wild-type DNA to
transform a mutant into wild type. It is also used in identifying a clone containing one
specific gene.
Functional Genomics. Studying pattern of gene expression and interaction in the genome
as a whole.
Functional Resistance. Also called pseudo-resistance. It results from some temporary
shifts in the environmental conditions favourable to an otherwise susceptible host
plant. This mechanism is not necessary based on heritable traits. These mechanisms
are (a) host evasion, (B) induced resistance, and (c) escape.
Fusion Nucleus. The product of fusion of polar nuclei in the embryo sac of higher plants.
The fusion nucleus can be called the parent of the endosperm.
Fuzz. The dried florets with pappus hair and seeds (as in sugar cane). In cotton, the
remaining fibre (after ginning) along with seed is called fuzz.
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Gain-of-Function Mutation. A mutation that results in a new functional ability for a
protein, detectable at the phenotypic level.
Gamete. A cell of meiotic origin specialized for fertilization. It carries only one member
of a chromosome/gene pair (in basically diploid individuals). It fuses with any other
such cell from the opposite sex or mating type to form a diploid zygote. It may be an
egg cell or a sperm cell.
GATT (1948). A convention called General Agreement on Trade and Tariffs. It was set
up in January 01, 1948 to regulate the world trade of goods. Now more than 108
countries, including India are the members of this convention. Later on, it was
replaced by WTO.
Gamete Selection. A modification of early testing. It involves crossing a good inbred
line with a random sample of pollen from an open-pollinated variety followed by
advancement of superior F 1 from selfed seeds on the basis of test cross performance.
This leads to improvement in the existing inbred lines.
Gametic Selection. Selection against a genotype occurring in gametes (haploids). Of
course, there is no difference between dominant and recessive genes, since both
genotypes are phenotypically expressed. Meiotic drive or segregation distortion may
be considered as one form of gametic selection in which gametes carrying a particular
gene show preferential survival.
Gametic Number. The number of chromosomes present in the gametic cell of a species.
It is usually designated by n. It may differ from the basic number (x). In a species that
is basically diploid, both n and x are equal. However, in a ployploid, the gametic or
haploid number depends upon the number of basic sets (x) in the somatic cell. For
example, in H. vulgare n is equal to x, but in T. aestivum n amounts to 3x.
Gametocyte. The germ cell.
Gametogamy. The union of female and male gametes and the fusion of the gamete nuclei
to form the zygote; also see fertilisation.
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Gametogenesis. The formation of female and male gametes (sex cells). Oogenesis and
spermatogenesis refer to the formation of egg cell and sperm cell, respectively.
Genotype-Environment Interaction. Thechangeinorder, ranking, andrelativevalues
among genotypes across various environments, which are due primarily to macroenvironmental
differences. If genotypes behave consistently, G-E interaction comes to
zero, and consequently genotypes are judged stable. G-E interactions increase rapidly
even to unmanageable levels with the increase in the number of genotypes,
environments, and the level of classification. In low-input agriculture and/ or low
productivity environments in which most variables are unpredictable, high G-E
interactions are observed. Consequently, identification of superior genotypes becomes
very difficult.
Gametic Frequency. The frequency of a particular kind of gamete in the population. All
the three parameters, viz., gene frequency, genotypic frequency and gametic
frequency, are used to describe genetic variation at a locus (loci).
Gametophyte. The haploid gamete-producing stage in the life cycle of plants. It is
prominent and independent in some species, but reduced or parasitic in others.
Gametophytic Incompatibility. A mating system in which incompatibility reaction is
governed by the genetic constitution of the male gametes. It is also called
oppositional-factor system because stylar tissues prohibit or slow down the growth of
pollen-tube containing the same allele of the incompatibility gene. Although
controlled by one gene with multiple alleles (Nicotiana spp.), two-loci system has
also been found (e.g., rye).
Gene (Johannsen 1909). The fundamental and functional unit of heredity. In classical
sense, any inherited factor that determines a biological characteristic of an organism is
called a gene. The concept of the gene dates back to 1865 when Gregor Mendel
showed that certain well-defined attributes of Pisum spp. are governed independently
by a number of factors (genes). Biochemically, it is a segment of DNA, composed of
a transcribed region and a regulatory sequence that make transcription possible.
Gene Action. The way a gene(s) acts in producing a particular phenotype. It can be linear
or non-linear.
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Genealogy. A record of the descent of an individual, or a group of individuals, from an
ancestor or ancestors (or from older forms).
Gene Amplification. The process by which the number of copies of a chromosomal
segment is increased in a somatic cell.
Gene Conversion. A meiotic process of directed change in which one allele directs the
conversion of a partner allele into its own form.
Gene Deployment. A strategy of resistance management that involves the use of different
sets of resistance genes in the various sub-areas of an epidemiological unit. In this
case, the production area is subdivided into several zones, and each zone is planted
with a variety having a specific gene(s) for resistance. It is a planned way to avoid
monoculture of a particular resistance gene over long geographical areas. This type of
geographical gene deployment has been termed geographical multiline by Nelson
(1973). However, gene deployment requires a number of genes with similar
effectiveness for control of prevalent races, and the cooperation between breeders and
pathologists. The absence of major genes with equal resistance against the prevalent
races limits its feasibility because when one gene is clearly superior to others, most
breeders prefer to use it.
Gene Disruption. Inactivation of a gene by the integration of a specially engineered
introduced DNA fragment.
Gene Dose. The number of copies of a particular gene present in the genome.
General Resistance. Non-specific host plant resistance.
Generation. The phase in the life cycle of plants, which extends from one to the
immediately successive reproduction (alternation of generation). In genetics and plant
breeding, it corresponds to whole reproductive cycle, and comprises those individuals
of a population who are equally removed from a common ancestor.
Generation Time. The time between two successive generations.
Generative. Reproductive.
Genetic. Of heredity; hereditary.
Genetic Cost. Also called selection cost; applies to deleterious recessive genes. It is the
loss of fitness per individuals affected (s) times the frequency of affected individuals
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(q2), that is, selection cost equals to selection coefficient times frequency of
deleterious recessive genes (genetic cost = s x q 2 ). Suppose a population in which
there are three genotypes (AA, Aa, and aa) and their fitness are W AA =W Aa =1.0, W aa
=0.8. Letp=q=0.5, thengeneticcostwillbe0.2x 0.25 = 0.05.
Genetic Death. Expression of genetic load carried by a population that is accompanied
by the loss of a portion of its individuals. It is not necessarily an actual death before
reproductive age but can also be expressed through sterility, inability to find a mate,
or by any means that reduces reproductive ability relative to the optimum genotype.
Numerically it equals to sq 2 N, where ‘s’ is selection coefficient against deleterious
homozygote, q 2 is the frequency of such homozygotes, and N is the number of
individuals in a population.
Genetic Drift. Random fluctuations in allelic or genotypic frequencies resulting from
small population size.
Genetic Engineering. Genetic manipulation bypassing sexual reproduction so that
individuals with a new combination of heritable properties are established. Two tools
of genetic engineering are rDNA technology and protoplast fusion. Tobacco is the
first crop, which was genetically engineered in 1986.
Genetic Erosion. The gradual and persistent loss of plant genetic resources (different
alleles) due to different processes. This may be the result various breeding methods
such as inbreeding. Modern agriculture that follows planting of a few superior
varieties may eventually lead to genetic erosion.
Genetic Extinction. See genetic death.
Genetic Inertia. See genetic homeostasis.
Genetic Information. The sum total of instructions, which direct all the activities of the
cell and are stored in the nucleotide sequence of nucleic acids (DNA and RNA).
Genetic Isolate. A breeding population, which does not exchange genes with any other
such group.
Genetic Mobility. The ability of biological species to change their habitat or (in plants)
the range of distribution of pollen and seeds. It largely determines the manner
whereby the mechanism of geographical isolation becomes effective.
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Gene Family. A set of genes in one genome all descended from the same ancestral gene.
Gene Frequency. Also called allele frequency; the proportion in which different alleles
of a gene occur in a population. The term ‘allelic’ frequency is most appropriate,
although used less frequently. One should properly use the term “allele frequency”;
however, the usage of the term “gene frequency” is too widespread to enable an easy
change.
Gene Fusion. The accidental joining of DNA of two genes (as can occur in a
translocation). It can give rise to hybrid proteins or to the misregulation of the
transcription unit of one gene by the cis-regulatory elements (enhancers)ofanother.
Genetic Gain. The expected genetic superiority of the progeny of the selected
individuals compared to the base population. If the original population consisted of a
mixture of pure lines/clones, the genetic advance predicted on the basis of broad
sense heritability amounts to genetic gain. If, however, the base population consisted
of a mixture of segregating materials, then genetic gain amounts to the genetic
advance based on narrow sense heritability. Under such a situation, genetic gain is: Gs
= k x σ P x h 2 , whereas, h 2 = narrow sense heritability.
Genetic Homeostasis. The stability of a group of genotypes (plants) that exceeds that of
its individual members; also called population buffering. Heterogeneous cultivars
generally have more stability on the average than do homogeneous ones. In genetics,
it is the maintenance of genetic variability in a population in the face of all the forces
acting to reduce it. Over the long period of evolution in a particular environment, a
population has built-up a particular array of gene frequencies, which confers a very
high degree of fitness upon the population. Therefore, any departure from these
frequencies may reduce fitness, and thus the population opposes any change and
maintains such frequencies in it. According to Lerner (1954), homeostasis may also
arise from the necessity for the maintenance of certain level of heterozygosity to
ensure normal development.
Genetic Load. The proportion by which fitness of a population at the given locus is
decreased owing to the presence of deleterious (lethal, semilethal, sub vital) genes. It
is expressed as the average number of potential genetic deaths per individual. At a
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locus, if the fitness of a genotype (aa) carrying deleterious genes is 1-s, the incurred
genetic loss is sq 2 . The value sq 2 also equals the mutation rate at equilibrium. There
are two principal components of genetic load: mutational load and segregational
load. The former is the load maintained by recurrent mutation to deleterious alleles at
loci that are homozygous for non-deleterious alleles. The latter is the load maintained
by segregation from advantageous heterozygotes.
Genetic Material. A substance that carries genetic information. For a substance to be
called as the genetic material, it should: (a) replicate, (b) carry all kinds of necessary
biological information, (c) transmit these information to the cell, and (d) be relatively
stable. Nucleic acids fulfil all these conditions, and are therefore called the genetic
materials.
Genetic Resources. An assembly of divergent gene complexes that can be exploited to
improve a crop species. Based on utility, it can be primary, secondary, etc. gene pools.
Genetic Value. A measure of the inherent potential of a genotype that cannot be assessed
exactly. Largely, differences in the genetic values can be judged by differences in
phenotypic values particularly when environment affects the phenotypes uniformly in
the same direction.
Genetic Variance. A parameter which measures genetic differences among individuals/
lines/families of a population. Since all measurements are necessarily made on
phenotypes, it is measured indirectly.
Glucosinolates. A compound in the seed meal/cake of rapeseed and mustard that breaks
into toxic sulphur compounds in the intestines of animals. It has been found to impair
growth and reproduction of the animals. Excessive and continuous feeding results into
enlargement of thyroid glands and ultimately poor growth of ruminants.
Gene-for-Gene Hypothesis (Flor 1956). The interaction of gene(s) conditioning
resistance in the host with the gene(s) governing pathogenicity in the pathogen, finally
determining the disease expression by the host. Flor (1956) established it in flax in
relation to flax rust. He found that hybrids between races of flax rust segregate for
pathogenicity in accordance with the number of genes for resistance in the differential
host. Thus if a variety is PP, the ratio of avirulent to virulent segregates in the hybrids
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between an avirulent and virulent strains is found to be 3:1. If the host variety is
PPRR (that is, it carries two dominant genes), the ratio of avirulent to virulent will be
15:1, and so on. This suggests that complementary genic systems in the host and
parasite control reaction to the flax rust. Resistance occurs when complementary
genes in both host and parasite are dominant; if either or both the pairs of
complementary genes are recessive, susceptibility results. Thus a variety that carries
no dominant genes for resistance is susceptible to all races of the parasite and a
variety carrying one dominant gene for resistance is resistant to all races carrying the
dominant complementary gene for pathogenicity. The gene-for-gene relationship
probably also operates for host-specific insects but not for insects that have many
hosts. However, the gene-for-gene hypothesis has been criticized on several grounds;
some are: (1) it does not recognize intermediate reaction, and (2) it does not convey
any idea about the strength of the resistant gene. According to this hypothesis, for
each gene conditioning resistance in the host, there is a corresponding gene governing
virulence in the pathogen. Therefore, a minimum of two resistance genes in the host
and two complementary virulence genes in the pathogen are necessary to demonstrate
a gene for gene gene hypothesis. In addition, the disease reaction as demonstrated by
Flor is the most common pattern observed with biotrophic pathogens. With
pathogens, that produce host-specific toxins (such as victorin by Drechslera
victoriae), an opposite pattern is found.
TABLE - 1. Designation of complementary genes for rust resistance in the
host and for virulence and avirulence in the pathogen.
_______________________________________________________________________
Genotype of the host Genotype of the pathogen Host reaction
nnpp A N A N A P A P susceptible
NNpp a N a N A P A P susceptible
NNpp A N A N A P A P resistant
nnPP A N A N a P a P susceptible
nnPP A N A N A P A P resistant
NNPP a N a N a P a P susceptible
NNPP A N A N A P A P resistant
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Genetic Male Sterility. A type of male sterility that is governed by the gene(s) carried
on the chromosomes. The male sterile condition is ordinarily monogenic recessive. Its
exploitation in hybrid seed production is rather difficult due to problems involved in
the maintenance of male-sterile line. Male-sterile stocks are maintained by crossing
with heterozygous male-fertile line (Ms ms). This necessitates identification of malesterile
(and male-fertile) plants since the progeny contains the two types in equal
proportions. To circumvent this problem, genetic markers such as tightly linked genes
or pleiotropic effects of male-sterile genes can be used.
Gene Pyramidying. A strategy for management of resistance that involves incorporation
of an increasing number of resistance genes into the same cultivars likely to be
released in succession. Thus, with the available number of genes, the resistance can
be used for a long time. However, there are several disadvantages that limit its use: (a)
considerable efforts are required to incorporate many a major gene into one
genotype, (b) the use of back cross method to incorporate the major genes into one
genotype restricts the agronomic characteristics of the new cultivar, and (c) resistance
of the cultivar may encourage evolution of new virulent races of the pathogen,
particularly if the same major genes are used singly in other varieties being grown in
the adjacent area.
Genetic Assortative Mating. Mating together of individuals more closely related (by
ancestry) than individuals mating at random. Genetic assortative mating at all loci is
called inbreeding. (Inbreeding deals with the entire genome whereas genetic
assortative mating is specific to a locus). It results in an increase in homozygosity.
Genetic Advance. The genetic superiority (and not phenotypic) of selected individuals /
lines/families over the base population. It measures the difference between mean
genotypic values of selected lines (As) and of the original lines (Ao). Thus, Gs = As –
Ao = k × σ P ×H 2 . Therefore, it depends on: k = standardized selection differential
and its value is fixed for a given proportion of lines saved from the base population
(2.06 at 5% selection intensity), σ P = phenotypic standard deviation of the original
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population, and H 2 = heritability coefficient (broad sense). However, heritability
could be high even when genetic variance is low; but, genetic advance is high if the
estimates of genetic variance and k are high.
General Combining Ability. The performance of an inbred line in a cross with an openpollinated
variety. The average performance of an inbred line in a series of single
crosses is called average combining ability. There is a strong correlation between
performance of an inbred in inbred-variety cross and average performance of the
same inbred in a number of single crosses. Thus the two terms, general and average
combining ability are used interchangeably and synonymously. However, the term
general combining ability is used more commonly and frequently than the latter. GCA
is critically important in adaptedness and yield. It results from additive effects of
many alleles of many different loci that enhance performance cumulatively in
multiple environments within a given ecogeographical regions; thus it is amenable to
improvement by repeated cycles of selection.
Genetic Disassortative Mating. Mating between individuals less closely related (by
ancestry) than the average. In the broadest sense, it is called outbreeding. Its real
application is only in connection with breeding hybrid varieties. Also called negative
assortative mating.
Genetic Enhancement. Also called genetic improvement; increasing frequency of
desirable alleles at individual loci. Several mechanisms such as dominance,
overdominance, epistasis, linkage, and the like may interact towards genetic
enhancement in a given genotype. Once superior genetic make-up is established, it
leads to significant increase in yield. For example, the proportion of yield gain by
growing single cross hybrids (from 1960-1990 in USA) resulting from genetic
improvement has been estimated to be > 60%, whereas < 40% was ascribed to
improved cultural practices, including increased use of fertilizers.
Genetic Homeostasis. The tendency of a population to maintain its dynamic state of
equilibrium of its gene pool, which results in optimum balance to the population due
to self-regulatory mechanisms; also called population buffering or collective
homeostasis. Selection leads to disruption of such state; however, relaxation in
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selection pressure results in at least partial restoration of the original genetic
composition.
Genic Hybrid Sterility. Failure to gamete formation in the hybrid due to abnormal
developmental processes caused by specific gene complexes. In this case, meiotic
pairing is completely normal; sterility seems to be caused by incompatibility between
genes derived from two parents.
Genocopy. The production of the same phenotype by different genes.
Genospecies. A genetically defined species; a group of individuals potentially able to
contribute to or share in a common gene pool.
Genotype. The genetic constitution of an organism as distinguished from its appearance
(phenotype). It is the specific allelic composition of an individual or of a cell, which
may be of the entire cell or, more commonly, of a certain gene or a set of genes. It is
stable (except for rare changes called mutations) compared to the phenotype, which
changes with time.
Genetics (Bateson 1905). The study of heredity and variation. The term has been derived
from a Greek word, which means, “to generate”. It is the study of genes through their
variation.
Genotypic. Of or concerned with genotype.
Genotypic Frequency. The frequency at which a genotype occurs in a population.
GISH. Genomic In Situ Hybridisation; a technique that may provide a clue to the
structure and evolution of alloploid species and genome origins. It can also be used
for monitoring introgression of genes and detection of chromosomal aberrations, not
possible by other approaches.
GP4. Genetic resources that contain specific useful gene(s) from different types of
organisms across texa that can be transferred through rDNA technology. Such a gene
pool may be called a gene ocean.
Germ Cell. Any of the reproductive cells in multicellular organisms. It covers both
ancestral forms and their ultimate differentiated products.
Germ Line. The cell lineage from which the gametes are derived in a multi-tissued
eukaryote.
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Germinal Mutations. Mutations occurring in the cells that are destined to develop into
gametes.
Germplasm Theory (Weismann 1834-1914). An accepted theory of evolution that
disproved pangenesis theory of inheritance. The theory states that multicellular
organisms give rise to two types of tissues: somatoplasm and germplasm. Only
germplasm enters into reproductive processes and continuity of germplasm between
all descendant generations account for the many observed similarities that are
inherited. Somatoplasm, on the other hand, consists of tissues that are essential for the
functioning of the organisms but that lack the property of entering into sexual
reproduction.
Gene Interaction. Modification of gene action by a non-allelic gene(s). it may be the
collaboration of several different genes in the production of one phenotypic character
(or related group of characters).
Gene Map. A linear designation of mutant sites within a gene, based on the various
frequencies of interallelic (intragenic) recombination.
Gene Mutation. A point mutation that results from changes within the structure of a
gene.
Gene Pair. The two copies of a particular gene present in a diploid cell (one in each
chromosome set).
Gene Pool. The sum total of genes that occur in the reproductive gametes of a population.
It can be considered as a gametic pool from which samples are drawn at random to
form the zygotes of the next generation.
Gene Rearrangement. The process of programmed changes in the DNA structure of the
somatic cells, leading to changes in gene number or in the structural and functional
properties of the rearranged gene.
Gene Replacement. The insertion of a genetically engineered transgene in place of a
resident gene that is often achieved by a double crossover.
Gene Tagging. Mapping of genes of economic importance close to known markers. Thus
a (molecular) marker very closely linked to a gene of interest can act as a tag. This tag
(molecular marker) can be used for indirect selection of the target gene in the
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breeding programme. Several genes of economic importance such as disease
resistance, fertility restoration, stress tolerance, insect resistance, yield attributing
traits, and the like have been tagged. Gene tagging is a pre-requirement for markerassisted
selection and map based gene cloning.
Gene Therapy. The correction of a genetic deficiency in a cell by the addition of new
DNA and its insertion into the genome.
Genetic Code. The set of correspondences between nucleotide pair triplets in DNA and
amino acids in polypeptides.
Genetic Dissection. The process of identifying a particular biological structure or
function through the use of recombination and mutation.
Genetic Markers. Alleles used as experimental probes to keep track of an individual, a
tissue, a cell, a nucleus, a chromosome, or a gene; also see morphological and
molecular markers.
Germplasm. The sum total of hereditary materials present in a cultivated species
including related wild ones. It can also be defined as the genetic material forming the
material basis of inherited qualities with capability of being transmitted from one
generation to the next by the germ cells.
Germplasm Collection. A collection of genotypes of a particular species, from different
sources and geographical locations, used as source materials in plant breeding.
Genome. A set of chromosomes usually tending to inherit together as a group. It
corresponds to the haploid set in a basically diploid species. Also, the basic number
(x) in a species constitutes a genome. It is the entire complement of genetic material
in a set of chromosomes.
Genome Analysis. Genetic dissection of genomes of a cultivated species along with
related ones. It is carried out in order to: (a) determine phylogenetic relationship
between species, (b) trace phylogeny of a species, (c) provide a guideline to the plant
breeders for transferring an alien gene(s) into a commercial cultivar, and (d) produce
synthetic allopolyploid. The criteria that are used in genome analysis may be
geographical distribution, morphology of parents, cytological and biochemical
characteristics, and the like.
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Genomics-Assisted Breeding. The integration and use of genomic tools in breeding
practices for developing superior lines with enhanced biotic or abiotic stress tolerance
and improved yield. The objective of GAB is to establish and utilize relationship
between genotype and phenotype for crop improvement. GAB includes a range of
approaches including genomics, transcriptomics and proteomics to identify the
molecular markers associated with traits of interest to the breeders that help prediction
of phenotype from the genotype to assist breeding.
Genomic Library. A library encompassing an entire genome.
Genomics. The cloning and molecular characterization of entire genomes. In other words,
it is cloning and mapping of all the genes present in a genome.
Genome Wide Selection. Also called genomic selection (GS); targeting identification of
superior lines with higher breeding value in a breeding programme based on genomewide
marker profile data. As breeding values are estimated using the genome wide
marker data, these are generally referred as genomic-estimated breeding values
(GEBVs). In brief, GS employs two populations: (i) ‘training population’, that is
generally comprised of breeding lines that were/ are in use in a breeding programme
and phenotyping data, not for some traits, but for overall performance (e.g., yield and
yield components) are available across the environments, and (ii) ‘candidate
population’, which is generally being used currently by breeders.
Genetic Equilibrium. The condition in which successive generations of a population
contain the same genotypes in the same proportions with respect to particular genes or
combinations of genes. The consequence of such a condition is that mean of the
population remains unchanged owing to constancy of gene and genotypic frequencies.
Genus. A systematic unit that comprises a group of species having a common
phylogenetic origin and appearing to be distinctly differentiated from other such
groups.
Glycolysis. Lysis or splitting of glucose. Under anaerobic conditions, glucose is degraded
into lactate. If glycolysis is carried out under aerobic conditions, the final products are
pyruvate and coenzyme NADH. Whether aerobic or anaerobic, the process of
glycolysis is completed in a number of steps with the aid of 10 enzymes, all of which
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are located in the cytosol. The net energy yield of this chain of reactions is the
production of two molecules of ATP from one molecule of glucose.
Golden Rice (Ye et al. 2000). Genetically engineered rice capable of synthesizing provitamin
A (ß carotene) in the endosperm thus giving golden yellow colour to the rice
grain. It contains two genes from daffodil (Narcissus pseudonarcissus) and the other
one from a bacterium, Erwinia uredovora. The biosynthetic pathway is:
IPP
GDP (Geranylgeranyl diphosphate)
Phytoene synthase (daffodil gene1, psy)
Phytoene
Phytoene desaturase (Bacterial gene, crt1)
ß-carotene desaturase (Bacterial gene, crt1)
Lycopene
Lycopene ß-cyclase (daffodil gene2)
ß-carotene (in endosperm giving yellow colour)
Geologic Time. Divisions and history of life on earth.
Glossary. A partial dictionary of a particular subject, explaining words, terms, and
concepts.
Glume. The outer husks or bracts of each spikelet in grasses.
Gossypol. A polyphenolic compound present in darkly-pigmented glands that occur
throughout the cotton plant. It reduces the availability of lysine and certain other
essential amino acids in cottonseed protein, and may cause toxicity if cottonseed meal
is fed in excess to young poultry stocks. The best way to reduce or remove gossypol is
to breed for glandless varieties. The glandless trait is expressed in the presence of two
recessive genes, gl 2 and gl 3 . These genes do not reduce lint yield or fibre quality, but
insects have a preference for glandless cotton.
GR 50 . The dose of the mutagen that reduces growth of irradiated materials or individuals
to 50%. Like LD 50 , it also varies with genotypes (within species) and plant species.
Empirical evidences do suggest that numerical values of these two parameters (GR 50
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&LD 50 ) go high for genotypes that are fairly tolerant to abiotic stresses such as heat,
drought, cold, etc.
Grade. A unit of biological improvement from evolution point of view consisting of a
group of individuals similar in their level of organization.
Gradient. A gradual change in some quantitative property over a specific distance.
Grafting. A process of producing a chimera by vegetative fusion of parts (the scion and
stock) from different individuals.
Graft-Hybrids. Plants in which tissues of different genotypes exist side by side. In such
hybrids, some tissues (cell layers) come from the scion and some from the stock.
Gray. The SI unit of absorbed dose, typically associated with ionising radiation such
as X-rays or gamma particles or with other nuclear particles. Gray (Gy) is defined as
the absorption of one joule of such energy by one kilogram of matter (usually human
tissue). Gray, named after the British physicist Louis Harold Gray, replaces the
traditional unit, the rad (equivalent to 0.01 Gy).
Green Revolution. Sudden and quantum jump in the production and productivity of
agricultural crops. Dr William Gadd of the USDA coined the term in 1968. That year,
India experienced its first quantum jump (almost double to that of preceding year, that
is, about 17.0 million tons) in wheat production, leading to Indian Government
releasing a special stamp titled “wheat revolution”. Significantly, the stamp had the
portrait of the library of IARI, New Delhi to mark the importance of scientific and
knowledge inputs in farming. The Green Revolution started at first in the Punjab-
Haryana regions owing to the presence of several prerequisites such as owner
cultivation, land consolidation and levelling, rural communication and electrification,
existence of a dynamic research and extension system, assured and remunerative
marketing opportunities, and the like.
Gridding. A technique to reduce the selection error caused by environmental effects. It
consists of dividing the experimental area into small plots (grids) and selecting equal
number of superior plants within each grid regardless of the fertility and moisture
gradients. Thus it aids to efficiency of mass selection.
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Growth. The increase in mass of the organism, which is intimately associated with
cellular metabolic activity.
Gynoecious. Of plants having only pistillate flowers.
Gyanogenesis. The development of a haploid embryo (individual) from the egg cell.
Gynandromorph. An individual that is a mosaic of male and female structures. The
underlying cause is frequently sex chromosome mosaicism, such that some cells are
chromosomal females, whereas others are chromosomal males.
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H4 (Patel 1971). The first commercial hybrid of cotton in the world. It was developed at
Cotton Research Station, Surat (India) by crossing two upland cotton (G. hirsutum L.)
varieties namely ‘Gujrat 67’ and ‘American Nectariless’. The commercial seed
production was performed manually.
Habitat. That place inhabited by a population, species, or a group of species.
Haploid (Blakeslee et al. 1922). A cell or organism with gametic chromosome number. It
has also been defined as a sporophyte with the gametophytic chromosome number.
Haploids, when viable, are typically smaller and less vigorous than diploids. They are
characteristically sterile. The reason that accounts for sterility in monoploids holds
true in this case also. Haploids are produced spontaneously in natural populations or
they can be produced artificially through a number of techniques. Haploid embryos
can arise from an egg cell (gyanogenesis), from a gametophytic cell other than the egg
cell (apogamy) or from a male gamete (androgenesis). However, no one method of
producing haploids is effective with many species. They have special significance in
the plant breeding as they can provide complete homozygous line after doubling
chromosome number in a single generation. They are also useful in basic genetic
investigation as genetic segregation is less complex compared to diploids (which have
two chromosomes of a particular kind).
Haploidization. Production of a haploid from a diploid (or a di- or poly-haploid from a
polyploid) by progressive chromosome loss.
Haplontic. Of organisms (called haplonts) with a life cycle in which meiosis occurs in
the zygote giving rose to four haploid cells. The diploid stage is short-lived.
Hardy-Weinberg Equilibrium. A state of population in which the same proportion of
genotypes is carried forward to successive generations with respect to any given locus
or combination of loci. It is also called genetic equilibrium or Hardy-Weinberg-
Tschetverikov equilibrium after the name of its propounders. If a population is not in
equilibrium, equilibrium at a locus is established after one generation of panmixis
(random mating). If, the gene frequency p or q in males is not equal to that of females,
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then it takes one generation to equalize the frequency between the sexes, and the
second generation to reach the equilibrium. For the similar reason, the equilibrium is
not attained even after two generations for the sex-linked genes if males and females
start with unequal gene frequencies. One consequence of this equilibrium is that rare
alleles are virtually never in homozygous condition. Most copies of such rare alleles
are found in heterozygotes. The relative frequency in heterozygotes as opposed to
homozygotes is p/q, which for q = 0.001 is a ratio of 999:1. For two genes that are
independent in inheritance, if we start with a population of individuals of which all
are AaBb so that p = q = r = s (that is, all gametes AB, Ab, aB, ab are equally
present), equilibrium is reached in one generation of random mating as occurs for one
gene.
Hardy-Weinberg Law. A law dealing with constancy of gene frequency across
generations in a panmictic population in the absence of disturbing forces. In an
infinitely large random mating population, the frequencies of adaptively neutral
alleles of a gene and the corresponding genotypic frequencies (at equilibrium) remain
constant from one generation to the other unless there are disturbances due to
selection, non-random mating, differential migration and differential mutation.
Harlequin Chromosomes. Sister chromatids that stain differently, so one appears dark
and the other light.
Harvest Index. The proportion of dry matter that is accumulated in economically usable
parts. If the crop is grown primarily for grain purpose, then, harvest index (HI) is the
proportion of the dry matter that is accumulated in grain. As HI is a ratio of economic
yield and biological yield, a genotype having low grain yield could have high HI and
vice-versa. Thus selection entirely on the basis of HI is not reliable.
Heaving. Lifting effect of the soil due to alternate freezing and thawing. It may result in
the lifting up of plants and may tear them loose from the soil, or sheer off roots.
Helicase. An enzyme that breaks hydrogen bonds in DNA and unwinds it during
movement of the replication fork after initiation of process of DNA replication.
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Helix. The natural conformation of biological polymers (DNA and proteins)
characterized by a spiral structure with a repeating pattern described by rotation and
translation.
Hemizygous. Of genes and chromosomes present in only one copy in a diploid organism
(X-linked genes in a male mammal, for instance). Such genes and chromosomes do
not occur in pairs of alleles and homologues, respectively.
Heredity. The biological similarity of offspring and parents; the property by virtue of
which offspring resemble their parents.
Heritability (broad sense, H 2 ). Capability of being inherited; the proportion of total
variability that is due to genetic causes (heredity). Quantitatively, it is expressed as the
ratio of the genetic variance to the phenotypic variance; H 2 = [genetic
variance/phenotypic variance] x 100 (in percentage) = [σ 2 G /σ 2 P] x 100. For
instance, if H 2 is 80%, it implies that one-fifth of the total variance is environmental
and four-fifths genetic in origin; in other words, only 80% of the phenotypic
superiority of selected lines can be realized in the next generation. It can be high even
when σ 2 G is low. It is called broad sense heritability because it takes the total genetic
variance into account. As such it has little significance in plant breeding, and its
estimate in one population in a given environment cannot be extrapolated either to
another population or the same population in a different environment. With lower
heritability, considerable overlapping is encountered between genotypes; this poses
tremendous difficulty in selection.
Heritability (narrow sense; h 2 ). The proportion of total variance that is due to additive
gene action. It is the proportion of additive genetic variance to the total variance. h 2 =
[additive variance/phenotypic variance] = [σ 2 A/σ 2 P]. Since additive variance better
indicates the degree to which the progeny of the segregating individuals will resemble
their parents, h 2 is more reliable compared to the H 2 . High heritability means high
regression of offspring on their parents.
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Hermaphroditism. A condition concerning presence of reproductive organs of both
sexes in the same individual or in the same flower in higher plants. (Hermaphrodite
animals and monoecious plants are analogous!).
Heterocaryon. A cell having genetically differentiated nuclei; the result of
heterocaryosis. Two different nuclear typesarecontainedinacommoncytoplasm. It
is apparent that a heterocaryon containing different types of nuclei behaves like a
genetic heterozygote, and hence possesses heterocaryotic vigour analogous to the
heterotic advantage of heterozygotes in the cross-fertilized sexual species.
Furthermore, the system is clearly capable of providing a type of somatic segregation
based on exchange of entire nuclei during hyphal fusion, and thus creates adequate
variation in pathogenic asexual fungi.
Heterocaryosis. The coming together of genetically distinct nuclei in the same cell of
mycelium to form heterocaryon. This may change pathogenicity of fungal races. It
may also lead further to segregation and recombination of different nuclei and
eventually to creation of new pathotypes at least in asexual fungi and the fungi
imperfecti.
Heterochromatin. Densely staining condensed chromosomal regions that are believed to
be genetically inert for the most part. Heterochromatin can be divided into two
classes: (a) constitutive heterochromatin, which may be an entire chromosome or a
specific segment that stain densely in every preparation from a given species, and (b)
facultative heterochromatin, which may also be an entire chromosome or a specific
segment that appear as euchromatin in some preparations and heterochromatin in
others. It may be responsible for a variety of meiotic alterations. It can be used for: (1)
increasing crossing over (through B-chromosomes in maize), (2) changing the
position of crossing over (through B-chromosomes in maize), (3) reducing pairing
between homeologous chromosomes (through B-chromosomes in common wheat and
Lolium spp.), and (4) increasing pairing between homologous chromosomes (5B
chromosome in modern wheat).
Heterochromosomes. Sex chromosomes or allosomes.
Heterodimer. A protein consisting of two nonidentical polypeptide subunits.
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Heteroduplex. A DNA double helix formed by annealing single strands from different
sources. If there is a structural difference between the strands, the heteroduplex may
show such abnormalities as loops or buckles.
Heterogametic Sex. The sex having heteromorphic sex chromosomes (XY, for example)
and thus producing two different kinds of gametes with respect to the sex
chromosomes.
Heterogeneity. A description of the state of a population containing a number of pure
lines or genetically different individuals (in case of a random mating population).
Heterogeneity in such a population (especially in selfers) often leads to: (a)
phenotypic interactions providing gains in performance, and (b) mutual buffering or
homeostasis resulting in steadier performance. Therefore, such a population is
expected to contain cooperative or synergistic neighbours (genotypes). In other words,
heterogeneity in this kind of population is favoured for synergistic genotypes.
Heterogeneous Nuclear RNA. Assortment of RNA types found in the nucleus, including
mRNA precursors and other types of RNA. It is designated as HnRNA.
Heterokaryon Test. A test for cytoplasmic mutations, based on new associations of
phenotypes in cells derived from specifically marked heterokaryons.
Heteromorphic Chromosomes. A chromosome pair with some homology but differing
in size, shape, or staining properties.
Heteromorphic Incompatibility. An incompatibility system which operates through
differences in the morphology of flowers on different plants coupled with sporophytic
control of incompatibility reaction of the pollen. The best-known case is found in
Primula species; however, it is practically unimportant in higher plants.
Heteromultimer. A protein consisting of at least two polypeptide subunits, with at least
two of the subunits being nonidentical (e.g., haemoglobin).
Heteroplasmon. A cell containing a mixture of genetically different cytoplasms and
generally different mitochondria or different chloroplasts.
Heteroploidy. A condition of deviation from the normal chromosome number by a few
chromosomes (2n ± a few chromosomes).
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Heterosis (Shull 1908). A phenomenon in which an F 1 derived by crossing genetically
dissimilar parents may fall outside the range of their parents with respect to a
character (s). Improved biological fitness of such an F 1 hybrid is often called hybrid
vigour. In strict sense, it describes a different genetic make-up of the F 1 compared to
its parents. Thus heterosis is the process and hybrid vigour is the consequence of that
process. However, for all practical purposes, these two terms are used synonymously.
Historically, it was attributed to heterozygosity (dominance and/ or overdominance,
that is, intra-locus interactions at individual loci) in F 1 hybrids. However, the high
proportion of homozygous loci (79% of the 23 loci studied) of the 4 most widely
grown single cross hybrids of maize (in USA) casts doubt on the single locus
heterozygosity and associated overdominance in promoting high performance of such
hybrids. More recently, multi-locus epistasis has been shown to be the major
mechanism of heterosis in a leading hybrid variety of rice (Shanyou 63) in China.
Thus epistasis (especially multi-locus epistasis) plays a major role in the genetic basis
of heterosis.
Heterostyly. A mechanism to promote outbreeding in certain angiosperms. It implies the
presence in a species of two (distyly) or three (tristyly) different types of individuals,
distinguished by the relative positions of stigma and anthers in the flowers. Also see
heteromorphic incompatibility.
Heterothallic Fungus. A fungus species in which two different mating types must unite
to complete the sexual life cycle. “Heterothallic” literally means, “different bodied”.
Mating types are physically identical but physiologically different.
Heterozygosity. A measure of the genetic variation in a population. It refers to the
frequency of heterozygotes (that carry dissimilar alleles of a gene) with respect to a
given locus.
Heterozygotes. Organisms carrying dissimilar alleles (Aa) of a gene pair. They are
sometimes called hybrids.
Heterozygous Balance (Mather 1943). A type of genetic organization adjusted to
heterozygosity. This adjusted state confers high adaptive value upon associated
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individuals. Heterosis is supposed to be the outcome of such genetic organization.
Autogamous species too show heterosis since it has also appreciable level of
heterozygous balance. This balance in self-fertilized species is supposed to be
evolutionary remains of their outbreeding ancestors.
Hexaploid. A cell having six chromosome sets or an organism composed of such cells. If
all the sets are identical, the individual is called an autohexaploid (e.g., sweet potato).
If, however, genomes are only partially homologous, it is referred to as allohexaploid.
Hierarchy. An arrangement into a graded series.
High-Input Agriculture. Agriculture augmented with a variety of inputs, such as
irrigation and improved mechanization, and/ or with such inputs as fertilizers,
pesticides, herbicides, and fungicides. Breeding programmes, in general, have been
very successful and efficient in high-input agriculture (resource rich areas), perhaps
because agricultural conditions in farmers’ fields and agricultural research stations in
which most new varieties are bred and developed are closely similar. Furthermore,
most of the variables of the production environments are predictable and manageable.
High Nutrient Rice. Genetically engineered rice with enhanced iron and cysteine
content. It has a ferritin gene from P. vulgaris, andthermotolerantphytase gene from
A. fumigates.
Histocompatibility Genes. The genes that encode the histocompatibility antigens that in
turn determine the acceptance or rejection of a tissue graft.
Histogenesis. The process of formation and development of tissues; also called
histogeny.
Histone. A type of basic protein that forms the unit around which DNA is coiled in the
nucleosomes of eukaryotic chromosomes. Histones are a group of five basic proteins
that associate with DNA in the cell nucleus to form chromatin.
Holandric Inheritance. A type of inheritance governed by genes completely linked to
the Y-chromosome. Such genes (often called holandric or Y-linked genes) are
transmitted exclusively from father to the sons and thus manifestation is limited to the
male sex only.
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Holliday Junctions. The structures formed during chiasma formation when
chromosomes exchange information during meiosis. Holliday junctions form four
double helical arms (branched DNA) flanking a branch point. The branch point can
relocate throughout the molecule by virtue of the homologous sequences.
Holocentric Chromosomes. Chromosomes having diffused kinetochores (centromeres)
with microtubules attached along the length of the chromosomes. These may be found
in certain species or strains of a species (as in some strains of maize, for instance).
Homeologous Chromosomes. Partly homologous chromosomes originating from
different but related genomes, between which there does not occur point-to-point
pairing at meiosis. The partial homology between them usually indicates some
original ancestral homology.
Homeosis. The replacement of one body part by another. It can be caused by
environmental factors leading to developmental anomalies or by mutation.
Homeostasis. The tendency of a physiological system to resist to an external disturbance
such that the system is not displaced from its normal values.
Homeotic Genes. Genes that control the fate of segments along the anterior-posterior
axis of higher animals.
Homeotic Mutations. Mutations that can change the fate of an imaginal disk.
Homodimer. A protein consisting of two (more than two in case of homomultimer)
identical polypeptide subunits.
Homogametic Sex. The sex producing the same type of gametes with respect to the sex
chromosomes. Such sex contains homologous sex chromosomes (XX, for example).
Homogamy. The preference of a mating individual for another with similar phenotype or
genotype.
Homolog. A member of a pair of homologous chromosomes.
Homologous Chromosomes. The chromosomes (or parts of chromosome) that synapse
or pair during meiotic prophase; chromosomes with point-to-point pairing between
them at meiosis. They are also defined as the chromosomes in different but related
species that have retained most of the same genes during their evolution from a
common ancestor.
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Homomorphic Incompatibility. A type of incompatibility identified only by breeding
tests, and not by differences in floral morphology. The biochemical reaction of the
incompatibility may be determined either by genetic constitution of the pollen itself
(gametophytic) or by the genotype of the parent plant (sporophytic).
Homothallic Fungus. A fungus species in which a single sexual spore can complete the
entire sexual cycle (compare it with heterothallic fungus).
Homozygotes. Individuals containing identical alleles of a gene pair. They may be either
homozygous dominant (AA) or homozygous recessive (aa).
Homozygous Balance (Mather 1943). A condition in which genetic components of
individuals are adapted to homozygosity. Inbreeders and certain cross-pollinated
species do not show inbreeding depression by virtue of this kind of genetic
organization.
Honey Comb Selection (Fasoulas 1983). A design for selection. It was suggested as a
means to reduce the adverse effects of interplant competition on selection. As the
name implies, plants are grown in a fashion similar to honeycomb. Each plant is
surrounded by other six plants forming a hexagon. A plant that outperforms all the six
neighbouring plants is selected. It may improve selection during segregating
generations when individuals are grown in a non-replicated manner.
Hordicale (Qutuke 1940). A synthetic cereal obtained by crossing H. vulgare and S.
cereale. Plants are more like paternal parent. However, seeds are devoid of embryo
and endosperm because of their early abortion (14 days after pollination). The
incompatibility can be circumvented by spraying the spikes with growth regulators,
combined with culturing of embryos in vitro.
Horizontal Resistance (Van der Plank 1963). A kind of resistance in epidemiological
terms. It is evenly effective against all genetic variants (races) of a particular
pathogen. It can arise in two ways: (a) when the host genes do not operate in a genefor-gene
way vis-à-vis pathogen genes, no differential interactions are possible, and
(b) when several to many host genes with small effects (polygenes) operate on a genefor-gene
basis with an equivalent number of genes in the pathogen populations,
differential interactions are too small to be detectable, and the result appears to be
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horizontal resistance. Owing to race non-specificity, the resistance is only partial,
albeit durable and stable.
Hormone. A molecule that is secreted by an endocrine organ into the circulatory system
and that acts as a long-range signalling molecule through activating receptors on or
within target cells.
Host. A cell or organism whose metabolism is used for the growth and reproduction of a
virus, bacteria, pathogen, or any other parasite.
Host Evasion. A mechanism of functional resistance in which phenologies of the host
and insect do not synchronize. It takes place when plant growth pattern is modified so
as to bring in asynchrony of insect-host phenologies. For example, early planting of
rice saves the crop from gall-midge attack.
Host Range. The spectrum of strains of a given plant species that a given strain of pest
can infect.
Hot Spot. A place where an endemic disease(s) or pest(s) occurs regularly during
cropping season. In genetics, it is a part of a gene that shows a very high tendency to
become a mutant site, either spontaneously or under the action of a particular
mutagen.
Hybrid Breakdown. DistortedsegregationintheF 1 hybrid leading to the recovery of
parental types (cultivated and wild relatives) in the F 2 or later segregating or
backcross generations. It may be ascribed to centromeric affinity, cryptic structural
hybridity, gene substitution or unfavourable nuclear-cytoplasmic interactions. In
certain interspecific hybrids of rice, hybrid breakdown has been reported due to
duplicate recessive genes in the F 2 and later segregating generations.
Hybrid Corn Makers. An acronym used in recognition of pioneer works of early maize
breeders like Shull, East, Jones, Hays, Rickey and Jenkins.
Hybrid Cotton. The cross product of genetically different parents of cotton. The term is
also used to designate transgenic cotton, the lint of which would have the properties
of original cotton as well as polyester. Clothes made from such cotton would behave
like present cotton-clothes besides zero shrinkage. This way farmers may put
challenge to polyester industry.
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Hybrid Dysgenesis. A syndrome of effects including sterility, mutation, chromosome
breakage, male recombination in the hybrid progeny of crosses between certain
laboratory and natural isolates of Drosophila.
Hybrid Incapacity. A collective term that includes sterility and inviability of usually
interspecific or intergeneric hybrids. It is the result of genetic imbalance which may
affect either metabolism during early or later vegetative stages of development or
gametogenesis leading to failure in gamete formation.
Hybrid Inviability. Inability of a hybrid (usually derived from wide crosses) to survive
due to abnormal developmental processes. Death of hybrids can occur from the
zygotic stage up to the final differentiation of reproductive organs and formation of
gametes. It may be due to activation of lethal genes or disharmonious interactions
between cytoplasm of one species and nucleus of the other. It can be overcome by
embryo rescue, in vitro/vivo embryo culture, ovule culture, ovary culture, grafting
hybrids, reciprocal crosses or altering genomic ratio.
Hybrid Sterility. Partial or complete reproductive incapacity of an F 1 or later generation
hybrids between genetically different populations (usually belonging to two different
species). It is clearly an isolating mechanism, which prevents flow of genetic
information between populations. The basis of hybrid sterility may be genomic
disharmony, nuclear instability, hybrid break down and reversion to parental types,
lack of recombination and presence of deleterious genes or undesirable linkages. It
may appear as embryo sac sterility, pollen sterility, anther dehiscence, and asynchrony
in the development of male and female gametes in the same spikelet. It may be
overcome by chromosome doubling or backcrossing.
Hybrid Variety. An F 1 population obtained by crossing genetically dissimilar parents
and used for commercial plantings. It exploits heterosis to the greatest extent. A
hybrid variety must display hybrid vigour to the extent, which makes it profitable to
the farmers. Seeds of hybrid variety must be purchased afresh every year because the
superior gene combination of the F 1 generation breaks down in the F 2 generation
owing to Mendelian segregation and recombination. As a consequence, a large
number of genetically different F 2 individuals are produced; none of which is nearly
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as productive as the F 1 itself. The yield reduction in the F 2 generation is nearly always
at least 10%, and often more than 30%, compared to the parental hybrid variety.
Hybrid. The product of a cross between genetically dissimilar parents; an individual
carrying dissimilar alleles at one, two, three,….., etc. loci is called mono-, di-, tri-,….,
polyhybrids, respectively; a heterozygote. It is a progeny individual from any cross
between parents with differing genotypes.
Hybrid Equilibrium. The equilibrium state of hybridity in a stable breeding population.
Hybrid Swarm. A collection of hybrids produced by complete or local breakdown of
isolating barriers between two sympatric species. Hybrid swarm replaces the parental
species and serves as a continuous bridge between two parental extremes.
Hybrid Vigour. See heterosis.
Hybridisation In Situ. Finding the location of a gene by adding specific radioactive
probes for the gene and detecting the location of the radioactivity on the chromosome
after hybridisation.
Hybridisation. A process of making a hybrid by crossing genetically unlike parents of
the same species or of a different species. Its application in self-pollinated crops relate
to the creation of enormous amounts of variability through segregation and
recombination of genes in the hybrid. Its main object in breeding self-pollinated crops
is to obtain a superior genotype (recombinant) in segregating generation, which
results from recombination of desirable genes, which are found in two or more
genotypes. In cross-pollinated crops, this is an inseparable operation in population
improvements and in the development of hybrid varieties.
Hybridity. The union of genetically dissimilar gametes; the state of being heterozygous
or hybrid.
Hybridize. Forming a hybrid by performing a cross. Also annealing nucleic acid strands
from different sources.
Hydrogen Bond. A weak bond in which an atom shares an electron with a hydrogen
atom. For example, it is formed after a hydrogen atom is shared between two atoms
having negative charge such as between oxygen and nitrogen or between two nitrogen
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atoms. Hydrogen bonds are important in the specificity of base pairing in nucleic
acids, and in the determination of protein shape.
Hydrophobic Interactions. Exclusion of water by non-polar groups of a macromolecule,
causing compaction with hydrophobic groups inside and hydrophilic groups outside.
Hyperploid. An aneuploid containing a small number of extra chromosomes (2n + a few
chromosomes).
Hypersensitivity. The increased sensitivity of the host cells in the vicinity of infected
site, which leads to the death of infected cells ultimately restricting the spread of
obligate pathogens through separating them from the living host (e.g., resistance to
late blight of potato). Hypersensitivity is now thought to involve several different
mechanisms such as antibiosis, presence of mechanical barriers (the presence of cell
walls, which cannot be easily degraded by the pathogens’ enzymes), or the absence or
insufficiency of nutrients or other essential compounds.
Hypoploid. An aneuploid with a small number of missing chromosomes (2n – a few
chromosomes).
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IJ
I-Line. Inbred line.
I 1 , I 2 , I 3 … symbols used to designate first, second, third, etc. inbred generations.
Identity. Individual values of variables. It differs from correlation in that values can be
correlated without being equal. Thus correlation is not the same thing as identity. The
difference between these two statistics arises when we consider the effect of
environment on heritable traits. Parents and offspring could be perfectly correlated in
some trait such as height; yet, because of environmental difference between
generations, every child could be taller than its parents.
Ideotype (Donald 1968). A biological model expected to perform in a predictable
manner within a defined environment. Such an ideotype can be a plant with model
characteristics known to influence photosynthesis, growth and (in cereals) grain
production. It can be seen as a specified model of a plant, which is to produce a
desired product in a required amount and quality in a way that minimizes the effect of
environmental variability during growth and subsequent losses. In practice, such a
model is usually described in terms of phenotype, but theoretically, it could also be
visualized as a genotype. There is no doubt that when a plant breeder defines a new
superior variety in terms of phenotype, whether defined by morphology or physiology,
he is contriving a new ideotype. We should note that ideotype breeding is a
hypothetical ideal concept that is unfortunately dogmatic and unrealistic. In practical
terms, the ideal may not be attainable or even desirable. However, it is a logical step
towards new levels of yield and stability. It provides a guideline to the breeder in the
selection of parents to be crossed and in the selection pressure to be imposed in the
later generations.
Idiotype. The sum total of hereditary determinants of an individual. It includes both
genotype (nuclear genes) and plasmatype (cytogenes) of the individual.
Imago. An adult insect.
Immune System. The animal cells and tissues that recognize and attack foreign
substances within the body. Such a system is likely to exist for plant species also.
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Immunogenetics. An area of genetics that deals with genes governing antigens,
antibodies and their reaction.
Immunity. An absolute term that refers to complete freedom of plants from disease(s). It
is the extreme case of disease reaction in which host-parasite interaction does not
occur. Thus there is no question of disease symptoms on the plants. For example,
potato is immune to wheat rusts.
In Situ. “In place”.
In Vitro Mutagenesis. The production of either random or specific mutation in a piece of
cloned DNA. Typically, the DNA will then be repackaged and introduced into a cell
or an organism to assess the results of the mutagenesis.
In Vitro. In an experimental situation outside the organism (literally “in glass”).
In Vivo. In a living cell or organism.
Inbred. The result of inbreeding; same as the inbred line.
Inbred Line. The product of inbreeding; in plant breeding, a nearly homozygous line
usually originating by continued self-fertilization accompanied by selection for
desired traits.
Inbred-Variety Cross. The F 1 cross of an inbred line with a variety (usually an openpollinated
one). The purpose of such a cross is to know general combining ability of
inbred lines.
Inbreeding Coefficient. A quantitative measure of the intensity of inbreeding. It is a
measure of the probability that any two alleles of a locus will be identical by virtue of
descent from a common ancestor. It shows proportional reduction in heterozygosity
under various systems of genetic assortment. The rate of reduction in heterozygosity
per generation due to inbreeding is 0.5 with self-fertilization, 0.25 with sib mating,
0.125 with half sib mating and 0.0625 with cousin mating. Starting from a
heterozygote Aa, 50% heterozygosity is lost in the first generation of selfing, thus
inbreeding coefficient (F) is 0.5; in the second generation, 75% heterozygosity is lost,
and thus F is 0.75. Said in another way, the amount of homozygosity at initial stage is
zero and hence F = 0; after second generation of self-fertilization, amount of
homozygosity is 75%, and thus F = 0.75. This is why I.C. is also defined as the
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probability that the two alleles at a locus is homozygous by descent. F varies from
zero for random mating population to unity for a completely inbred system. It is a
very useful parameter to describe the property of any other population relative to
Mendelian population:
AA Aa aa
p 2 + 2pq + q 2 = 1.0 (Mendelian population)
[p 2 + F (p)(q)] + [2pq - F (2pq)] + [q 2 + F (p)(q)] = 1.0 (Inbred population)
Inbreeding Depression. A reduction in biological fitness and vigour following
inbreeding (in individuals, which are normally cross-pollinated). The degree of ID is a
function of number of deleterious recessive genes kept concealed in the
heterozygotes. These harmful alleles are exposed in homozygotes upon inbreeding
through segregation and recombination. Thus ID may vary among the inbred lines
derived from the same population. Numerically, it is expressed: ID = [(F 1 -F 2 )/F 1 ] x
100 (in percentage).
Inbreeding Load. The extent to which inbreeding, which ordinarily increases genetic
load, impairs an equilibrium population under random mating.
Inbreeding Superiority. A condition wherein mean F 2 performance is greater than the
average performance of the respective F 1 hybrid. Under stress conditions, even a
small difference in physiology of individuals can make a big difference. Contrary to
F 1 population, F 2 population has better buffering capacity owing to genetic differences
among individuals which may result in superior performance under stress conditions.
Inbreeding Tolerance. The ability of a genotype or a population or a species (belonging
to outcrossing group) to endure with the stress imposed by inbreeding. Self-pollinated
species are highly tolerant to inbreeding. Among outcrossing species, cucurbits and
sunflowers are most tolerant, whereas alfalfa is highly susceptible. Genotypic
variations for inbreeding tolerance have been noticed in maize.
Inbreeding. Mating between relatives; mating of individuals more closely related than
individuals mating at random. The degree of inbreeding is a function of genetic
correlation between the mating partners. The closest form of inbreeding is selffertilization;
however, in self-incompatible species, sib mating is the closest possible
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form of inbreeding. According to Allard (1999), the term inbreeding in plant breeding
is usually applied when matings are made between closely related individuals, and
particularly when mating is by selfing (as compared to genetic assortative mating).
There are two general causes of inbreeding: (a) restriction in population size, and (b)
various mechanisms that affect the mating systems.
Incompatibility. A genetically controlled physiological hindrance to fruitfulness; failure
of self- or cross-fertilization owing to antigenic differences that act between
pollination and fertilization. Self-incompatibility is a mechanism that enforces crosspollination
through providing physiological barriers to self-fertilisation even though
both male and female gametes are functional.
Incomplete Block Designs. Experimental designs in which each block (replication)
contains only a fraction of the treatments to be tested. These designs are exemplified
by lattice block design, group balanced block designs, and the like. These designs are
suited for experiments with a large number of treatments.
Incomplete Dominance. The situation in which a heterozygote deviates from midparental
value towards one of the homozygotes on the phenotypic scale of
measurement. (Exact intermediacy or mid-parental value implies no dominance or
lack of dominance).
Independence. The relationship between two or more variables such that the variation of
each one is uninfluenced by that of others, that is, correlation is zero.
Independent Culling. A method of selection for multiple resistance within the random
mating population. It involves screening of the population independently for each of
different pests/diseases, thus leaving plants only with multiple resistance in the
population. The next cycle of selection starts in the population derived by intermating
plants that carry resistance to all the multiple pests/diseases.
Independent Assortment. Free matching or combinations of different alleles of two or
more genes following segregation at gametogenesis. However, linkage hinders
independent assortment of genes; the degree to which it affects is a function of the
mapping distance between the two genes in question.
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Indeterminate. Descriptive of an inflorescence in which the terminal flower is last to
open. The flowers arise from axillary buds, and the floral axis may be indefinitely
prolonged by a terminal bud. It is a dominant monogenic trait in fenugreek, cluster
bean, broad bean, peas, etc.
Index Selection (Matzinger et al. 1976). Simultaneous selection for two or more traits
that are antagonistically associated. The proponents found that simultaneous selection
for both short height and more number of leaves in tobacco could be useful.
Indica Rice. Tropical ecotypes of rice, traditionally characterized by tall stature, weak
straw, photoperiod sensitivity, easy shattering, broad droopy leaves, and grain
dormancy.
Individual. A unit of breeding population that is expected to contribute to the gene pool
of that population.
Induced Mutation. Heritable genetic change resulting from treatment with a mutagen.
Induced Resistance. A term that may be used for temporarily increased resistance
resulting from some condition of plant or environment. For example, aphids are
particularly negatively correlated with the level of potassium in the soil.
Inducer. An environmental agent that triggers transcription from an operon.
Inflorescence. A cluster of flowers; the arrangement and mode of development of the
flowers on a floral axis.
Inheritance. The transmission of genetic information from parents and ancestors to their
offspring.
Inheritance of Acquired Character (Lamarck 1809). A disproved theory related to
evolution. According to it, the changes that accrue in the phenotype of an organism
during its lifetime can be passed on to its offspring.
Inoculate. Placing inoculum where it is likely to produce an infectious disease.
Inoculum. Spores, bacteria, or fragments of mycelium of pathogens that can infect plants,
or soil.
Inosine. Ararebasethatisimportantatthewobble (third) position of some tRNA
anticodons.
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INSA. International Seed Testing Association founded in 1924 for ensuring uniformity in
seed testing. ISTA rules for seed testing are used all over the world by its member and
non-member countries. It has a system of international and regional referee testing
through which the participating stations can appreciate their shortcomings. It issues
different kinds of certificates, which (especially orange international certificates) are
used extensively, and are of great importance for the international trade in seeds.
Insertion Sequence. A mobile piece of bacterial DNA that is capable of inactivating a
gene into which it inserts. It may be several hundred nucleotide pairs in length.
Insertional Translocation. The insertion of a segment from one chromosome into
another nonhomologous one. The best-known classical example is transfer of a small
segment (gene?) from Aegilops umbellulata into our cultivated bread wheat for leaf
rust resistance. The detailed analysis by Sears (1956) showed that the segment
insertion occurred near to kinetochore of one of the chromosomes, and the genotype
was slightly later in maturity in addition.
Instability. Inconsistency in performance of a genotype(s) across years and locations. It is
a type of variation that appears to be random in direction. It may be due to
developmental noise. It has no genetic basis.
Integrated Pest Management. Exploitation of several pest suppression tactics to keep
the pest population below damaging threshold. It is a multidimensional approach
involving a range of biological, cultural, physical, biochemical and chemical
techniques to hold pest population below economically damaging levels without
disrupting the agro-ecosystem. In fact, the initial factor in IPM is pest resistance in
plant cultivars. Its use as a basal component in IPM has several advantages: (a) it
permits lower level of resistance in plant varieties, (b) by doing so, we can increase
the number of cultivars available for commercial cultivation, and (c) it reduces the
selection pressure for the development of pesticide resistant biotypes of insects, and
other components of IPM reduce the selection pressure for development of biotypes
that would infest resistant cultivars.
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Integrated Plant Nutrition. A system to maintain soil fertility, sustaining productivity
and improving farmers’ profitability through judicious and efficient use of mineral
fertilizers, organic manures and biofertilizers.
Interbreeding. Hybridisation between genetically dissimilar individuals capable of
exchanging genetic information (genes). It holds individuals together in a population,
or populations together in a sub-species or species.
Intercalating Agent. A chemical that can insert itself between the stacked bases at the
centre of the double helix, possibly causing a frame-shift mutation.
Interchange Trisomic. A trisomic developed in such a way that two members of the
triplicate are interchanged chromosomes. It differs from a tertiary trisomic in having
two interchange chromosomes (instead of one in TT).
Interchromosomal Recombination. Recombination resulting from independent
assortment of nonhomologous chromosomes.
Interference. The effect of one crossover on the probability that another will occur in the
immediate vicinity. It is a measure of the independence of crossovers from each other,
and is calculated by subtracting the coefficient of coincidence from 1.
Interphase. The stage between nuclear divisions of the cell cycle, when chromosomes
are extended and functionally active.
Interploid Crosses. Crosses involving parents having different ploidy levels [e.g., a cross
between S. tuberosum (2n = 4x = 48) and S. phureja (2n = 2x = 24)]. Interploid
crosses are made sometimes to produce haploids, which may develop through
parthenogenesis or haploid apogamy.
Interspecific Hybridization. A method as well as design of evolution of crop plants. It
consists of crossing distinct taxonomic species with the preservation of improved
types from the segregation products. As many of the segregants have lower biological
fitness, its major application as a method of breeding has been mostly in vegetatively
propagated species.
Interstitial Region. The chromosomal region between the centromere and the site of a
rearrangement.
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Intervening Sequence. An Intron; a segment (comprising several nucleotide pairs) of
largely unknown function within a gene. The segment is initially transcribed, but it
gets eliminated from the functional mRNA transcript.
Intrachromosomal Recombination. Recombination resulting from crossing-over
between two gene pairs.
Introgression. The transfer of desirable genes from the wild species into the cultivated
ones.
Introgressive Hybridisation (Anderson 1949). A kind of interspecific hybridisation. It
consists of recrossing of interspecific hybrids with the parental species in such a way
that both the species are enriched with desired features of each other without loosing
their taxonomic identity. It aims at broadening the genetic base of different species.
Sometimes, introgression is difficult to detect because genetic contamination is very
slight.
Intron. A sequence of a gene that is transcribed into nuclear RNA but is removed from
functional transcript before translation of the message. After removal from mRNA, it
is rapidly degraded.
Intuitive Approach. An approach of choosing parents for crossing programme on the
basis of phenotypic assessment of agronomic, pathological and quality traits in
contrast to predictive approach that is based on genetic analysis. Breeders most
frequently use it.
Inversion. A rearrangement of a chromosome segment so that its genes are in reversed
linear order. It is a kind of chromosome mutation consisting of the removal of a
segment, its rotation through 180° and its reinsertion in the same location.
Inverted Repeat Sequence. A sequence found in identical but inverted form (for
example, at the opposite ends of a transposon).
Ionizing Radiations. Very high-energy radiations that can remove electrons from atoms
and attach them to other atoms, thereby producing positive and negative ion pairs (α,
β, γ, χ-rays, for example).
IPRs. Intellectual Property Rights. It is a monopolistic right, granted under law to
inventions for a fixed period to exclude others from imitating, manufacturing, using
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or selling a protected matter or process. Various forms of IPRs are patents, copy
rights, trademarks, trade secrets, PBRs, PVP, etc.
IR 8. A semi-dwarf variety of rice introduced in India from IRRI, Manila (Phillipines). Its
introduction was a major event in the history of rice breeding in the country. It had
very good plant type, and high productivity. It was developed from a cross between
‘Peta’ and ‘Dee-geo-woo-gen’. It carries sd 1 gene (for dwarfness) from Dee-geo-woogen.
Irradiation. Exposure of plants, plant parts, seeds, etc. to any kind of radiation to
increase mutation rates.
Isoaccepting tRNAs. The various types of tRNA molecules that carry a specific but the
same amino acid to the site of protein synthesis.
Isoalleles. Alleles indistinguishable except by special tests. Such alleles act within the
same phenotypic range of each other. Effects of isoalleles are ordinarily not
identifiable individually. Such alleles produce only slight differences in phenotypic
expression, making their identification bound to the use of special methods.
Isochromosome. A chromosome with equal and genetically identical arms. The two arms
of the chromosome are mirror images of each other.
Isodirectional Dominance. Dominance occurring in the same direction at all loci. It
could be one of the reasons of skewness observed in the distribution of genotypes
with respect to traits conditioned by four or five genes. However, its effect in
producing skewness decreases as the gene number increases as some may show
dominance in opposing direction.
Isoelectric Point. The pH at which the sum of positive and negative charges is zero. At
isoelectric point, the net charge is zero, and the molecule will not migrate in an
electrical field. This is the basis for isoelectric focussing, a technique used to separate
proteins.
Isogenic Lines. Identical lines differing at only one locus. Such lines are produced
usually through back cross method. True isolines (isogenic lines) are difficult, perhaps
impossible, to produce with back cross breeding owing to linkage between the gene of
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interest and those influencing other traits. The transfer that occurs during back
crossing involves a block of closely linked genes instead of a single one.
Isolation Distance. The minimum distance required to separate seed-producing field
from other varieties of the same or related species in order to prevent outcrossing or
undesired mating. This is an indispensable endeavour to maintain the genetic purity of
a variety during the process of its multiplication.
Isolate. A group of individuals (population) or of populations, who because of their
isolation, choose mating partners from that group. Because of their isolation, one
isolate is completely separated from other such groups.
Isolation. The separation of one group from another so that mating between or amongst
them is prevented. In genetic term, it is the restriction to exchange of genetic
information between distinct populations due to barriers to interbreeding. If the gene
flow is prevented through differences in reproductive mechanisms, it is called
reproductive isolation.
Isomeric Genes. Genes which can each produce the same phenotype. If a number of such
genes are present in one individual, they may be cumulative or non-cumulative in
their action.
Isotope. One of several forms of an atom having the same atomic number but differing
atomic masses ( 12 C 6 and 14 C 6 , for example).
Isozymes. Multiple molecular forms of a single enzyme that differ by minor variations in
amino acid composition and sometimes in regulation. Lactic dehydrogenase, which
catalyses the conversion of pyruvate to lactate, is a tetramer, and have five isozymes
(isoenzymes) derived from all the combinations. They differ in their electrophoretic
mobility in starch gels. The relative proportions of the isoenzymes are characteristic
for each tissue and for each stage of development.
Iteration. A procedure that makes use of repeated trials to find the best-fitting value of a
parameter from observed data.
Japonica Rice. The temperate ecotypes of rice, characterized by more leaves, fewer
tillers, relatively shattering resistance, and having broad and short grains with lower
amylose content.
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Javanica Rice. An intermediate (between indica and japonica types) type of rice selected
from tropical varieties in Indonesia. This type is characteristically tall, thick culmed,
low tillering, resistant to shattering, and have broad stiff leaves, long awns and large
bold grains. Also called bulu rice, it is of recent origin.
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Karyogamy. The fusion of nuclei of male and female gametes. It is the ultimate event
(culmination) of fertilization. The result of the karyogamy is the zygote.
Karyokinesis. The division of nucleus; the nuclear division (mitosis).
Karyolymph. The karyoplasm; nucleoplsm.
Karyotype. The whole group of characteristics that allow identification of a particular
chromosome set, for example, the number, relative size, position of centromere,
length of the arms, secondary constrictions, satellite, and the like. It is characteristic
of an individual, species, genera, or a larger group.
Kilobase. 1000 nucleotide pairs.
Kinetochore. A complex of proteins to which a nuclear spindle fibre attaches. Cell
biologists use the term more frequently instead of centromere, which is synonymous
to kinetochore.
Klinefelter Syndrome. An abnormal human male phenotype due to an extra X
chromosome (XXY).
Knobs. The darkly stained heterochromatic regions present in normal maize
chromosomes. These are repeated sequences of nucleotides. The number of knobs
varies from variety to variety, and from plant to plant. Some have reported that their
standard number may be 23. They are not randomly distributed; rather they occupy
fixed positions on different chromosomes. Most of the knobs are intercalary.
Terminal knobs are rare. They have been used as markers to establish phylogenetic
relationship between maize and its wild relatives.
Knockout. Inactivation of one specific gene; same as gene disruption.
Krebs Cycle. A common pathway for degradation of “fuel” molecules such as
carbohydrates, fatty acids, and amino acids. Also called tricaboxylic acid cycle or
citric acid cycle, it takes place in mitochondrial matrix. The fuel molecules are acted
upon metabolically (in the cytoplasm through the process of glycolysis) to produce
acetyl groups, which are taken into the Krebs cycle by acetyl coenzyme. It takes two
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turns of the cycle to metabolise the two acetate molecules that are produced by
glycolysis from one molecule of glucose.
Kurtosis. The excess or deficiency of frequencies in the centre and the two extremes
compared to the flanks of a frequency curve. Higher kurtosis enhances value of
selection intensity; the reverse is true for low kurtic traits compared to normal
distribution. (Also see skewness).
Laggards. Chromosomes or pairing configurations that show slow movement or no
movement at all during nuclear division (anaphase). Such chromosomes may not be
included in one of the daughter nuclei giving rise to aneuploidy.
Lagging Strand. The strand of the DNA double helix that is synthesized by the ligation
of short fragments synthesized individually in the 5 to 3 direction during DNA
replication.
Lamarckism. An invalid concept of organic evolution, advocated by Lamarck. It states
that adaptive changes induced by the environment within an individual is transmitted
to its progeny. Also see inheritance of acquired characters.
Land Race. A line/strain/group of individuals evolved directly under influence of natural
selection; the earliest cultivated types; the old-established local stocks of a crop
species; any distinctive race of a cultivated species that has become genetically
differentiated as a result of natural and/or human selection operating in ecologically
different circumstances in various regions to which cultivated species have been
introduced; a heterogeneous mixture of genotypes having mutually beneficial
associations with each other and with environmental conditions. One of the most
important uses of such land race is to provide genetic resources for improvement of
our present-day varieties. They are usually tolerant to any pests or diseases probably
because they have had co-evolved at a given point of time. A series of local land races
is collectively called square heads.
Lateral Inhibition. The signal produced by one cell that prevents adjacent cells from
acquiring its fate.
Latin Square Design. An experimental design known for its capacity to handle
simultaneously two known sources of variation among experimental units. It makes it
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possible to estimate variation among row-blocks as well as among column-blocks and
to remove them from experimental error. However, this is possible only when the
number of treatments is equal to the number of blocks (replication), and thus is
completely a square. This enables the treatments to individually appear in each rowblock
and in each column-block. This design becomes impractical if the number of
treatments increases. On the other hand, if the treatment number is small, the degree
of freedom associated with the experimental error becomes too small for the error to
be estimated reliably. This design is used for field trials in which the experimental
area has two fertility gradients running perpendicular to each other or has a
unidirectional fertility gradient but also has some residual effects from the previous
trials.
Law of Homologous Series in Variation. Characters found in one plant species may
also be expected in similar species. Characters occurring in one collections made in
one place may abound in those from other places within the species. Vavilov (1951)
expressed this concept in genetic terms and used this principle as a clue to characters
remaining to be discovered. Also called law of parallel variation.
Law of independent Assortment. Mendel’s second law. During gametogenesis, the
segregation of one gene pair is independent of other gene pairs (or the inheritance of
one character is independent of the others). Notice that the phenomenon of gene
linkage is an important exception to this law.
Law of Minimum (Liebig 1840). A law stating that the growth of a plant is proportional
to the foodstuff, which is available in minimum quantity; also called the law of
limiting factor. The two key points for its practicability are: steady-state conditions
and factor interaction. Steady state implies a balance between inflow and outflow of
energy and materials. Factor interaction includes involvement of other condition with
the essential element. For instance, some plants have been shown to require less zinc
when growing in the shade than when growing in full sunlight; therefore, a low
concentration of zinc in the soil would less likely be limiting to plants in the shade
than under the same conditions in sunlight.
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Law of Segregation. The first law of Mendel. It states that the two dissimilar alleles of a
gene pair separate from each other during gametogenesis and distributed equally and
randomly into gametes. The law is true in all those organisms which utilize meiosis at
least in some part of its life cycle. It does not hold true if there is non-disjunction
during meiosis.
LD 50 . The dose of a mutagen at which 50% of the mutagenized individuals die (before
reproduction). It is used as an important parameter for effectively inducing mutation.
It varies with genotypes, plant species, and the like.
Leader Sequence. The sequence at the 5 end of an mRNA that is not translated into
protein.
Leading Strand. The strand of the DNA double helix that is synthesized continuously in
the 5 to 3 direction during DNA replication.
Leaky Mutation. A mutation that still retains a low but detectable wild-type function
besides displaying mutant phenotype.
Lesion. A damaged area in a gene (a mutant site), a chromosome or a protein.
Lethal. Of a gene (genotype) which, when expressed, is fatal to its carrier.
Lethal Gene. A gene whose expression results in the death of the carrier individual.
Leucoplast. A plastid containing no visible pigments. It is found in embryonic, germ,
and fully differentiated cells. Leucoplasts are also found in meristematic cells and in
those regions of the plant not receiving light. Leucoplasts in certain differentiated
zones of the root produce starch granules, called amyloplasts. They do not have
thylakoids and ribosomes.
Level of Significance. Also called confidence limit; the frequency at which chance errors
in performance are allowed to occur beyond which any variation is supposed to be
real one and hence significant. In other words, the probability chosen for deciding a
deviation to be significant (or non-significant) is called level of significance.
Conventionally, 1% or 5% (or in some genetical experiment, even 50%) error is
allowed. Thus, 99% or 95% is the confidence limits, respectively. In other words, we
can assert with confidence that in 99% or 95% cases, variation observed among
treatments is real (genetic) and not due to chance error. In all biological experiments,
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the level of significance is arbitrarily chosen at 5%. It means a chance deviation is
considered real one once in 20 times on an average. If we choose a more stringent
probability level, i.e., 1%, it implies a deviation arising from chance would be
interpreted as a real one once in 100 times. However, this level may let us miss some
good types when non-significant results are found. The less easily we reject a
hypothesis, greater is the chance that what we have accepted as valid may be false and
vice-versa. Thus as a compromise between the two extremes, a commonly accepted
level of significance for testing hypothesis is 5%.
Life Cycle. All the significant events that lead a particular life form to beget of its own
kind. It is species-specific and controlled by a particular genetic system. In higher
plants, it involves a regular alternation of generations.
Ligase. An enzyme that can rejoin a broken phosphodiester bond in a nucleic acid.
Limits of Tolerance (Shelford 1913). A range in between ecological minimum and
maximum of organisms. It led to the birth of toleration ecology; and thus limits within
which various plants and animals can exist are known.
Line Breeding. The mating, usually in successive generations, of individuals having a
known common ancestor. It is also a system of breeding in which a group of progeny
lines is composited on the basis of desired traits. Since pollination is not controlled, it
remains essentially a form of random mating with selection, and thus genetic
composition of the population is subject to a very slow change for a quantitative trait
even when there is adequate genetic variability.
Lineage. A group of individuals descended from a common ancestor; descendants of an
ancestor.
Line x Tester Analysis (Kempthorne 1957). An extension of top-cross method. It
involves crossing a number of lines, each with the same set of testers and growing the
resulting crosses in a suitably laid-out experimental design to provide for information
about various genetic parameters such as general and specific combining ability, gene
effects, and the like. The number of tester must be greater than one (contrast it with
top cross method that involves crossing with a single broad-based tester).
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Line. A group of individuals/plants with similar appearance and breeding behaviour. The
group is pure-breeding (or nearly so) and may be diploid or polyploid, and can be
distinguished from any other such group of the same species by some unique
phenotype and genotype. It is a more narrowly defined group than a strain or a variety.
Linkage Disequilibrium. A state of population in which there is a difference between
repulsion and coupling phase gametes, that is, (Ab) x (aB) – (AB) x (ab) 0.
However, under the condition of random mating, the difference (± d) is reduced to
one-half each generation so that after 4-5 generations, only 10% or even less of the
disequilibrium value remains in the population.
Linkage Drag. The transfer of undesirable genes along with useful gene(s) because of
their close association on the whole (or a segment of) chromosome. The problem of
linkage drag is often encountered in wide/distant hybrids, which largely restricts its
application in crop improvement.
Linkage Equilibrium. A state of population in which the difference between repulsion
and coupling phase gametes is zero, that is, (Ab) x (aB) – (AB) x (ab) = 0. Consider a
population in which frequencies of both A & B are 0.6 each and of both a & b are 0.4
each, then (0.6 x 0.4) (0.4 x 0.6) – (0.6 x 0.6) (0.4 x 0.4) = 0 (that is, population is in
equilibrium). This state of population will conserve the existing proportion of
genotype frequency generation after generation of random mating in absence of other
disturbing forces.
Linkage Group. A group of genes known to be linked. As linkage occurs in two
conformations (either in coupling or in repulsion phase), the number of linkage group
always equals the haploid number of chromosomes of the given species.
Linkage Map. A map showing position of genes on chromosomes as determined by
recombination relationships.
Linkage Value. Recombination fraction expressing the proportion of crossovers versus
parental types in a progeny; also called the recombination value. It varies from zero to
one half. The reason for this upper limit is that at a time only two out of four (½ =
50%) chromatids are involved in a crossing over.
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Linkage. A situation of coexistence of two or more genes on the same chromosome
owing to their proximity to each other. If the genes are contiguous, there is a complete
linkage that is a rare phenomenon. On the other hand, there is frequent occurrence of
partial or incomplete linkage, which is often broken down by crossing over. Its main
effect relates to an increase of in the proportion of parental gametes and a
corresponding diminution in recombinant gametes, eventually leading to deviation
from Mendelian expectancy. If linked traits are desirable, it is an aid to plant breeders;
if however, desirable characters are negatively correlated, then undesirable linkage
needs to be broken. For example, gene for rust resistance in wheat is closely linked to
that for late maturity, which can be broken. However, such opportunity is precluded in
the case of pleiotropy.
Linked. Of two genes showing a recombination value of less than 0.5. Linked genes tend
to be transmitted together.
Lint. Long fibres of cotton separated from the seed after ginning.
Linters. The short fibres of cotton that generally remain attached to the seed in ginning;
also called fuzz.
Lipids. A large group of different compounds characterized by their relative insolubility
in water and solubility in organic solvents. There are simple lipids (alcohol esters of
fatty acids), steroids (cholesterol), and conjugated lipids (glycolipids). Phospholipids
are the main components of biological membranes. They have a hydrophilic
(phosphate containing region) and two hydrophobic (fatty acid) tails.
LMOs. Living Modified Organisms; see transgenics.
Local Control. The principle of reducing experimental error. The principle comprises
randomisation and replication.
Locus. The position occupied by a gene on a chromosome.
Lodging. The bending or breaking-over of a plant before harvest. It causes yield losses in
small grain crops, soybeans, cereals, and other crops. Unlike measurement of yield,
the evaluation of lodging resistance is almost entirely a visual appraisal. Genotypic
differences for lodging resistance have been noted within several crop species.
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Lodging resistance may be directly related to the strength of the main stem, number of
nodes, height of the plant, and other traits.
Lodicule. One of two scale-like structures at the base of the ovary in a grass flower.
Low Productivity Environment. A crop production environment characterized by harsh
and erratically unpredictable climatic conditions with the high risk of crop failure.
Such environments are prone to large errors, less differentiation between genotypes,
and less repeatability across years. Various components of such environments are
highly unpredictable and intractable. (Also see subsistence agriculture).
Ludwig Theorem. A theory which states that new genotypes can be added to a
population if they are able to utilize new components of the environment, even if
these are inferior to the ancestral niche. The phenomenon is also called annidation.
Luxuriance. An accidental condition produced by complementary gene action in the F 1
hybrid between species, and varieties or strains of both self- as well as crosspollinated
species. The excessive size and vigour is not connected in any way with the
adaptive superiority of such hybrids compared to their parents.
Lysis. The rupture and death of a bacterial cell on the release of phage
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M. A general term for mutagenic treatment. M 0 refers to the parent material likely to be
mutagenized. M 1 , M 2 , M 3 ,…are symbols used to designate first, second, third, etc.
generations after treatment with a mutagenic agent. M 2 is often assumed to be
analogous to F 2 generation so far as segregation events and relative degree of genetic
variability is concerned. However, M 1 cannot be compared to F 1 ; while there is 100%
heterozygosity in F 1 , mutant loci in M 1 are random and not so frequent, hence,
heterozygosity is variable.
Macro-evolution. Evolution above species level.
Macromolecule. A large molecule with colloidal properties; a long polymer composed of
monomers. The molecular weight of such a molecule is greater than 1000. Polymers
such as DNA, RNA, a protein or a polysaccharide are macromolecules.
Macro-mutation. A mutation with the recognizable effect on a single plant.
Macrophytes. Rooted or large floating plants generally growing in shallow water only
(as in a pond).
Maisinte. A maize-teosinte hybrid fodder crop. It has been produced by backcrossing
maize-teosinte hybrid with maize. Its main features are quicker growth, earlier
maturity, higher fodder yield and high crude protein content than parents. Owing to
teosinte genes, it may give good response to stress conditions.
Major Genes. Genes with large, easily recognizable and relatively stable effects. There
are numerous examples of well defined morphological or physiological characters
which are governed by single genes, and are little affected by either the genetic or
physical environments, e.g., differences between field and sweet corn, indeterminate
vs. determinate growth habit in fenugreek, etc. It is, however, interesting to mention
that genes are neither major nor minor; nor are they qualitative/quantitative; rather it
is their effects which appear as either major or minor. However, the term such as
major genes is used frequently, and it is accepted usage.
Male Sterility. Absence or non-functioning of pollens in plants. This provides a barrier
to self-fertilization. However, it is advantageous in hybrids seed production since it
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makes mechanical emasculation unnecessary. It occurs spontaneously in almost all
crop plants irrespective of their pollination behaviour as a consequence of mutation at
any one of the several loci, which condition vital steps in the formation of functional
pollen.
Malthusian. Relating to Thomas Robert Malthus and his population theory (1798). The
theory states that population numbers tend to increase at a faster rate (geometric
progression) than means of subsistence (arithmetic progression), leading to
competition for environmental resources in short supply.
Map Unit. A unit of distance in a linkage map. It is the “distance” between two linked
gene pairs, recombination value for which is one per cent.
Mapping. The linear arrangement of genes on a chromosome.
Mapping Function. The relationship between distance in a linkage map and
recombination value (between the two linked genes) or frequency. It is expressed by a
formula: true map units = -ln [(1-2 R.F.)/2] x 100, whereas, R.F. stands for
recombination frequency.
Marcotting Technique. A technique to induce rooting of nodes by covering them
selectively with suitable materials such as mud, soil-coconut fibre mixture or peat
mixtures. The rooted nodes are then cut and replanted in the soil. This facilitates
reduction in the height of the prospective parents to be crossed (as in sugar cane).
Marker Assisted Back Crossing (MABC). Introgression of specific trait(s) from a
donor parent into the genetic background of a recurrent parent (generally leading
variety) using molecular markers (Hospital 2005). The product of MABC is a
line/cultivar containing only the major gene/QTL from the donor parent, while
retaining the whole genome of the recurrent parent. MABC approach generally
involves transfer of a limited number of trait loci including transgenes from one
genetic background (donor genotype) to the other genetic background (elite variety).
MABC approach can also be used to generate near-isogenic lines (NILs) or
chromosome segment substitution lines (CSSLs) for genomics research, which are
populations that are often used for genetic analysis of genes/QTLs. Gene pyramiding
is an important application of MABC in which a few different genes for the same trait
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(e.g., resistance to different races) or for different traits are brought together in one
genetic background using molecular markers.
Marker-Assisted Recurrent Selection (MARS). Estimation of marker effects from
genotyping F 2 or F 3 population and phenotyping F 2 derived F 4 or F 5 progenies,
followed by two or three recombination cycles based on presence of marker alleles for
small effect QTLs (Eathington et al. 2007). In the first step of MARS, de novo QTL
identification is carried out initially, that is, QTLs are identified in the breeding
population itself, generally derived from good × good crosses. Subsequently, the lines
carrying superior alleles for maximum QTLs are crossed to pyramid superior alleles
in one genetic background. Recombined lines are then subjected to a final phenotypic
screening to select the best lines for multi-location field traits to release them as
varieties. MARS is particularly useful for capturing the several genomic regions
especially to target more number of minor as well as major QTLs. Therefore, genetic
gain achieved by MARS as compared to the MABC programme is higher (Bernardo
and Charcosset 2006). The recurrent-selection method is routinely used mainly in
cross-pollinated crops like maize, and this process can be improved with the help of
molecular markers; therefore, the process is called marker-assisted recurrent selection
or MARS.
Marker-Assisted Selection (MAS). The selection of target gene(s) aided by tightly
linked markers. The essential requirements for MAS in a breeding programme are: (a)
co-segregation or very close linkage (<1cM) of markers with the target gene/ trait, (b)
availability of an efficient means of screening large populations for molecular
markers, and (c) high reproducibility across laboratories of the screening techniques.
Besides, markers and screening techniques should be economical and user-friendly.
Of course, the population can be screened at any growth stage and in any
environment. It is also independent of interaction effects. Markers can also provide
additional information on the breeding value of a genotype, and thus accelerate
selection and breeding cycles. It can be practiced especially for traits whose
phenotypic selection is difficult. It may be of special value in breeding species that
have large stature or long generation time (arbour crops) where fewer individuals
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might save several hectares, and fewer generations may save several decades. MAS is
being used efficiently: (a) in gene pyramiding, (b) in marker-assisted alien gene
introgression, and (c) for simultaneous identification and pyramiding of QTLs from
primitive cultivars and alien species.
Markers (Morphological). Traits or genes (variant alleles), which are used to label a
biological system throughout the course of experimentation. These are usually mutant
alleles, which may be either dominant or recessive. These are limited in number,
influenced by environments, and developmental stage specific compared to molecular
markers. Further, they could appear as pleiotropic effect of other major genes.
Masking Action. Gene action such that a gene hides the effect of another non-allelic
gene (when both are present). In oats, for instance, Y produces yellow seed coat and B
produces black seed coat. The gene Y will have no effect in presence of B, since black
colour masks yellow ones. It gives a ratio of 12 black (B-Y-, B-yy): 3 yellow (bbY-): 1
white (bbyy).
Mass Pedigree Method. A system of breeding in which a genetically diverse population
is propagated in mass until conditions favourable for artificial selection occur, after
which pedigree selection is practised. The bulking may end as early as F 2 generation
or may be continued for many generations if the advent of conditions suitable for
selection is long-delayed.
Mass Selection. A form of selection in which individual plants are selected depending
upon their better phenotypic values (usually without progeny test) and the next
generation is propagated from aggregate of their seeds. In autogamous crop species, it
has basically two uses: First, it can effect improvement in land varieties very safely
and rapidly, and the second, it can be used to purify existing varieties. In the crosspollinated
species, it is used to improve characters having high heritability. It is
virtually powerless to effect any improvement in a trait like yield (low heritability)
obviously for three reasons: (a) selection is exclusively based on phenotype, (b) there
is lack of pollen control on female parent, and (c) there is no progeny test. Therefore,
simplicity of mass selection is the weakness in itself. However, some form of mass
selection is almost always practiced in most breeding methods.
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Maternal Effect. The environmental influence of the mother’s tissues on the phenotype
of the offspring. It may be result of nuclear genes as observed for cases like shellcoiling
example in the water snail Limnaea.
Maternal Inheritance. A type of uniparental inheritance in which all progeny have the
genotype and phenotype of the parent acting as the female. It was first discovered by
Carl Correns (1909) in his studies on four-o’clock plants (Mirabilis jalapa) forleaf
variegation (patches of green and white tissues in the leaf).
Maternally Expressed Gene. A gene that contributes to the phenotype of an offspring on
the basis of its expression in the mother.
Mating System/ Mating. Any of a number of schemes by which individuals are assorted
in pairs leading to sexual reproduction, and thus formation of zygotes.
Mating Types. The analogous terms used in lower organisms for sexes in higher
organisms. Mating types differ only physiologically, and not in the physical form.
Matriclinous. Having predominantly maternal hereditary traits.
Matriclinous Inheritance. A type of inheritance in which all offspring have the nucleusdetermined
phenotype of the mother.
Maximum Likelihood. A statistical procedure for estimating values of population
parameters from sample data. The method identifies values that have maximum
probability of being the best fitting for any given set of observations.
Mean Deviation. The sum total of average deviations (d) of individual observations
from their arithmetic mean without regard to the sign {d/N}. Though not used
frequently, it finds its greatest application in calculating combining ability effect(s) of
a given parent of/or a cross in plant breeding experiments.
Mean. Also called arithmetic mean. It is the arithmetic average and is obtained when the
number of individuals in the data divides the sum of the values of the observations (in
that data). It is usually designated by μ and x in the population and sample,
respectively. Unlike median and mode, it is affected by the extreme values. However,
it is most commonly used and preferred as it provides more information about the
central tendency than other measures.
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Mechanical Errors. Errors occurring in the execution of an experiment. These can occur
during layout, management and care of the experiment and data keeping and
recording. These are primarily human errors, and thus cannot be effectively controlled
by statistical techniques. These can make a substantial difference in the experimental
results. Their prevention depends largely on the awareness and the skill of the
researcher in relation to the various operations of the experiment.
Medium. Any material on which experimental cultures are grown. Although composition
of a medium varies according to the objectives of a programme, it usually consists of
sugar (as a source of energy), certain inorganic salts, one or a few phytohormones and
vitamins, and agar (as a solidifying agent if a solid medium is to be prepared).
Median. The middle value of a series of observations arranged in ascending or
descending order of their magnitudes. It divides the series into two equal halves, half
the number of the observations lying above it and half below. If the number of
observations in a series is odd, then (N+1)/2 is the middle item of the series (where N
is the total number of observations). However, in case of an even series data, two
middle items [(N/2) and (N/2+1)] will represent the median.
Mega Base Pairs. One million nucleotide pairs.
Megaenvironment. Broad (not necessarily continuous often transcontinental) area with
similar biotic and abiotic stresses, cropping systems and consumer preferences.
Analysis of megaenvironment is performed by AMMI model.
Megasporangium. A sporangium producing megaspores.
Megaspore. One of the four haploid spores originating from the meiotic division of the
diploid megaspore mother cell in the ovary, and which gives rise to the mega
gametophyte.
Megaspore Mother Cell. The diploid (2n) cell in the ovary that gives rise to four haploid
megaspores through meiosis.
Meiocyte. A cell embarking upon meiosis (or a cell in which meiosis takes place).
Meiosis. A kind of nuclear division occurring in reproductive cells, which results in
production of gametes (animals) or sexual spores (in plants) with haploid (n)
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chromosome number. Thus, it maintains chromosome number of sexually
reproducing species.
Meiospores. Products of meiosis in plants.
Meiotic Drive. Any mechanism that affects the genetic composition of a population
through unequal contribution of gene(s) [and thus chromosome(s)] to the gametes
during meiosis. This preferential segregation makes a disproportionate contribution to
the gene pool of the next generation. It is an exception to the rule of 1:1 recovery of
segregating alleles in heterozygous organism. This unequal or preferential segregation
has also been termed segregation distortion.
Melting. Denaturation of DNA.
Melting Pot Technique. A modified polycross method as in sugar cane. It consists of
placing arrows from a number of prospective parents in small containers (added with
preservative solution) and effecting random intermating by changing the position of
containers (along with arrows) in relation to each other every day. Seeds so obtained
include selfs and all possible crosses including reciprocals.
Mendelian Inheritance. Inheritance of chromosomal genes in contrast to extrachromosomal
inheritance, which deals with cytogenes.
Mendelian Ratio. A ratio of progeny phenotypes based on Mendel’s laws of inheritance.
Mendelian Variation. The diverse types resulting from gene mutations as well as
recombinations following hybridization between types carrying these mutations. This
is the major kind of variation exploited by plant breeders.
Meristem. An area of rapidly dividing plant cells. It may be a single cell (as in lower
plants) or it may include many cells (as in higher plants).
Meristem Tip Culture. The in vitro culturing of plant tissue from the meristem tip
region for the purpose of regenerating pathogen-free plants.
Mesolithic Age. The cultural period between the Paleolithic and Neolithic Ages;
appearance of the bow and cutting tools.
Metabolism. The chemical reactions occurring in a living cell.
Metabolite. A product of metabolism.
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Metacentric Chromosome. A chromosome with centromere placed in the middle so that
its both arms are equal, and take V-shape at anaphase.
Metaphase. The stage of mitosis or meiosis at which chromosomes align along the
equatorial plane of the cell.
Metaxenia. The influence of pollen on the maternal tissues of the fruit. It greatly
influences fruit quality in date palm.
Methylation. Modification of a molecule by the addition of a methyl group. Many a
mutagen acts through methylation (e.g., MMS), causing mispairing between bases,
and eventually leading to GC AT transition.
Metroglyph Analysis (Anderson 1957). A semi-graphic method of studying variability
in a large number of germplasm lines taken at a time. For variability assessment, the
first two characters, which are highly variable, are depicted on X and Y-axes,
separately. The mean values of X (say, character-1) for each genotype is plotted
against the mean values of Y; thus, each line occupies a definite position on the graph,
called glyph. Variation for the remaining characters of each genotype is displayed on
the respective glyph by distinctive rays. Each character occupies a definite ray
position. The length of the ray for a particular character on the glyph may be short,
medium, or long depending on the index value of a genotype. It is a very easy
technique to study the pattern of morphological variation among genotypes, and can
be applied to both replicated and unreplicated data. However, inclusion of a large
number of genotypes sometimes leads to overlapping of glyphs on the graph.
Microcentres (Harlan 1951). The small regions enriched with tremendous plant
diversity and rapid rate of plant evolution. These microcentres may offer excellent
opportunities to collect valuable types, and to study evolution of cultivated plants
experimentally. (Also see ecological regions).
Micro-mutation. A mutation with a small effect that can be detected only by
measurement on a group of plants.
Micron C 7 . The smallest known gene, also designated by Mcc C 7 . It produces a
heptapeptide inhibiting protein synthesis in the Enterobacteriaceae. The heptapeptide
is synthesized from 21 bp open reading frame.
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Micronaire. An instrument that measures the rate of airflow through a standard volume
of cotton. From its readings, fibre fineness and maturity of cotton samples may be
compared.
Micropyle. The usual point of entry for pollen tube in the embryo sac.
Micropropagation. In vitro propagation of plants from very small tissues (parts). It is
one of the best and most successful examples of the commercial application of tissue
culture technology. Advantages of micropropagation include: (a) identical progeny
true-to-the mother plant, (b) rapid and large-scale multiplication of elite genotypes,
and (c) rejuvenation of old clones/varieties. Besides, it is season-neutral and
facilitates germplasm exchange by avoiding the risk of spreading pathogens and
insect-pests.
Microsatellite. A type of repetitive DNA based on very short repeats such as
dinucleotides.
Microspecies. A small breeding population with limited variability. It may be a biotype
of the species.
Microspore. One of the four haploid spores originating from meiotic division of
microspore mother cell in the anther and which gives rise to the pollen grain.
Microspore Mother Cell. The diploid cell (2n) in the anther, which gives rise, through
meiosis, to four haploid microspores.
Microsporocyte. Themicrosporemothercell(or pollen mother cell).
Midparent Value. The mean value of a quantitative phenotype obtained from two
specific parents; the arithmetic mean of two parents with respect to a trait(s). For
instance, if a parent is 20 unit on the phenotypic scale of measurement and the second
one 30 unit, the midparent value will be 25 unit [MP=(20+30)/2].
Migration. Any form of introduction of genes (individuals) from one population into
another. It can be responsible for introducing new genes or moving up or downwards
the frequency of genes already present in the breeders’ or natural populations. The
change in frequency of the gene already present is expressed as: p =m(P-p t ),
whereas, m is the proportion of migrants, P is frequency in donor population, and p t is
frequency in the recipient population. Clearly there will be no change in the gene
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frequency, if there is no difference in gene frequency between donor and recipient
population.
Mini Core. See core collection.
Minimal Medium. A medium containing only a carbon source (sugar), inorganic salts
and water.
Minisatellite. A type of repetitive DNA sequence based on short repeat sequences with a
unique common core. It is used for DNA fingerprinting.
Mispairing. The presence of a non-complementary nucleotide at a given position in one
of the polynucleotide chains of a DNA double helix. It may lead to a gene mutation.
Missense Mutation. A mutation that alters a codon in such away that it encodes a
different amino acid, for example, GAA (glutamic acid) to GUA (valine). This
change, which occurs in the B polypeptide chain at 6 th amino acid position, causes
sickle cell anaemia in human population.
Missing Plot Technique. A technique to calculate the value of any missing treatment in
any block (replication). The calculation for any missing value (x) is done as: x = [r Bj
+ t Ti – GT / (r-1) (t-1)]; whereas, r = no. of replication, t = no. of treatments, Bj =
known jth block total where x is missing, Ti = known treatment total for which x is
missing, and GT = grand total with missing observation. Missing plot technique is an
advantage for a randomised complete block design.
Mitochondrion. A eukaryotic cell organelle that is the site of ATP synthesis, and of the
citric acid cycle. It is a small body usually with distinct shelf like internal layers. It
provides energy through respiration and oxidation.
Mitogen. Any substance that stimulates cells to undergo mitosis.
Mitosis. A kind of cell division in which the nucleus is divided into two daughter nuclei
with equivalent chromosome complements. It is a conservative process that maintains
the same chromosome number in the daughter cells compared to the parental one. It
occurs in both somatic as well as reproductive cells.
MitoticCrossover(Stern 1943). A crossover resulting from the pairing of homologs
during mitosis; also called somatic crossover. Like meiosis, it takes place in fourstrand
stage of chromosomes. The outcomes of somatic crossing-over depend upon:
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(a) the site of the crossing-over, and (b) the mode of orientation and, thus distribution
of the centromeres of the chromosomes engagedinsomaticcrossing-over. Italways
occurs in the diploid cells. It can be an important source of variation in asexual
pathogenic fungi. It is also thought to be important in allowing recessive cancercausing
mutations to become expressed.
Mixoploidy. A term that covers all types of chimeras in which the heterogeneity between
different elements involves differences in chromosome number (2n = x, 2x, 3x, 4x, 2x-
1, 2x-2, etc)
Mode. The value of the variate, which occurs mostfrequentlyinadataset. Inafrequency
table, the modal class is the class that has the greatest frequency.
Modified Doak Method (Mehta and Patel 1983). A method of producing hybrid seed in
cotton without emasculation. The top portion of the corolla is impressed deeply to
expose only the tip of the stigma. The anthesis is prevented through placing lint
dipped in mud over the remaining portion of the corolla. The stigma is pollinated in
the next morning. The boll setting in crosses has been found greater (specially in the
diploid cotton) than with Doak Method.
Modified Convergent Cross (Mackey, 1954). A convergent cross that involves a very
good variety as persistent crossing partner at every stage of the crossing procedure.
This results in 50% of the genes from the persistent good variety. For example:
A × B A × C A × D A × E
↓ ↓ ↓ ↓
F 1 × F 1 F 1 × F 1
↓
↓
F 1 × F 1
↓
F 1
(50% genes from the parent A)
The procedure can be combined further with different degrees of back crossing
(convergent back cross) of F 1 ’s with the desired parent (A) to ensure even higher
percentage of genes from the very good variety (A).
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Modified Single Cross. The progeny of a cross between a single cross, derived from two
related inbred lines, and an unrelated inbred line, (A 1 × A 2 )×B.
Modifier. A gene(s) that affects the expression of other gene(s) primarily in a quantitative
manner. It can act as enhancer or inhibitor depending upon the situation. Its effect
may be dominant or recessive. The degree of dominance or recessiveness of a gene
may also be influenced by modifying genes. Modifiers may have a large or small
effect (but the mode of action is almost always quantitative). Some of them may have
their own primary function and modifying actions appear as secondary effects, while
others have solely enhancing or inhibitory effects on the expression of other genes.
The well-documented example is that of spotting in mice. Similar cases in plants do
exist. Actually further improvement in any character (wherein major genes have
already been exploited) will depend upon manipulation of modifiers.
Molecular Biology. A branch of modern biology in which biological phenomena are
studied by physical, chemical and biochemical investigations at the molecular level.
Molecular Cytogenetics. A correlated study of cytology and genetics at molecular level.
It may provide clues about the stability and expression of trans genes.
Molecular Genetics. The study of genetic systems that can be described at the molecular
level. It is, therefore, the study of molecular processes underlying gene structure and
function.
Molecular Markers. Agents that mark genetic variation at DNA level. There are
numerous molecular markers, for example, RFLP, RAPDs, AFLP, STS, EST, SSRs
(also called microsatellites), SCARs, and the like. Except RFLP (that is southern
hybridisation based molecular marker), all are PCR-based markers. AFLP combines
the properties of RFLP (restriction fragment length polymorphism) and PCR
(polymerase chain reaction). However, STS (sequence tagged sites) is more desirable
because of high degree of reproducibility and large-scale automation, which is
essential for handling a large number of samples. These are numerous, permanent and
uninfluenced by environments/developmental stage of plants. Molecular markers can
be reliably used in the selection of superior genotypes. However, these are expensive
compared to morphological or biochemical ones. These can be used to: (a) develop
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saturated maps, (b) fingerprint DNA for varietal characterization, (c) study
phylogenetic and evolutionary relationship among related species, (d) characterize A,
B, and R lines and find mechanisms of heterosis, (e) tag gene/ genes of interest, (f)
practice marker-assisted selection, and (g) map orthologous gene(s). Markers may be
of two types: screenable and selectable. The former allows screening of transformed
cells through expression of specific enzyme to produce phenotype enabling us to
identify transformed cells. But selectable markers are genes that confer resistance to
some compounds like herbicides and antibiotics usually toxic to normal plants; thus
only cells having these markers survive under selective conditions. It is emphasized
that the importance of marker loci is solely that they identify “short chromosome
segments” that carry predominantly favourable alleles. However, such chromosome
segments must be small so that it may remain intact through many cycles of crossing
over and segregation. This is true for marker loci marking even QTLs.
Monad. An individual cell produced by a meiocyte (instead of tetrad) as a result of
meiotic abnormality.
Monocistronic mRNA. An rnRNA that encodes only one protein.
Monoclinous. Having male and female germ cells in one and the same flower.
Monoecy. A condition/system wherein staminate (male) and pistillate (female) flowers
are borne separately on the same plant (for example, corn).
Monogenomic Species. Species which are basically diploid containing only a single kind
of genome. B. campestris (AA), B. nigra (BB), and B. oleracea (CC)aresomeofthe
examples. Such species are also called primary species.
Monogeny. The production of only male or female offspring.
Monogerm. A sugar beet seed with a single germ, in contrast to a multigerm seed.
Monohybrid. A hybrid that is heterozygous with respect to one gene only, Aa, for
example. The hybrid at two loci (AaBb) is called a dihybrid, andsoon.
Monohybrid Cross. A cross between two individuals identically heterozygous at one
locus, Aa × Aa, for example.
Monomer. A polypeptide.
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Monophyletic Evolution. Evolution of individuals from a single interbreeding
population.
Monoploid. An organism having a single complement of a basic chromosome set of the
species; an individual having a single genome of the species. For instance, monoploid
of bread wheat may have either A, B, or D genome. It (monoploid) may arise
spontaneously in nature. The surviving monoploids look lean and thin. Since
chromosomes do not have pairing-partners, random segregation occurs during
meiosis. Gametes are nearly always deficient in one or more chromosomes, and thus
monoploids are usually sterile. For instance, a monoploid of barley has seven
chromosomes. A functional gamete may be formed if it has all the seven
chromosomes. However, the probability of going all the seven chromosomes to one of
the poles during meiosis I is (½) x-1 = (½) 7-1 = (½) 6 . This amounts to the probability
of a functional gamete. Thus the probability that a monoploid of barley sets one seed
is (½) 6 ×(½) 6 =(½) 12 . This explains why a monoploid plant is almost sterile.
Monosome. A chromosome with no homolog to pair with; a single chromosome in an
otherwise diploid individual (2n-1).
Monosomic. An individual lacking in one chromosome of the diploid complement,
henece, having 2n-1 chromosomes. The missing chromosome is called a monosome.
In monosomics, the unpaired chromosome passes at random to either pole during
meiosis; however, it frequently lags at anaphase, and is not included in either daughter
nucleus. For this reason, gametes with n-1 chromosomes are frequent compared to
those with n chromosomes. However, this bias is not reflected strikingly in the
zygotic chromosome numbers because gametes with n-1 chromosomes often do not
function (especially gametes). Furthermore, zygotes with 2n-2 chromosomes are
inviable except in a few polyploid species. Thus most of the progeny of monosomics
are either normal diploids (2n) or monosomics (2n-1).
Morph. Any one of the genetic forms that account for polymorphism.
Morphogenesis. A developmental process that leads to changes in gross form, cellular
fine structure or both; processes giving final shape to an adult organism by
establishment of specific pattern of tissues and organs.
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Mosaic. Also called a chimera (more frequently in plant species). A tissue containing two
or more genetically distinct cell types or an individual composed of such tissues.
Moving Means Method (Knott 1972, Townley-Smith and Hurd 1973). A method of
testing the genetic potential of early generation lines (F 2 /F 3 /F 4 ) in unreplicated plots.
It involves taking mean of a number of adjacent plots (excluding the plot in question)
as an environmental index. The performance of a test genotype is expressed as the
difference between its own value and the moving mean, or as the percentage of
moving mean.
12.00 13.00 15.00
18.00 20.00 17.00
14.00 19.00 15.00
In the above example, the value of moving mean is
{(12+13+15+18+17+14+19+15)/8} = 15.38. Thus, performance of the test genotype
(middle plot) = (20.00/15.38) × 100 130%. Its control over experimental error is
comparable to the use of intermittent check plots; however, it has an added advantage
since no plots are wasted for accommodating the check entries.
mRNA. An RNA molecule transcribed from the DNA of a gene under the influence of an
enzyme transcriptase. The genetic message is translated into a polypeptide by the
action of ribosomes. During the process, only the genetic message flows from DNA to
RNA (there is no physical transfer of any material).
Multiline (Jensen 1952). A blend of compatible lines, each selected for similarity of
height, maturity, and other agronomic or horticultural characteristics, but carrying a
different gene(s) for resistance. However, Norman Borlaug (1954) has suggested a
similar but somewhat more sophisticated approach based on lines developed by
standard back cross breeding. In the present context, a multi-line cultivar is a mixture
of isogenic lines. In this approach, genes for specific resistance (that reduce the initial
amount of inoculum) are considered in the component lines. However, in the
aftermath, the overall result appears as horizontal resistance, since the rate of
pathogen increase is also considerably reduced. Browning and Frey (1969) have
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called it synthetic horizontal resistance or population resistance. It is only a concept;
now-a-days, it is not used practically.
Multilineal Hybrids (Borlaug 1965). A mixture of several modified single crosses that
differ in relation to a single gene for resistance to a specific race of the pathogen. In
this approach, the advantages of multilines and of heterosis are exploited
simultaneously. At first, two parents A and B are chosen to produce a hybrid. One
resistant gene along with cytoplasmic male sterility is incorporated in parent A. Other
resistant genes are separately incorporated in the parent A to produce component
lines. The last resistant gene along with the fertility restorer one is combined in the
parent B. Other steps are as follows:
A B Component of multilineal
hybrids
↓
[{A 1 (S) × A 2 } × B 8 (F)] 1
[{A 1 (S) × A 3 } × B 8 (F)] 2
[{A 1 (S) × A 4 } × B 8 (F)] 3
[{A 1 (S) × A 5 } × B 8 (F)] 4
[{A 1 (S) × A 6 } × B 8 (F)] 5
[{A 1 (S) × A 7 } × B 8 (F)] 6
_____________________________________________________________________
Seeds of above six hybrids are mixed to form a multilineal hybrid variety. Note that subscript number
indicates the presence of particular resistance genes. The letter S and F in the parenthesis represents
sterility and fertility conditions, respectively of the lines concerned.
Multiline Variety. A variety developed through a composite of isolines. The term
“multilines” sometimes is applied to mixtures of genetically diverse lines produced in
various ways to buffer against environmental stresses. More accurately, these
populations should be called composites instead. Such a variety is now rare in
cultivation.
Multimer. A protein consisting of two or more subunits (monomers or polypeptides).
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Multimeric Structure. A structure composed of several identical or different subunits
held together by weak bonds.
Multiple Allele. A member of a series of several known allelic forms of a gene. The
existence of such a situation is called multiple allelism. The presence of multiple
alleles has been observed for the self-incompatibility system in the Nicotiana spp.
Please notice that the presence of all the allelic forms of a gene in any natural
population is a rare event. As the no. of allelic forms increase, the possible kinds of
gametes (and homozygotes) and total no. of genotypes increase exponentially. For
example, for a single gene pair with four alleles, the kinds of gametes (homozygotes)
and total no. of genotypes will be 4 1 (= 4) and (4 ×5)/2 (= 10), respectively. For two
genes with same no. of alleles at the two loci, the values will be 42 (= 16) and
(16×17)/2 (=136), respectively.
Multiple Correlation Coefficient. A statistic measuring the joint association of all the
independent variables with the dependent one. It thus tells how much of the variation
in the dependent (Y) could be accounted for by reference to these independent ones.
Numerically, it is the square root of the ratio of the regression sum of squares to the
total sum of squares, and is denoted by R.
Multiple Cross. A cross involving many parents. Theoretically its objective is to obtain
recombination from many a parent; the practical utility of multiple cross, however, is
limited in plant breeding for two reasons: (a) the size of the population increases
exponentially with increasing no. of parents to ensure occurrence of the desired
genotype in the segregating generation, and (b) it is often difficult to find many well
adapted parents having different useful genes. (Also called convergent cross).
Multiple-Factor Hypothesis (Nilsson-Ehle 1908). A hypothesis to explain quantitative
variation. Many genes, each with a small but equal effect, segregate to produce a
continuous variation in a quantitative character. Conclusive evidences for this
hypothesis proved beyond doubt that all genetic traits (whether qualitative or
quantitative) follow Mendel’s laws of heredity.
Multiple Resistance. A strategy in resistance breeding that involves placement of two,
three, or more new and still effective resistance genes into a new cultivar to impose
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barriers of several resistance to the pathogen population simultaneously. It should be
an effective strategy because a new race, to overcome multiple resistance genes, must
have two or more simultaneous changes towards the virulence, whereas a new race
needs only one change towards virulence to overcome a single gene for resistance. It
can be effective if the breeding is coordinated centrally, and the production area is
isolated from other areas where the system is not applied.
Multivalent. A pairing configuration of three or more completely or partially
homologous chromosomes observed during meiosis-I. Multivalent formation is a
characteristic feature of autopolyploids.
Mutagen. An agent (physical, chemical or even biological) that is capable of increasing
the mutation rate.
Mutagen Effectiveness. The number of induced mutations per unit dose of a mutagen.
Mutagen Efficiency. A ratio of specific desirable mutagenic change(s) to the undesired
effects such as plant damage, sterility, lethality, and the like.
Mutant. An organism or cell carrying a detectable mutation. The term is an adjective;
thus it must precede a noun (a mutant individual, for example).
Mutant Allele. An allele differing from the allele found in the standard or wild type.
Mutant Hunt. The process of collecting different mutants showing abnormalities in a
certain structure or in a certain function, as a preparation for mutational dissection of
that function.
Mutant Site. The damaged or altered area within a mutated gene.
Mutagenesis. Treatment of plants or plant parts with a mutagen to increase mutation
rates.
Mutation Breeding. A system of breeding in which plants/plant parts/seeds or any kinds
of propagules are treated with any kind of mutagen accompanied by selection for
desired types in succeeding generations.
Mutational Equilibrium. The product of frequency of mutations per individual per
generation and average number of generations during which mutated gene is
maintained in the population before it is eliminated.
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Mutation Pressure. The continued recurrent mutation tending to increase the frequency
of the mutated allele in the gene pool of a population.
Mutation. In broadest sense, any heritable variation in a gene or in chromosome structure
and/or number. However, the use of the term has now been restricted to include only
gene mutation, which refers to a process by which new allele(s) is produced through
heritable structural changes in the gene(s). It may be the process or even the result of
the process. It may occur in nature spontaneously, but the frequency is very low
(about one per million). The rate of mutation can be increased in breeders’
populations through chemical or physical agents called mutagens. The molecular
basis of mutation relates to mispairing between nucleotides of the DNA molecule
irrespective of whether the mutation is natural or artificial. In populations of a species,
it the ultimate source of all variation. If an allele A mutates to a (ignoring back
mutation) with the mutation rate for a given number of generations n, then
frequency of the allele A after n th generation, p n =p 0 e -n (whereas, eisbaseofthe
natural logarithms). Thus the frequency of the A allele decreases with time but the
rate will be very slow. Therefore, the process of mutation cannot drive the process of
evolution unless supplemented with recombination or migration. The statement that a
mutation, if not favoured, is lost from a population is not true absolutely. The
probability that it could be lost is (2N-1)/2N (N = size of the population); if not, then
the probability that it is fixed is 1/2N. Thus a mutation can become established in a
population even though it is not favoured by natural selection simply by a process of
random genetic drift.
Mutation Event. The actual occurrence of a mutation in time and space.
Mutation Frequency. The proportion of mutants in a population.
Mutation Rate. The number of mutation events per gene copy in a population per unit of
time (for instance, per cell generation).
Mutational Dissection. The study of the components of a biological function through a
study of mutations affecting that function.
Mutator Gene. Any gene that increases mutation rate of other genes.
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Mutein. A protein with one or more mutationally altered amino acids in the
polypeptide(s). it is analogous to normal or wild type protein. It may or may not lack
enzymological, immunological, or physio-chemical activity.
Muton. The smallest part of a gene that is involved in a mutation event. It is now known
to be a nucleotide pair.
Mycoplasma. Small bacteria that produce infectitious diseases in plants and animals.
Among living organisms, they have the smallest mass. Mycoplasma can be cultured in
vitro like any bacteria. Mycoplasma genitalium with a genome size of 580 kb DNA is
the first complete life.
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Nanotechnology. A technology to direct the assembly of highly structured materials with
specific nanoscale features (as in DNA) for material purposes. It presents a new
chapter in the history of the molecule.
Narrow Genetic Base. Having little genetic variation as in an inbred line.
National Agricultural Policy (2001). A policy related to rejuvenation of Indian
agricultural economy. The document is a result of a series of consultative processes at
various levels that deal with technology, institutions and policy. For accelerating
growth in agricultural sector, three components such as technology, policy and
institutions have been visualised indispensable.
National Agricultural Technology Project (1998). A project started jointly by the ICAR
and DAC to promote integration of research, technology generation, assessment and
refinement, and dissemination in area-specific and production system mode. It was
visualised as an important component of the overall strategy for combining food
security, poverty alleviation, rural development and conservation of natural resources.
Natural Crossing. Spontaneous crossing between genetically different plants/
lines/strains being grown in proximity to each other. The extent of natural crossing
varies with time of flowering, wind direction, temperature, and interaction of these
factors among themselves and with the particular genotypes of the plants.
Naturalist. A student of field biology, in contrast to a laboratory worker.
Negative Assortative Mating. Preferential mating between unlike parents or partners. If
the parents are genetically contrasting, it is called negative genetic assortative mating
(AA × aa). If unlike parents are chosen on the basis of phenotypic appearance, it is
referred to as negative phenotypic assortative mating (tall × dwarf). Negative
assortative mating is also called disassortative mating.
Negative Control. Gene regulation mediated by factors that block or turn off
transcription. Repressor blocks the expression of structural gene(s).
Neo-Darwinian Evolution. A modified Darwinian paradigm developed during 1930s
and 1940s that incorporated concepts from modern population, ecological and
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evolutionary genetics into descent with modification; often referred to as the Modern
Evolutionary Synthesis.
Neolithic Age. Cultural period beginning ~12,000 years ago; appearance of technically
advanced stone tools.
Neurospora. A pink mould that is commonly found growing on old food.
Neutral Mutation. A mutation that has no effect on the biological fitness of its carriers.
It is also defined as a kind of mutation that has no phenotypic effect. It may be so
because the altered codon specifies functionally equivalent amino acid in the
polypeptide [e.g., AAA (lysine) to AGA (arginine); both are basic amino acids].
Neutral Polymorphism. A kind of polymorphism which is sustained owing to neutral
effects of alleles on survival and reproduction of genotypes.
NHI. Nitrogen Harvest Index. It refers to partitioning of nitrogen between grain and the
remaining plant-parts. In semi-dwarf wheat, the new varieties have a greatly improved
NHI.
Niche. A group of multidimensional environmental factors of an ecosystem to which a
population fits. According to Mayr (1963), it deals with the outward projection of
needs and the specific way of utilizing the environmental factors by the individuals of
the population.
Nicking. Nuclease action to sever the sugar-phosphate backbone in one DNA strand at
one specific site.
nif Genes. A cluster of genes orchestrated to produce the enzyme nitrogenase. More than
22 such genes are known. Out of these, only three (H, D, and K) are structural; some
are regulatory (A, L, etc.), and the rest may assist in making the expressed product
fully functional.
Nitrocellulose Filter. A type of filter used to hold DNA for hybridisation.
Nitrogen bases. Types of molecules that form important parts of nucleic acids. These are
composed of nitrogen containing ring structures. The two strands of a DNA double
are linked with each other through hydrogen bond established between
complementary bases.
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Nitrogenase. An enzyme that reduces nitrogen into ammonia. It has two sub-units: MoFe
bigger sub-unit (synthesized by nif K&D), and Fe smaller sub-unit (synthesized by nif
H).
No Dominance. A condition in which the heterozygote falls exactly the mid-way
between the two homozygotes [for example, AA (3), Aa (2.5), and aa (2)].
No Selection. Relaxation in selection pressure such that no individuals are rejected, that
is, 100 per cent individuals are carried forward merely to maintain the population
(selection intensity is lowest, i.e., 1.0 or 100%).
Nobilisation. A term coined by Dutch breeders in Java for the improvement of sugar
cane. It refers to crossing and recrossing schedule to effect improvement of hardy and
disease resistant but inferior wild canes (S. spontaneum) by crossing it with more
attractive, sweeter noble canes (S. officinarum) as the female one. However, the term
has been extended to include crossing of any wild species of Saccharum or its related
genera to S. officinarum. When S. officinarum is used as the female, it uniquely
transfers 2n gametes to its progeny (in case of backcrosses, up to 2 nd backcrossing);
however, in reciprocal crosses, only n+n transmission has been observed.
Noble canes. The canes belonging to the species Saccharum officinarum. Canes are socalled
because they are tall, handsome, large barrelled shaped and colourful.
Nodule. The rounded irregular mass of cells that appear as a small knot. It is a nitrogen
fixing machinery on the roots of legumes. Root nodules are most common; however,
stem nodules (Sesbania rostrata) and leaf nodules (Psychotria sp.) have also been
reported.
Nonadditive Variance. The variance caused by non-additive gene action. It includes
dominance, and dominance × dominance type of gene interaction.
Nondisjunction. The failure of homologs (at meiosis) or sister chromatids (at mitosis) to
separate from each other so that the two go to the same pole at anaphase.
Non-linear Tetrad. A tetrad in which the products of meiosis are arranged in no
particular order.
Non-Mendelian Inheritance. Extrachromosomal inheritance.
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Non-Mendelian Ratio. An unusual ratio of progeny phenotypes that does not conform to
the simple operation of Mendel’s laws of inheritance. For example, mutant: wild
ratios of 3: 5, 5: 3, 6: 2 or 2: 6 in tetrads indicate that gene conversion has occurred.
Non-Preference. An insect response to characteristics of plants that make them
undesirable for use by insects as a site for reproduction, food, shelter, or any
combination of these three. Two types of non-preferences have been reported : (a) one
that is manifested only in the presence of the preferred host, and (b) the second even
in the absence of the preferred host. Such a mechanism is strong enough to force the
insects to starve, particularly in early instar stages. Therefore, the extreme nonpreference
may be mistaken for antibiosis and vice-versa. The plant characters that
influence non-preferences include colour, light reflection, type of pubescence, leaf
angle, odour, taste, etc. It has been observed that nectariless cultivars of cotton are
resistant to cotton ball worm. Similarly, yellow-green varieties of peas are less
desirable to the pea aphid than are blue-green cultivars. Solid stem has been
associated with resistance to stem saw fly and stem borer in wheat and maize,
respectively. However, solid stem is often associated with low yielding capacity, and
precisely for this reason, resistant cultivars are not grown where these insects are
problems in the respective crops. It seems likely that non-preference is polygenically
determined.
Non-Random Mating. Deviations from random mating based either on mating of like to
like or on mating of unlikes. Both assortative and disassortative mating are nonrandom
mating systems.
Nonsense Codon. A codon for which no formal tRNA molecule exists, and the presence
of which causes termination of translation of genetic message in the form of ending of
the polypeptide chain. Three such codons are UAG, UGA and UAA. Though these
bear the sense of termination of polypeptide chain, this misnomer still exists. (The use
of the term such as stop codon instead of calling it nonsense one seems to be the most
appropriate).
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Nonsense Mutation. A mutation that alters a gene in such a way that it produces a
nonsense codon in place of a “sense” one, for example, UGG (tryptophan) to UGA
(stop codon).
Nonsense Suppressor. A mutation producing an altered tRNA so that an amino acid is
inserted in translation in response to a stop codon.
Nonsister Chromatids. Chromatids derived from separate homologous chromosomes or
from non-homologous chromosomes of the chromosome complement. See sister
chromatids.
Norin 10. A Japanese semi dwarf variety of bread wheat discovered in 1946. It carried
two recessive genes for dwarfness, designated as Rht 1 and Rht 2 on 4A and 4D
chromosomes, respectively. These genes facilitate greater mobilization of dry matter
towards economic yield. They also do not produce negative correlation between
biological yield and harvest index. The exploitation of these genes has provided
correlated response to a host of characters related to yield.
Norm of Reaction. The pattern of phenotypes produced by a given genotype under
different environmental conditions.
North Carolina Designs (Comstock and Robinson 1948,’52). Mating designs developed
to overcome problems of limited progeny relationships of biparental mating design,
and to provide estimates for both additive and dominance variances. In all three such
designs (NCD I, II, and III), both full and half-sib progenies are produced by
attempting biparental matings in F 2 generation of a cross between two pure lines.
Both NCD I and NCD II are influenced by the presence of maternal effects, and
depend on the assumption of gene frequency. NCD III is the most informative design
as it also reflects some light about the effect of linkage on the average degree of
dominance. It is also economical, as it requires much less data.
Northern Blot. Transfer of electrophoretically separated RNA molecules from a gel onto
an absorbent sheet, which is then emersed in a labelled probe that will bind to the
RNA of interest.
Nucellar Embryony. A form of apomixis in which the embryo arises directly from the
nucellus.
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Nucellus. A parenchymatous tissue that extends from the chalaza at the base to the
micropyle at the apex of the embryo sac.
Nuclear Division. The division of the cell nucleus that may follow different patterns such
as amitosis, mitosis, or meiosis.
Nuclear Instability. Any deviation from normal nuclear behaviour producing a nucleus
of abnormal structure, karyotype or behaviour. It may be attributed to higher DNA
content and large telomeric segments of heterochromatin. It affects seed fertility and
endosperm development. It may lead to bridge formation at anaphase and shrivelled
grains.
Nuclear Membrane. The outer boundary of the nucleus. Also called nuclear envelope.
Nuclear Segregation. The segregation of genetically different nuclei contained in a
heterokaryotic multikaryon. It is one of the important sources of variation in asexual
pathogenic fungi.
Nuclease. An enzyme that can degrade DNA/RNA by breaking its phosphodiester bonds.
Nucleases may attack a polynucleotide either from an end (exonucleases), or at
random (endonucleases).
Nucleic Acid. A polymer of nucleotides (deoxyribonucleotides in the DNA or
ribonucleotides in the RNA). Nucleic acid (DNA, and in some cases RNA) acts as the
genetic material in living organisms. The presence of one free acid group in each
phosphoric acid molecule confers on the polynucleotide its acid properties and
enables the molecule to form ionic bonds with basic proteins (histones). The free acid
group also causes nucleic acids to be highly basophilic.
Nucleoid. A DNA mass within a chloroplast, mitochondrion, bacterium or a prokaryotic
cell. It has variable shape but uniform texture. It lacks a membrane (nuclear envelope)
and does not undergo processes comparable to mitosis and meiosis of eukaryotes.
Nucleolar Organiser. A region of the chromosome physically associated with the
nucleolus and containing rRNA genes.
Nucleolus. An organelle found in the nucleus, containing rRNA and amplified multiple
copies of the genes encoding rRNA.
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Nucleoside. A nitrogen base bound to a sugar molecule. A phosphorylated nucleoside is
called nucleotide.
Nucleosome. The basic unit of eukaryotic chromosome structure. It is a “ball” of eight
histone molecules wrapped about by two coils of DNA. It is an octamer of two units
each of histones H2A, H2B, H3 and H4; the DNA is wrapped twice around the
octamer (of four histones). The additional histone, H1, (fifth one) is running down the
centre of the coil (of DNA), probably acting as a stabiliser. It is worth-mentioning that
H1 is not the part of the nucleosome.
Nucleotide Pair. A pair of nucleotides (one in each strand of DNA) that are joined by
hydrogen bonds.
Nucleotide. A molecule composed of a nitrogen base, a sugar and a phosphate group; a
phosphorylated nucleoside. It is the basic building block of nucleic acids.
Null Allele. An allele whose effect is either an absence of normal gene product at the
molecular level or an absence of normal function at the phenotypic level.
Null Hypothesis. A hypothesis that there is no discrepancy between observation and
expectation based on some set of postulates.
Null Mutation. A mutation that results in complete absence of function for the gene.
Nulliplex. The condition in which a polyploid is recessive in all chromosomes with
respect to a particular gene (e.g., aaaa is called a nulliplex tetraploid). Simplex
denotes recessiveness at all loci except one (Aaaa), duplex two (AAaa), triplex three
(AAAa), quadriplex four (AAAA), etc.
Nullisomic. An otherwise 2n plant that lacks both members of one specific pair of
chromosomes, hence, with 2n–2 chromosomes. The missing ones are called
nullisomes.
Nullisomy. A condition of having individuals with 2n-2 chromosomes. Such individuals
are referred to as nullisomics. Nullisomy is normally a lethal condition except in a few
polyploid species. In crops like modern wheat, nullisomics produce gametes with n-1
chromosomes. Since male gametes with n-1 chromosomes frequently tend to be nonfunctional,
the degree of sterility associated with nullisomics is very high.
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Nutritional Value. The value of a food as judged by its taste, calorific value, quantity
and quality of proteins, presence of vitamins and minerals, absence of antinutritional
factors, etc.
Nutritious Diet. A diet that promotes health and prevents disease through supply of all
the essential nutrients, minerals and vitamins. Variations in nutritive content of food
depends on soil nutrients and cultivars.
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Ochre Codon. One of the stop codons, UAA.
Ocimene. A volatile terpene found in certain legumes. Its amount varies among clones,
and thus ocimene content may be altered by breeding, suggesting that clones with
improved attractiveness for honeybee visitation could be developed.
Octad. A group of eight ascospores contained in an ascus. It is produced in species in
which the tetrad normally undergoes a post-meiotic mitotic division.
ODV. Other Distinguishable Varieties; seeds of other varieties with distinct appearance
present in a seed lot of designated variety.
OECD. Organization for Economic Cooperation and Development. OECD was
developed after the Second World War in Europe. The scheme developed for uniform
technology and standard methods and practice for seed production and certification
for forage moving in international trade was taken over by OECD in 1962. Since then,
it adopted and implemented schemes for certification of different kinds of seeds
including those of cereals, sugar beet, forestry, and the like. The basic objective of
OECD seed scheme is to encourage use of seed of high quality in the participating
countries. The scheme authorizes use of labels and certificates for seed produced and
processed for international trade according to the seed principles. The scheme is open
on voluntary basis to members of organization as well as to other member countries
of United Nations. If a country adopts OECD seed schemes, it is obliged to ensure
that the rules of the scheme are strictly observed.
Off - Season Nursery. A place wherein a breeding strain is grown in a season not suited
to its normal rearing. The off-season planting serves the purpose of advancing
generations, e.g., growing of off-season wheat and chickpea at Wellington (Tamil
Nadu) and Dharwad (Karnataka), respectively.
Okasaki Fragment. A small segment of single-stranded DNA synthesized as a part of
the lagging strand in DNA replication. The strand of the original DNA being used as
the template for lagging strand is unfolded in 3’ to 5’ direction; however, replication
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always proceeds in the opposite direction (5’ to 3’ direction). This is why lagging
strand is synthesized in fragments (while the leading strand continuously).
Oligogene. Major gene.
Oligogenic. Of characters controlled by one or a few genes.
Oligonucleotide. A short segment of synthetic DNA.
Oncogene. The cancer-causing gene; a gene whose expression leads to the production of
a cancer. Oncogenes are usually mutated forms of normal cellular genes.
OneGene–OneEnzymeHypothesis(Beadle and Tatum 1941). A hypothesis stating
that each gene controls the synthesis or the activity of only a single protein with
catalytic activity (enzyme). With the developments in biochemical genetics, the
principle has been verified and modified. The modified version is one cistron – one
polypeptide.
Ontogeny. The course of development of an individual organism, organ, or an organelle.
On-Types. Plants that conform to the standard type in question with respect to a
characteristic or group of characters. For example, several on-type progenies are
bulked to form the pure-seed lot during purification of a variety through mass
selection.
Opal Codon. One of three stop codons, UGA.
Open-Pollinated Corn. Corn propagated from seed produced by uncontrolled
pollination.
Open-Pollinated Progeny Test. A test for GCA by evaluating progenies derived from
seed produced on selected plants outcrossed with other plants of the same population.
Usually used in forage species.
Operator. A DNA region at one end of an operon that acts as the binding site for
repressor protein.
Operon. A set of adjacent structural genes (whose mRNA is synthesized in one unit) and
the adjacent regulatory signals that affect transcription of the structural genes.
Oppositional Factor Hypothesis (East and Mangelsdorf, 1925). A hypothesis put
forward to account for gametophytic incompatibility in Nicotiana spp. It states that
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the incompatible allele in the style opposes the penetration of pollen tubes with the
same allele.
Order Effects. The effects associated with the sequence in which parents are crossed to
produce three-way (triallel) or double cross (quadriallel) hybrids. A critical analysis of
the combining ability effects of higher order provides evidence for the relative
significance of the order in which the parents have been arranged to produce double
cross hybrids. It results as a consequence of epistasis. For instance, if two inbred lines,
A and B, are derived from one source and lines C and D from another, the highestyielding
double cross is likely to be obtained from pairings of the type (A × B) ×(C ×
D).
Order. A systematic category that follows the class but precedes family.
ORF. Open Reading Frame. A section of sequenced piece of DNA that begins with a
start codon (AUG) and ends with a stop codon (any one of UAG, UGA and UAA). It is
presumed to be the coding sequence of a gene.
Organelle. A subcellular structure of characteristic size, shape, and specialized function,
for example, mitochondria, chloroplast, endoplasmic reticulum or nucleolus.
Organism. The unit of a continuous lineage with an individual evolutionary history.
Organogenesis. The process of the production of organ systems in animal
embryogenesis.
Osmoregulation. The adjustment of osmotic potential. Under water deficit condition,
there is reduction in the osmotic potential of crop plants. This reduction may stem
from either a net increase in the cell solute concentration or the loss of water from the
cell. Thus a genotype having capacity for osmoregulation can maintain turgor pressure
which is necessary for normal cell function and growth. Low osmotic potential can
also result in lowering of the level of leaf water potential, which triggers stomatal
closure in response to water deficit. Osmotic adjustment during seed-filling period
could help maintain photosynthesis under water stress, resulting in improved drought
resistance and yield. Considerable genotypic differences for osmotic adjustment have
been found in sorghum and other crops.
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Outbreeding. Mating between relatives occurring less commonly than would occur by
chance. Said in another way, it is a mating system in which mating is between
individuals less closely related than individuals mating at random. The overall
consequence of outbreeding is to prevent differentiation of populations. Also called
negative inbreeding.
Outcross. A cross, usually natural, to a plant of different genotype.
Overcompensatory Interaction. An interaction between two or more genotypes such
that they cooperate each other to mutual advantage. Cooperating genotypes have a
history of mutual selection. This kind of interaction is expected to approach towards
stable equilibrium. This system is something akin to a system involving heterozygote
advantage over homozygotes, and properties similar to that of favourable epistatic
interactions between or among different loci.
Overdominance Hypothesis. An assumption based on the superiority of heterozygote
compared to homozygous parents. The superiority increases in proportion to the
amount of heterozygosity. The implication is that the two different alleles in the
heterozygote (A 1 A 2 ) perform different functions, and that the sum of their different
products is superior to the single product produced by either allele in homozygous
state (A 1 A 1 / A 2 A 2 ). There are in fact at least some evidences at which the
heterozygote (for blood group) is indeed superior to homozygotes. However, for
metric traits, what is observed as overdominance is not the real one because the level
of dominance is overestimated due to repulsion phase linkages.
Overdominance. Dominance such that the heterozygote falls outside the range of a
parent with greater phenotypic value, AB blood group compared to AA and BB groups
in humans, for example. True overdominance (single locus heterozygote advantage
over homozygotes) is almost certainly rare; however, dominance and/ or epistasis
together with linkage are capable of simulating overdominance.
Oxidative Phosphorylation. ATP synthesis by phosphorylation of ADP using the energy
provided by electron transfer during aerobic respiration.
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P. A shorthand to designate parental generations in crosses.
P 1 , P 2 , P 3 ,……. First, second, third, etc. generations from a parent; also used to designate
different parents used in making a hybrid or series of hybrids. P 1 also refers to the
immediate parents of a first filial generation or F 1 .
Pachytene. The four-strand stage of meiosis during which crossing over occurs between
non-sister chromatids of a pair of homologous chromosomes.
Paleolithic Age. Cultural period beginning with the earliest chipped stone tools
(~750,000 years ago) until the beginning of the Mesolithic Age, ca. 15,000 years ago.
Palindrome.
Pangenesis Theory (Darwin, 1809-1882). An obsolete theory of evolution. According to
it, pangenes (gemmules), which are very small, exact, but invisible copy of each body
organ and component, assemble in gametes and upon fertilization, they separate out to
different parts of the body during development to constitute a mixture of maternal and
paternal organs and tissues. Although a disproved idea, however, it suggested a
physical basis of heredity.
Panmictic Population. A random mating population.
Panmictic Index. A measure of the relative amount of hetrozygosity that is reduced by
inbreeding in a random mating population of limited size. It is a complementary
parameter to inbreeding index or coefficient. Mathematically, P = 1 – F; the initial
value of this parameter in a large population is taken as unity as F = 0 in such a case.
The loss of heterozygosity in a small sized population is equal to the factor 1/2N
(where N = population size = number of diploid individuals).
Panmixia. Random mating without restriction (usually extended to include random
mating under the restrictions of sex or incompatibility).
Paracentric Inversion. An inversion that excludes the centromere of the chromosome. It
does not change the morphology of the chromosome. It is very difficult to detect, as it
does not change the arm ratio. There is selective recovery of parental type gametes in
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an inversion heterozygote for crossover types are always deletion products for one or
afewgenes.
Paragenetic. Of chromosomal changes that affect the expression but not the constitution
of the genes concerned (position effect, for example). The term genetic is concerned
with the constitution of any one of genes concerned).
Paralogous Genes. Two genes within a species that have evolved by gene duplication.
Paramutation. An epigenetic phenomenon in plants wherein genetic activity of a normal
allele (paramutable) is heritably changed/ reduced in heterozygous condition by a
special “paramutagenic” allele. It was first noted in maize. The paramutational
alterations persist, but show a tendency towards reversion to the standard phenotype
over a number of generations. This may occur due to mutation in the paramutagenic
allele.
Parasexual. Of reproductive systems that achieve genetic recombination by any
mechanism other than sexual one (meiosis). Parasexual cycle is observed in lower
plants such as fungi.
Parasite. An organism that lives upon or within another living organism at whose
expense it obtains some advantage without any compensation to the host.
Parasitic Epistasis (Sidhu, 1984). The effect of one parasite (pathogen) on the
pathogenicity of another. For instance, Sidhu and Webster (1979) have shown that a
non-virulent strain of Fusarium sp. blots out parasitic effect of Verticillium on tomato
plants known to be separately resistant and susceptible to Fusarium and Verticillium,
respectively.
Parasitism. A type of interaction between two species in which one species (the parasite,
which is usually smaller) benefits at the cost of other (the host, usually larger).
Parental Generation. The generation in which plants from two contrasting pure lines
are crossed. It is usually designated by the letter P.
Parental Imprinting. An epigenetic phenomenon in which the activity of a gene is
dependent upon whether it was inherited from the father or the mother. Some genes
are paternally imprinted, others maternally.
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Parthenocarpy. The development of fruits without seeds or without embryo in seeds. It
results from a failure of pollination, a failure in fertilization, or a failure in embryo
development.
Parthenogenesis. Development of an organism from a sex cell, usually a female gamete,
but without fertilization. This occurs especially among some lower plants and
animals.
Parameter. A numerical quantity that specifies a population in respect to some
characteristics. Parameter is to population, what a statistic is to sample.
Partial Correlation Coefficient. A statistic that measures the association of two
variables after making allowance for their association with the third one (as against
total correlation coefficient). PCC between variables 1&2 = r 12.3 =(r 12 –r 13 xr 23 )/[(1-
r 2 13) (1-r 2 23)]½. The variable whose influence is allowed for in the calculation of PCC
is spoken as the eliminated variable.
Partial Diallel (Kempthorne and Curnow 1961). A set of crosses resulting from crossing
a given number of parents in only restricted number of combinations (not in all
possible combinations). In a normal diallel, each line is involved in n-1 crosses. But
in partial diallel, only a random sample of crosses ‘s’ is made (where, s < n-1).
However, either s or n should be odd from analysis point of view. Thus with n lines,
the total number of crosses to be analysed in a partial diallel is ns/2. Partial diallel
analysis provides information on both general and specific combining ability;
however, the precision of estimates depends on the sample size s vis-à-vis n (total no.
of lines). Various workers have clearly shown that bias is more common when s <
(n/2).
Partial Dominance. A condition in which the heterozygote deviates from the midparental
value towards one of the homozygotes [for example, AA (3), Aa (2.75), and
aa (2)]; also called incomplete dominance.
Particulate Inheritance. An accepted model of inheritance which states that the genetic
information is transmitted from one generation to the next in discrete units (particles)
that maintain their integrity in the resulting zygotes even after fusion of gametes
(without blending or mixing of units).
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Passive Resistance. The resistance mechanisms which may already be present in the host
before the attack by any kind of pathogens. For instance, resistance to a fungal
pathogen may be attributable to thick cuticle, thick hairs, wax, etc., which prevent
infection or impede sporulation of the fungus, or to a preformed antibiotic compound
(gossypol in adult cotton plants).
Patent. The legal right, which provides protection to the interests of the experimenter.
Indian patent act was enacted for the first time by the British in 1859, which was later
revised, and modified Indian Patent Act came in force in 1971. It excluded patenting
of inventions like atomic energy, agriculture and horticulture. Under the act, only
process patent was granted for seven years. However, in 1994, the patent act was
again modified keeping in view of GATT agreement. It included agriculture and
horticulture while safeguarding the interests of experimenters and farmers. But it
permitted patents of products also.
Path Coefficient. The ratio of the standard deviation due to a given cause (on the effect)
to the total standard deviation of the effect. For example, if y is the effect and x 1 is one
of the causes, the path coefficient for the path from the cause x 1 to the effect y will be
(x 1 y). This path coefficient due to a particular cause is referred to as the direct
effect of that cause on yield (effect).
Path Coefficient Analysis. An analysis of direct and indirect effects of various causal
factors on the dependent variable through diagrammatic representation. Path
coefficient analysis is simply a standardized partial regression coefficient that breaks
the correlation coefficient into measures of direct and indirect effects. The effect of a
causal factor (e.g., seed weight, x 1 ) on the dependent variable (e.g., yield) via the
second causal factor (x 2 ) is called indirect effect of that factor. Thus indirect effect of
x 1 on y through x 2 will be the product of path coefficient of (x 2 )andr x1.x2 . Path
coefficient analysis also includes effect of undefined variables (not included in the
study) on the dependent variable (yield, y); it is called residual effect, which can be
calculated by the formula: R = {1-Pi.
Pathogen. A parasite that causes a disease in the host plants; an organism causing
disease in another organism.
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Pathogenesis. The chain of events that lead to the development of a disease in the host.
For example, at first a pathogen (bacterium, for example) establishes contact with the
host. Thereafter, it multiplies and grows into a mass at the cost of nutrients and other
food materials of the host. These proliferating mass also block transporting tissues of
the host and thence symptoms of disease appear on the plants. This sequence of
events, that is, from infection to appearance of disease symptoms, is called
pathogenesis.
Pathogenicity. The ability of a pathogen to cause a disease.
Patroclinous Inheritance. A type of inheritance in which all offspring have the nucleusbased
phenotype of the father.
Pattern of Evolution. Different designs or models of evolution of plant species. These
different models are not mutually exclusive, that is, two or more methods may be
interwoven in a slow and gradual change eventually leading to formation of a new
species.
PCR. Polymerase Chain Reaction. A technique to amplify a sample of DNA aided by
primers and Taq polymerase (obtained from a bacterium called Thermus aquaticus)
through regulating temperature changes. Entire exercise is done in a machine called
PCR machine. Taq polymerase can withstand the denaturing temperatures. More than
one million and one billion copies of a given DNA sample can be produced in 20 and
30 cycles, respectively. Some PCR machines can process 96 samples at a time. PCR
has been used to create DNA fingerprints by amplifying microsatellite DNA. PCR
amplification can be done without restriction cutting. It gives the results directly upon
electrophoresis. It is now a routine tool in the laboratory of molecular geneticists who
use it to amplify rapidly DNA regions of interest for research purposes.
PCR Cycle. The Photosynthetic Carbon-Reduction Cycle; also called Calvin cycle. In
this cycle, the fixation of CO 2 in form of hexose (carbohydrate) occurs in the stroma
of chloroplasts with the aid of several enzymes and energy provided by ATP and
NADPH (produced during light reaction).
PDR. Pathogen derived resistance in plants.
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Pedigree Breeding. A system of breeding in which individual plants are selected in the
segregating generations from a cross on the basis of their desirability judged
individually and on the basis of a pedigree record. It has been a widely used method in
breeding self-pollinated crops.
Pedigree. A record of ancestry of an individual, family, or a strain. In pedigree method
of handling segregating generations, it makes it possible for the elimination of all
except one member of the closely related families at the final stage. It is useful
primarily in avoiding selection of closely related lines whose probable worth is nearly
identical. However, it does not establish retraceable route that can be followed in
developing the same variety by repeating the same cross.
PEG. Polyethylene glycol. A chemical that acts as fusogenic agent for the fusion of
protoplasts. It is non-specific in that it induces fusion between protoplasts of the same
as well as of different species. The fused product is a somatic hybrid (amphidiploid).
Penetrance. The frequency with which an allele of a single locus or alleles of several loci
are manifested on the phenotype of the carriers. Its value ranges from zero to hundred
per cent. Suppose, a gene is expressed in 10 individuals out of 100. This implies a 10
per cent penetrance of the gene in question. Thus penetrance is also defined as the
proportion of individuals with a particular genotype expressing a phenotype
associated with that specific genotype. Penetrance of a gene depends upon the
position of other genes in the genome, the presence of modifiers and the external
environment. Incomplete penetrance complicates the task of breeders working with
the specific gene.
Peptide. An amino acid.
Peptide Bond. A bond joining two amino acids.
Perfect Population Size. The minimum size of a population that permits expression of
each genotype expected in an F 2 population from a cross differing at one, two, etc.
loci. It is represented by 4 n , where ‘n’ is the number of segregating loci in a hybrid.
According to Comstock and Robinson (1940), the number of individuals in an F 2
population for a monogenic trait should be at least 40. However, according to Allard
(1999), it should be 12.
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Pericentric Inversion. An inversion that involves centromere of the chromosome. It
greatly alters the morphology (and hence, arm ratio) of the chromosome. The changed
arm ratio facilitates its easy detection. Like paracentric inversion, crossover types are
not recovered but for a different reason. Recombinants are simultaneously duplicated
(for one part) and deleted (for the other) products. Thus there is selective recovery of
parental types in an inversion heterozygote.
Permissive Conditions. Those environmental conditions under which a conditional
mutant shows the wild-type phenotype.
Ph.D. Doctor (docere: a Latin word, which means to teach) of philosophy (philo +
sophia; the Greek words, which imply love and knowledge, respectively). Thus it
denotes a person having love for knowledge and teaching.
Phenocopy. An environmentally induced phenotype that resembles the phenotype
produced by a mutation. Of course, this effect is not inherited.
Phenotype. A description of all aspects of individual’s morphology, physiology,
behaviour, and its ecological relationships. The phenotype, which is the observable
properties of an organism, is brought about by its genotype in concert with the
environment in which the organism develops. The term has been derived from a
Greek word that literally means “ the form that is shown”. In practice, however, it is
used in a more restricted sense; that is, we say phenotype with respect to a particular
trait. Contrary to genotype, it is a dynamic entity, which changes with time.
Phenotypic Assortative Mating. Mating between individuals which resemble each
other more closely phenotypically than the rest members of the population. The final
result of phenotypic assortative mating amounts to that for genetic assortative mating
so long as heritability is complete (that is , 100 per cent). Under condition of complete
heritability, the rate of approach towards homozygosity gets slower with the increase
in the number of loci involved. When heritability is not complete, 100 per cent
homozygosity is not possible (‘F’ cannot reach unity).
Phenotypic Disassortative Mating. Mating between individuals having contrasting
phenotypic traits. It tends to maintain (or even slightly increase) heterozygosity,
decrease the population variance, and reduce correlation between relatives. It may be
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used to compensate for defects by choosing contrasting parents each of which offset
weaknesses of the other.
Phenotypic Sex Determination. Sex determination by nongenetic means.
Phenotypic Variance. The total variance observed in a character.
Philadelphia Chromosome. A translocation between the long arms of chromosomes 9
and 22, often found in the white blood cells of patients with chronic myeloid
leukaemia.
Phosphodiester Bond. A bond between a sugar group and a phosphate group. Such
bonds form the sugar-phosphate backbone of DNA.
Phosphorylation. The addition of a phosphate group to ADP for producing ATP.
Photophosphorylation. ATP synthesis by phosphorylation of ADP using the light energy
in the chloroplasts of plants.
Phyletic Evolution. Gradual transformation of one species into another without
branching.
Phylogeny. The evolutionary history of an organism or a taxonomic group.
Physiological Drought. See drought.
Ph Gene (Pairing homoeologous gene). The gene(s) that suppresses homeologous
pairing and thus ensures homologous pairing in bread wheat. It results in diploid-like
genetics for this basically hexaploid species. The gene is situated on the long arm of
5B chromosome. Two such genes are known: Ph 1 and Ph 2 . However, Ph 1 is the
genetic system that determines exclusive homologous pairing in modern wheat.
Ph 1 Suppressor. The species (T. searsii) known to suppress the activity of Ph 1 in hybrids
with bread wheat, resulting in high homoeologous pairing.
ph Mutant (Wall et al. 1971). A mutant that carries a mutation in the Ph gene, and thus
permits homeologous pairing. The level of pairing in this mutant, however, is not as
high as in the nullisomic for 5B. Intercalary deficiency for Ph 1 is designated ph 1 b
mutant, which permits high level of homeologous pairing. The terminal deficiency of
3D is termed ph 2 a and is characterized by an intermediate level of homeologous
pairing. Further, ph1c mutant has been found in durum wheat and is used in
promoting homeologous pairing in intergenomic hybrids.
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Physiological Homeostasis. A type of buffering reaction of the organism against
fluctuating environmental conditions by internal self-regulatory systems that resist
those changes either by varying phenotypically or by remaining constant in the face of
environmental changes. It is frequently associated with heterozygosity of genotypes.
Physiological Races. Pathogens of the same species with similar or identical
morphology but differing pathogenic capabilities.
Physiological Threshold. The limit beyond which physiological functioning are affected
so adversely that individuals can no longer survive and thus selection is powerless to
effect any change even in the presence of substantial genetic variation in the
population.
Phytoalexins. The biochemical substances (phenolic compounds) formed by the host
plant in response to injury, physiological stimuli, infectious agents or their products
that accumulate to levels which inhibit the growth of microorganisms and repel pests
and pathogens, e.g., DIMBOA in maize and onions, pisatin in peas, phaseolins in
beans, rishtin and phytotuberin in potatoes. These biochemical substances confers
resistance on the concerned hosts against respective pests or pathogens.
Phytoplankton. Minute floating plants, usually algae, distributed throughout the pond as
deep as light penetrates.
Phytotoxins. Chemical compounds that kill plants. These compounds are produced by
pathogens. The resistant hosts in such cases are insensitive to toxins. For example, the
causal organism of victoria blight in oat, Drecheslera victoriae, produces a toxin
called ‘victorin’. Resistant varieties of oats are resistant to this toxin.
Phytotron. A controlled environment facility to assist plant scientists in conducting crop
breeding and crop management studies without any constraint of seasons and
locations. This unique facility has been created at IARI, New Delhi in 1997. It has an
area of 2500 m 2 , 22 growth chambers and 10 green houses where temperature,
relative humidity, CO 2 levels, and light can be controlled to produce a wide range of
simulated conditions. Further, by manipulating these components, plants can be raised
up to maturity whenever required. It is particularly suited to rain-fed and other noncongenial
production regimes inhabited mostly by resource-poor farmers. It can
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provide conditions to facilitate flowering of photosensitive and late maturing
genotypes and thus provides rapid generation advancement. Conducive conditions for
screening of genotypes against biotic and abiotic stresses can also be created.
Pima Cotton. Gossypium barbadense; also called Egyptian cotton.
Pinocytosis. The process of active intake of sizable droplets of fluids by the cell.
Plant Breeders’ Right. A system of intellectual property right (IPR) that provide patent
type protection to traditionally bred varieties. It is specifically designed to cope with
the inherent variability of plant materials. It gives to the holder the rights and ability
to recoup his development costs from the market over a period of years.
Plaque. A clear area on a bacterial lawn, left by lysis of the bacteria through progressive
infections by a phage and its descendants.
Plasma Membrane. A unit membrane surrounding the outer layer of the cell’s
cytoplasm. It is selectively permeable.
Plasmid. Autonomously replicating extrachromosomal DNA molecule found in bacteria.
It is not the essential component of bacterial genetic material. Its presence is
correlated with stress condition and confers resistance to certain antibiotics. These
properties of plasmid make it suitable for exploitation as a vehicle in genetic
engineering.
Plasmodesma. Any of the fibrillar plasma connections extending through the cell wall
and forming the bridge between adjacent cells.
Plasmogamy. The fusion of the cytoplasm of two or more cells. It precedes karyogamy in
the process of fertilization.
Plasmotype. The sum total of extrachromosomal hereditary determinants. Plasmotype
and genotype together constitute the idiotype or genetic system of the cell.
Plant Introduction. Bringing of a plant species or a variety from one place/environment
to a new place/environment where it has not been grown previously. When
successful, plant introduction serves the same purpose as developing varieties in a
well-developed breeding programme. In the past, it has played important roles in the
movement of plant species/varieties from one place to the other, for instance, the
introduction of soyabeans in India from China. These introductions are utilised
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principally in three different ways: (a) direct use by increase in masse (e.g., Taichung
Native 1ofriceandSonoro 64 of wheat), (b) selections made from introductions
(Sonalika from segregating materials of bread wheat), and (c) use in hybridisation.
However, the present role of plant introductions, owing to emphasis for area-specific
breeding programmes, is limited to providing a reservoir of germplasm at the disposal
of plant breeders.
Plasticity. The degree to which the expression of an individual’s genotype can be
modified by environmental factors, irrespective of the adaptive value of that
modification. The plasticity may be specific to a particular character, particular
environment, or specific in direction. It may be under genetic control, and can be
fundamentally changed by selection.
Plastid. A cytoplasmic organelle in plant cells, primarily concerned with the formation
and storage of soluble and insoluble carbohydrates. Plastids include chloroplast
(having chlorophyll pigments), chromoplast (containing carotenoids commonly found
in fruits and flowers), and leucoplast (with no visible pigments, such as amyloplast).
Plastogene. Any one of the hereditary determinants located in the plastids of plant cells.
Plastom. The sum total of plastogenes present in the plastid DNA.
Pleiotropy. A phenomenon of having more than one seemingly unrelated phenotypic
manifestation of one gene; such a gene is called pleiotropic gene. Mutation in such a
gene is called pleiotropic mutation. For instance, the recessive gene for white-eye
colour in Drosophila melanogaster also affects the testicular membrane, the shape of
spermatheca, longevity, and general viability. The well-known - Dee-geo-woo-gen -
has not only shortened the plant height but has also changed angle of inclination of
leaves, lodging tendency, fertilizer responsiveness, etc. in rice. The above examples
indicate that each “major” gene could have one or more side effects; and if side
effects were favourable, selection for major effect would be accompanied by
correlated response for all other side effects. However, unfavourable side effects
would preclude exploitation of pleiotropy in crop improvement. It should be noted
that most genes, if not all, affect more than one characteristic.
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Pleiotropic Mutation. A mutation that affects several different characters. For example,
a mutation in Sp gene (to sp one) changes height, position of inflorescence, number of
pods/cluster and maturity period in fenugreek (Trigonella foenum-graecum L) in
homozygous condition.
Ploidy. A term referring to the number of chromosome sets (or genomes) per cell.
Point Mutation. A mutation that can be mapped to one specific locus.
Poky. A slow-growing mitochondrial mutant in Neurospora.
Polar Mutation. A mutation that affects the transcription or translation of the part of the
gene or operon on only one side of the mutant site, for example, nonsense mutations,
frame-shift mutations, and the like.
Pollen Grain. The male gametophyte in flowering plants.
Pollination. The transfer of pollen from the anther to a stigma. Self-pollination is the
transfer of pollen from an anther to the stigma of the same flower or another flower
on the same plant, or within a clone. Cross-pollination is the transfer of pollen from
an anther on one plant to a stigma of a flower on a genetically different plant or clone.
Poly(A) Tail. A string of adenine nucleotides added to mRNA after transcription.
Polyacrylamide. A material used to make electrophoretic gels for separation of mixtures
of macromolecules.
Polycistronic mRNA. An mRNA that codes for more than one protein. Bacterial genes are
mostly polycistronic in the sense that more than one gene, which are contiguous and
related, are expressed together. Thus during transcription, polycistronic mRNA is
produced. Unlike bacteria, eukaryotic genes (and mRNA) are monocistronic.
Polycross. The progeny raised from seed of a line outcrossed with selected lines growing
in the same breeding nursery. The evaluation of such progeny is called polycross test
that provides information about GCA. To ensure that the line is pollinated equally
from other lines, the line should be replicated many times in the isolated crossing
block in which outcrosses are made. Although widely used owing to economy of
efforts with precise estimates of GCA, the inherent difficulty is non-randomness of
pollination.
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Polyembryony. A condition of simultaneous presence of two or more embryos within the
same seed.
Polyhaploid. A haploid of a polyploid containing more than two genomes (identical or
differentiated).
Polygenes (Mather, 1943). Genes whose effects are too slight to be identified
individually but which, through similar and supplementary effects, can have large
effects on total variability.
Polylinker. A vector DNA sequence containing multiple unique restriction-enzyme-cut
sites, convenient for inserting foreign DNA.
PCR. Polymerase Chain Reaction. A method for amplifying specific DNA segments
that exploits certain features of DNA replication.
Polymorphism. The regular and simultaneous presence of more than two distinct forms
in the same population at greater frequencies that cannot be explained by recurrent
mutation. It may occur at the level of protein, enzymes, genes or nucleotide sequences
of the DNA.
Polynucleotide. A linear sequence of nucleotides in which 3 position of the sugar of one
nucleotide is linked through a phosphate group to the 5 position of the sugar of the
adjacent nucleotide.
Polypeptide. A chain of amino acids linked by peptide bonds. It can be a protein as well.
Polyploid. A cell containing more than two sets of chromosomes (2n =3x, 4x, etc)orthe
individual composed of such cells. Such a condition is referred to as polyploidy. It is
usually grouped into two classes: (a) autopolyploid, and (b) allopolyploid. In
autoploids, each of the repeated sets is considered to be identical or nearly so (as in
peanuts), that is, chromosome sets are homologous. But in alloploids, chromosome
sets are dissimilar or homeologous (only partially homologous). Since they contain
genomes from two or more species, they are also designated hybrid polyploids.
Mackey (1970) suggested the use of the terms disomic and polysomic for alloploids
and autoploids, respectively as these terms give a more precise and conceptual picture
of the nature of polyploids from the viewpoint of cytogenetics and breeding. Disomic
polyploids (as exemplified by modern wheat) have regular formation of bivalents,
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pairing only between homologous chromosomes, disomic genetics, homozygosity at
each locus, and the chance for heterozygosity and its consequent heterosis from
interactions between homeologous loci. In contrast, a polysomic polyploid such as the
tetraploid potato can have meiotic pairing between any two or more of the
homologous chromosomes, tetrasomic genetics and again the most important, the
opportunity for up to four alleles per locus available for intra- and interlocus
interactions in optimising heterotic combinations. In general autoploids have not been
very important in natural evolution due to aberrant chromosome behaviour during
meiosis and associated high degree of sterility. However, from breeding point of
view, they bear significance in crop species that are propagated by asexual means,
such as potato, banana, and the like. In contrast, hybrid polyploidy has played
important roles in both natural evolution and plant breeding.
Polyploidy. A situation or condition describing occurrence of individuals with more than
two sets of chromosomes. It leads to a great increase in allelic diversity, and provides
an opportunity to utilize intra-locus interactions among alleles at the specific locus.
This intra-locus interactions for most loci is analogous to true overdominance. It also
provides even greater opportunity for exploitation of favourable epistatic interactions
among alleles of different loci (pseudo-overdominance). Polyploidy breeding seems
to be more successful in asexually propagated crops, crop species with low
chromosome number, and cross-pollinated crops.
Polysaccharide. A biological polymer composed of several monosaccharides (such as
glucose, fructose or galactose) or sugar subunits, for example, starch or cellulose.
Population Density. The number of individuals of a population per unit area in the
habitat.
Population Genetics. The study of inherited variation, and thus the behaviour of genes in
a population of organisms. The problem of population genetics is to relate the
heritable changes in population of organisms to the underlying individual processes of
inheritance and development. It is the translation of Darwin’s three principles (the
principle of variation, of heredity, and of selection) into precise genetic terms.
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Population Theory (Malthus 1798). A theory related to human population growth in
response to means of subsistence. It states that the increase in human population will
be controlled by hunger in absence of wars or any other forms of disaster. Modern
Malthusians maintain that the catastrophe has only been delayed, and that the misery
associated over population is still the fate of modern civilization. They believe that
population growth may proceed at a faster rate (geometric progression) than means of
subsistence (which may increase only at a arithmetic rate).
Population. A term originally coined to denote a group of people. However, it has been
broadened to include groups of individuals of any one kind of organism. The term as
used in Genetics & Plant Breeding includes a reproductive community of sexual and
cross-fertilizing organisms, which share in a common gene pool. Since Mendel’s laws
of inheritance still apply to the transmission of genes among individuals of such a
group, it is also referred to as Mendelian population. As a consequence of its
reproductive systems and its previous evolutionary history, each population in this
category is believed to possess an integrated genetic structure, which may be
described in terms of systems of gene and genotypic frequencies. In autogamous
crops, it refers to a group of segregating individuals, pure lines and/or families. It
tends to maintain their individual integrity by virtue of special reproductive
mechanism that precludes (within limits) free interbreeding among themselves. In
statistics, population is a hypothetical and infinitely large series of potential
observations among which observations actually made constitute a sample.
Position Effect. A term used to describe a situation in which the phenotypic influence of
a gene is altered by changes in the position of that gene within the genome.
Position-Effect Variegation. Variegation caused by the inactivation of a gene in some
cells through its abnormal juxtaposition with heterochromatin.
Positive Assortative Mating. A situation in which like individuals mate more commonly
than expected by chance. If the mating-individuals are related by descent/ancestry (AA
× AA), it is called genetic assortative mating. If mating of like-to-like (tall × tall) is
based on appearance, it is referred to as phenotypic assortative mating. The ultimate
effect of assortative mating is an increase in the homozygosity of the population.
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Positive Control. Regulation mediated by a protein that is required for the activation of a
transcription unit. Expression of the gene requires activating signal.
Potential Variability. A kind of bound variability that is released slowly in subsequent
generations when it gets converted into free variability following segregation. In most
cases, it occurs in the heterozygotes. However, if two or more genes affect the same
trait in completely additive manner, potential variability will occur in AaBb, AAbb
and aaBB. The latter two genotypes contain homozygotic potential variability that is
released very slowly because it is first converted into heterozygotic potential
variability by crossing and then converted into free variability.
Primary Structure. The sequence of amino acids in the polypeptide chain.
Primase. An enzyme that makes RNA primers during DNA replication.
Primer. A short single-stranded RNA or DNA that can act as a start site for 3’ chain
growth when bound to a single-stranded template.
Primer Walking. The use of a primer based on a sequenced area of a genome to
sequence into a flanking unsequenced area.
Prepotency. The capacity of a parent to impress its characteristics on its offspring so
they are more alike than usual. Differences in prepotency depend on homozygosity,
dominance, epistasis, and linkage. A homozygote dominant at all loci would excel in
prepotency because its close relatives would be unusually alike. Such an individual
produces only one kind of gamete; therefore, upon selfing or crossing inter se of
individuals with the same genotype will produce individuals of the same kind, that is,
genetic correlations among relatives will be very high. Multiple heterozygotes, which
produce many kinds of gametes (and hence, many kinds of individuals upon selfing),
are generally not highly prepotent.
Pre-Breeding. The sum total of all the activities related to germplasm enhancement for
future use in varietal development.
Precision Agriculture. Agriculture that makes an effective use of information
technology through data available on soil, water, environment and other related
variables.
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Preformation. The belief that the egg, sperm, or zygote contains a preformed adult in
miniature form, which becomes unfolded during development.
Primary Species. See monogenomic species.
Primary Gene Pool. Genetic resources of a crop that include both cultivated and wild
races wherein exchange of genetic information among individuals is possible through
routine breeding.
Primary Trisomics. Trisomics having a particular homologue in triplicate. Total number
of primary trisomics is equal to the haploid number of chromosomes of a species.
Primary Triticales. See triticales.
Primordium. An embryonic structure, which eventually forms an adult part or organ.
Probability. The number of times an event is likely to occur in the total number of trials.
For instance, the probability of rolling a four on a die in a single trial is 1/6 (because
the die has six sides).
Probe. Defined nucleic acid segment that can be used to identify specific DNA molecules
bearing the complementary sequence, usually through autoradiography.
Product Rule. A statistical rule to predict probability of occurrence of two or more
independent events. The probability of two independent events occurring
simultaneously is the product of their individual probabilities. For example, with two
dice we have independent objects, and thus probability of two fours is 1/36 (1/6 × 1/6
= 1/36).
Production Environment. A complex of several biotic and abiotic factors that affect
growth and production of a particular crop. Abiotic factors include various edaphic
and climatic factors. Edaphic factors are predictable, and breeding ideotype on this
ground is easy for this stress; however, it is not true owing to other reasons.
Productivity. The rate at which radiant energy is converted into organic substances by
photosynthetic and chemosynthetic activity of producer organisms (chiefly green
plants). This is also called gross primary productivity, total photosynthesis or total
assimilation. The rate of storage of organic matter in plant tissues exceeding the
respiratory use by the plants during the period of measurement is called net primary
productivity, apparent photosynthesis or net assimilation. Here the term productivity
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and the phrase rate of production have been used interchangeably. In agriculture,
production per unit area is called productivity.
Proflavin. A mutagen that tends to produce frame-shift mutations.
Progeny Selection. A type of selection in which individuals are chosen to produce the
next generation on the basis of performance of their progeny, rather than on their own
phenotypic appearance. The procedure is similar to ear-to-row selection.
Progeny Test. A test of the breeding value of a genotype based on the performance of its
offspring produced in some definite system of mating. It could be indicative of
whether the genotype in question is heterozygous (which will produce variable
offspring) or its obvious superiority was due primarily to better micro-environmental
conditions. Early progeny test is usually advocated for the monogenic or oligogenic
traits owing to two obvious reasons: (1) perfect population size for expression of each
genotype is usually small, and consequently, the chance of obtaining a desirable
genotype is more, and (2) such a small-sized population requires little space. Early -
testing has also been recommended for a large scale screening of inbred lines so that
resources can be saved for a final stage evaluation; however, objections have also
been raised on the grounds that it will lead to a loss of many worth-while inbred lines.
Early progeny test is also useful for traits having higher heritability. If a hybrid is
differing for many genes and the trait in question has lower heritability, progeny tests
in early segregating generations (F 2 /F 3 /F 4 ) are not conducted. Plant selections made in
the F5/F6 are likely to be homozygous, or nearly so, and the performance of their
progeny consequently more predictable than the performance of progeny of the
heterozygotes that earlier dominated the population and were under influence of
dominance and non-allelic interaction.
Progeny. The offspring of a particular mating or of a particular individual (in case of
apomictic reproduction).
Prokaryote. The kingdom (including viruses, bacteria and blue-green algae) lacking
membrane-bounded eukaryotic organization of the genetic material. It can also be
defined as an organism composed of a prokaryotic cell(s).
Prokaryotic Cell. A cell having no nuclear membrane and hence no separate nucleus.
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Promoter. A regulator region of a short distance from the 5’ end of a gene that acts as the
binding site for RNA polymerase (for the initiation of transcription).
Propagule. The means of propagation and/ or dissemination. It may be unicellular or
multicellular agent (seed, for example).
Proper Plot Technique. A technique to minimize experimental error as much as
possible. Some common plot techniques are proper choice of plot size and shape,
block size and shape, number of replication, and the like.
Prophage. A phage “chromosome” inserted as a part of the linear structure of the DNA
chromosome of a bacterium.
Prophase. The early stage of nuclear division during which chromatins condense and
chromosomes become visible.
Proplastid. An immature plastid.
Protandry. A mating system that involves maturation of anthers before pistils (pollen
shed first as in maize, carrots, etc.).
Protein-for-Protein Hypothesis (Van der Plank 1976, ’78). A hypothesis pertaining to
host-parasite interaction. The products of genes of both host and pathogen recognize
and interact with each other that lead either to incompatibility (resistance) or
compatibility (diseased condition). According to Van der Plank, the protein-forprotein
hypothesis is applied to diseases to which the gene-for-gene hypothesis has
been applied. The two hypotheses differ somewhat in emphasis; the latter centres
around genes for resistance, whereas the former is concerned primarily, albeit not
exclusively, with compatible host - pathogen combinations. Strobel (1975) objected
the explanation made by Van der Plank. According to him, it is the protein from the
susceptible allele that recognizes and binds the toxin produced by the pathogen, and
thus the explanation of Van der Plank (1978) that pathogen protein copolymerizes
with that of host protein giving rise to compatibility reaction, cannot be true.
According to Samborski (1978), it also does not account for the fact observed by Flor
(1956) that interaction of recessive alleles of host and dominant alleles of pathogen in
flax rust could lead to compatibility. Therefore, according to him, compatibility arises
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when either the host or the pathogen, or both produce wrong products or nothing at
all.
Proteome. The complete set of protein-coding genes in a genome.
Proteomics. The cloning and molecular characterization of the complete set of proteincoding
genes in a genome.
Protogyny. Just reverse of protandry. It is a mechanism in which stigma becomes
receptive first, (i.e., before pollen shed; for example, pearl millet, walnuts, etc).
Notice that both protandry and protogyny are analogous in that both fosters crosspollination.
Proto-oncogene. The normal cellular counterpart of genes that can be mutated to become
dominant oncogenes.
Protoplasm. The cytoplasm and nucleoplasm of the cell.
Protoplast. A plant cell devoid of the wall. The wall of the cell is digested by an enzyme,
called cellulase (and macerozyme). Enzymatic treatments yield a large quantity of
protoplasts without cell breakage and osmotic shrinkage. Viability of the protoplasts
can be tested by the presence of photosynthetic and respiratory activity; however,
fluoroscein diacetate is frequently used to test the viability. Cultured protoplasts can
be used not only for somatic cell fusion but also for taking up foreign DNA.
Protoplast Fusion. Also called somatic hybridisation; the fusion of protoplasts from two
diverse sources aided by suitable fusogenic treatments. It is one of the important uses
of protoplast culture. A variety of fusogenic treatments are available, which include
NaNO 3 , high pH with Ca ++ , polyethylene glycol and electrical impulse. Such agents
bring the protoplasts together, cause them to adhere with one another for bringing
fusion. It is used to hybridise otherwise sexually incompatible (cross incompatible).
Somatic hybridisation has been used in Nicotiana, Datura, Petunia, Solanum, and the
like.
Prototroph. A strain of organisms that will proliferate on minimal medium (without any
supplemented nutrient such as an amino acid, a vitamin or a hormone).
Provirus. A virus “chromosome” integrated into the DNA of the host cell.
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Pseudocompatibility. The occurrence of fertilization in otherwise incompatible plants
under exceptional environmental or genotypic conditions. Also see pseudofertility.
Pseudo-Self-Compatibility. Partial seed setting following self-pollination in an
otherwise self-incompatible plant. This usually occurs at the end-of-the-season
because the strength of self-incompatibility system weakens due to aging.
Pseudodominance. The sudden appearance of a recessive phenotype in a pedigree due to
deletion of a masking dominant gene (from one of the homologs in a deletion
heterozygote). It is one of the marking phenomena for the occurrence of a deletion
event.
Pseudogamy. Formation of seeds after stimulation by a male gamete (seeming
pollination). In this case, pollen does not make any genetic contribution to embryo
formation, but is necessary to initiate the growth of ovary or to form endosperm
nucleus. The progeny shows the characteristics of the female parent.
Pseudogene. An inactive gene derived from an ancestral active one.
Pseudofertility. The ability of an otherwise self-incompatible plant to produce viable
offspring under specific environmental condition (at the end of the growing season). It
so happens (e.g. tobacco) because the efficiency of the incompatibility reaction
weakens with age.
Pseudoheterosis. See luxuriance.
PSGMS. Photoperiod-Sensitive Genetic Male-sterility; photoperiod mediated malesterility
condition observed in certain genotypes of rice. Such genotypes may be
maintained (propagated) under short-day condition, and become male sterile during
long-day condition. Such lines offer opportunity for replacing the widely used threeline
system for producing hybrid varieties involving male-sterile, maintainer and
restorer lines. However, in PSGMS, temperature also plays an important role in the
fertility of PSGMS rice. Two loci have been identified; one controls PSGMS system,
and the other conditions temperature sensitivity.
Pulse–Chase Experiment. An experiment in which cells are grown in a radioactive
medium for a brief period (the pulse) and then transferred to a nonradioactive one for
a longer period (the chase).
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Punnett Square. A grid used as a graphic representation of the progeny zygotes (F 2 )
resulting from different gamete fusions in a specific cross.
Pure-Line Breeding. A system of breeding in which the new variety is made up of the
progeny of a single self-fertilized homozygous individual. Although agriculturally
advanced countries have already exploited its full potential, however, it has still a
place wherein land varieties still exist.
Pure-Line theory (Johannsen 1903). A theory of breeding based on progeny of single
self-fertilized homozygous individual. The proposed theory can be summarized as: (a)
a population of self-pollinated crop consists of a mixture of pure lines, (b) a pure-line
breeds true to the type, (c) a pure-line may become “impure” owing to natural
crossing, mutation and mechanical mixing, (d) variation between pure-lines is
heritable in nature, and (e) variation within a pure-line does not have a genetic basis,
and is caused due to slight differences in the environment. All plants within a pureline
are virtually homozygous for the same gene pairs, and thus selection within a
pure-line is not effective.
Pure-Line. Progeny of a single self-fertilised homozygous individual. A pure line breeds
true for the specific character being studied; that is, all offspring produced by selfing
or crossing within the progeny show the same form for this trait. It is a strain
homozygous at all loci, ordinarily obtained by successive self-fertilizations.
Purity. The ratio of sucrose to total solids (brix) dissolved in juice of sugar cane or sugar
beet.
Purine. Atypeofnitrogenbasewiththepresenceoftworingsinthestructure. The
purine bases of nucleic acids are adenine and guanine.
Pyrimidine. A type of nitrogen base with a single ring forming its chemical structure.
The pyrimidine bases in DNA are cytosine and thymine, whereas in RNA are cytosine
and uracil.
PVP. Plant Variety Protection; legal protection to newly bred varieties. The bill under
IPRs (India) was passed on 9. 8. 2001 and called Protection of Plant Varieties and
Farmers’ Rights Bill 2001. Under the new regime of PVP, for granting a legal right to
a variety, it must conform to the criteria of novelty and distinctness, uniformity and
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stability (DUS). The legal protection is valid for 9 and 6 years for trees and vines, and
field crops, respectively. It can be renewed; however, the total period will not exceed
18 and 15 years for trees and vines, and field crops, respectively.
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QPM. Quality Protein Maize.
QTLs. Quantitative Trait Loci; several loci (or genes) affecting the phenotypic variation
in continuously varying traits such as height, weight, yield, and the like.
Qualitative Character. A character for which variation between genotypes is discrete,
thus making easy classification into distinct categories. For this kind of trait, there
occurs a one to one correspondence between genotype and phenotype. As a
consequence, each genotype (ignoring ephemeral effects of dominance) is capable of
producing a unique phenotype; differences in environmental conditions have
relatively little effects on the expression of genotypes with respect to a particular
locus. Even characters conditioned by many a gene can show discrete variation
provided environmental effects are too small to permit each genotype to produce a
unique phenotype associated with it (under unambiguous norm of reaction).
Qualitative Resistance. A general term to describe plant resistance in which the
frequency distribution of resistant and susceptible plants in a population is
discontinuous. With this type of resistance, it is easy to classify plants either as
resistant or as susceptible or into well-defined classes of disease reaction.
Quality. Physical, chemical, and nutritional characteristics of the product that affect its
utilisation by human beings directly or indirectly. Sellers may be interested in the
market quality, but the millers are primarily concerned with the milling quality,
whereas the ultimate users may have primary concern for physical and nutritional
quality.
Quantitative Character. A character in which variation is continuous so that
classification into discrete categories is not possible. Such characters cannot be
analysed by standard methods of classical genetics, but can be specified only in terms
of metric system of measurements, and as such they are also called metrical traits. For
such characters, the norm of reaction is ambiguous, that is, one to many
correspondences between genotype and phenotype is possible. Characters of this kind
are usually governed by several genes, each usually with a small effect on the
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phenotype. However, multiple allelic series of a single gene can also produce a variety
of continuously varying phenotypes on the scale of measurement (but the presence of
all members of such a series is unlikely in any one population at a given point of
time). Expression of a quantitative character can be depicted as:
Envirn. 1 Environ. 2 Envirn.3 Environ. nth
↓ ↓ ↓ ↓
Gene A -------› Precursor A--------›Precursors AB ---------›ABC -----------› Final product
↑ ↑ ↑ ↓
Gene B Gene C Gene nth a wide range
of variation
owing to effects
of environment
at each step
Most characters with which plant breeders work are unfortunately of this type, e.g.,
protein content, cold or drought tolerance, etc. However, it must be understood that
even a one gene-two alleles system can result in a continuous distribution of
phenotypes if environment variation is large. Therefore, a quantitative trait is one for
which phenotypic difference between genotypes is small as compared to difference
between individuals within genotypes.
Quantitative Variation. The existence of a range of phenotypes for a specific character,
differing by degree rather than by distinct qualitative differences.
Quantitative Genetics. The genetics of quantitative traits or continuously varying
characters.
Quantitative Resistance. A class of resistance for which there occurs a continuous
gradation between resistance and susceptibility within a population of plants.
Therefore, no clear-cut distinction between resistant and susceptible plants is
observed, that is, they form a single spectrum consisting of different degrees of
resistance and susceptibility.
Quasidiploid. A cell with diploid chromosome number but aneuploid chromosome sets,
trisomic-monosomic (2n + 1 – 1), for example.
Quasiqualitative Character. An otherwise quantitative character which pretends to be
qualitative one, e.g., kernel colour in corn. Such traits show continuous distributions
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on the phenotypic scale; however, their inheritance is oligogenic like that of
qualitative traits. But so many minor genes regulate it. Continuous distribution may
be ascribed to the presence of several modifiers and regulatory genes affecting
expression of those oligogenes.
Quaternary Structure (of Protein). The multimeric constitution of the protein.
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R Plasmid. A plasmid containing one or several transposons that bear resistance genes.
Race. A population or aggregate of populations with characteristic gene frequencies that
distinguishes it from any other such groups within formally recognizable subspecies
or species. It is essentially an intraspecific category. Differences between races are
only relative and not absolute. Any race is able to interbreed freely with any other race
of the same species. There may be geographical races (subspecies occupying a
geographical subdivision of the range of a species), ecological races (ecotypes),
physiological races (differences are physiological rather than morphological), etc.
Races (pathological). Pathogens of the same species with or without identical/similar
morphology but are characterized by differing pathogenic capabilities. This implies
that the two races may have the same genetic complements except for their
pathogenic capacity. Zadoks (1966) defined a race as a “taxon” within a pathogen
species characterized by a specific combination of virulence genes. Differential series
of cultivars used to identify races, in fact, characterize particular virulence genes. In
older classification, races were used to be subdivided into biotypes. However, in
modern classification, the two terms are used interchangeably; but the term, biotype,
is used more frequently by entomologists than by plant pathologists. Terms like
pathotype, physiological race, and strain are also used as synonyms for the race.
RAD. Radiation Absorbed Dose; a measure of radiation dose. It can be defined as the
absorbed dose of 100 ergs of energy per gram of tissue. The Roentgen (R), an older
unit which, strictly speaking, is to be used only for γ and χ-rays. One Roentgen unit
provides 93 ergs of energy per gram of tissue.
Random Chromatid Assortment (Haldane 1930). A type of partition and segregation
(or assortment) of chromatids wherein sister chromatids at a given locus may end up
in the same gamete (in a higher order even polyploids). For example, in an
autotetraploid, when quadrivalent form and crossing over takes place betwixt
kinetochore and the locus in question, sister chromatids at this locus can end up
attached to different kinetochores. Consequently, sister alleles may be included in the
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same gamete. If quadrivalent formation is complete and crossing over results in 50%
recombination, we have to think in terms of chromatids (instead of chromosomes) to
predict gametic output. For instance, in a simplex (Aaaa), there are 2A chromatids and
6a chromatids. Three types of gametes are possible: AA, Aa and aa. The probability of
picking two A chromatids (out of AAaaaaaa) is 2/8×1/7 = 1/28AA. Similarly, the
probability for Aa is 2/8×6/7 + 6/8×2/7 = 12/28Aa. Obviously, the probability for aa
is 15/28. Thus the gametic output in a simplex will be in a ratio of 1AA: 12Aa: 15aa.
Similarly, gametic output in the duplex (3AA: 8Aa: 3aa) and triplex (15AA: 12Aa:
1aa) can be predicted under such conditions.
Random Chromosome Assortment (Muller 1914). A type of partition of chromatids in
which sister chromatids always go to different products of meiosis in an
autotetraploid (or in any higher order even polyploids). It happens if bivalents form
regularly during meiosis. This type of segregation also occurs, when quadrivalents
always form and the locus in question is inseparably linked to the centromere. The
effect of linkage is to cause the locus to separate reductionally at the first anaphase
and equationally at second anaphase, exactly as happens in case of bivalent formation.
In an autotetraploid, five genotypes namely AAAA, AAAa, AAaa, Aaaa and aaaa are
possible. Gametes produced by the first and the last will be AA and aa, respectively.
Gametic output in triplex (AAAa) andsimplex(Aaaa) will be 1AA: 1Aaand1Aa: 1aa,
respectively. Duplex (AAaa) produces three types of gametes in the ratio of 1AA: 4Aa:
1aa.
Random Fixation. The fixation (100% allelic frequency) of one of two alleles in a
population by chance events (e.g., genetic drift). Random fixation depends on a
number of factors such as selective value of the allele(s), mutation pressure, gene
flow, the degree of genetic relationship between the number of breeding individuals.
Random Genetic Drift. Random changes in allelic and genotypic frequencies in a small
random mating population owing to sampling errors. The genetic effect is similar to
that of inbreeding, that is, there is gradual loss of heterozygosity. The rate of this loss
is 1/2N, where N is the number of diploid individuals in the population. Thus, H t ≅
H 0 e -t/2N , whereas, H t and H 0 are proportions of heterozygotes in the t th and original
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generations, respectively. Which one of the original alleles (A or a) becomes fixed at
each locus is a chance matter.
Random Inbreeding. Mating between relatives occurring by chance owing to spatial
proximity, synchrony in flowering time, etc. in a random mating population.
However, there is no continuity of such inbreeding because of random distribution of
inbred and outbred progeny in the next generation. The genetic consequence is similar
to Hardy-Weinberg equilibrium but with more homozygotes.
Random Mating. A form of mating system in which each individual has an equal chance
of mating with any other individuals of the same population. (In case of self-fruitful
cross-pollinated species such as maize, random mating also includes self-mating, that
is, each male gamete has an equal opportunity to fertilize each female gamete). In an
infinitely large population, it leads to constancy of gene frequency, genotypic
frequencies, amount of heterozygosity and genetic variability, and genetic relationship
between individuals in succeeding generations in the absence of forces enumerated
under genetic equilibrium. Random mating of individuals is equivalent to random
combination or matching of male and female gametes during fertilization.
Random Sample. A random collection from a population such that all items of the
population are likely to be represented equally in the collection.
Random Sampling. The process of selecting a sample completely on the basis of chance.
Although a chance event cannot be individually predicted, the results of a large
number of such trials can be anticipated with considerable accuracy and confidence.
Its main advantage lies in the elimination of human biases and attainment of ultimate
precision. Another property is that the “mean” of the means derived from random
samples approaches the population value as the number of sample means averaged
increases.
Random. Arrived at by chance.
Randomised Complete Block Design (RCBD). An experimental design characterized
by the presence of blocks of equal size each of which contains all the treatments of
the experiment. It is one of the most widely used designs in plant breeding trials. The
design is especially suited for field experiments where the number of treatments is not
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large (40) and the experimental area has a predictable productivity gradient.
However, if becomes necessary, larger number of treatments can be included at the
cost of reducing plot size for each entry within the blocks.
Randomization. Random assignment of treatments to experimental plots. It ensures that
each entry or treatment will have an equal chance of being assigned to any
experimental plot, and consequently of being grown in any particular environment
existing in the experimental site. In essence, it is a technique to obtain an unbiased
estimate of experimental error.
Range. The simplest measure of dispersion. It is the difference between the largest (L)
and the smallest (S) of observations and provides, at least, some idea of the amount of
variability present. It is affected by fluctuations of sampling and is completely
unrelated to any type of frequency distribution. It is unit-dependent and expressed in
the unit of scale of measurement. Thus it cannot be used to compare variability of two
different traits. To make it comparable, a similar statistic called Range Coefficient
[(L-S)/(L+S)] is used.
RAPD. Randomly Amplified Polymorphic DNA; a set of several genomic fragments
amplified by single PCR primer. It is somewhat variable from individual to
individual. Heterozygotes (+ / -) for individual fragments can act as markers in
genome mapping.
Raphanobrassica (Karpechenko 1928). The classical allopolyploid synthesized by
making a cross between Brassica and Raphanus spp. The objective was to combine
the leaves of cabbage (Brassica) and the roots of radish (Raphanus). However, the
result came out to be just reverse. This clearly showed difficulty in predicting the
precise combination of parental characteristics that would appear in ensuing alloploid.
Reaction Norm. The range of phenotypic reactions of a genotype as manifested by the
variety of phenotypes that the specific genotype is able to produce in response to
environmental influences.
Reading Frame. The codon sequence that is determined by reading nucleotides in groups
of three from some specific start codon.
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Realised Heritability. The ratio of the single-generation progress of selection to the
selection differential of the parents. Numerically, it is tantamount to narrow-sense
heritability (h 2 ).
Reannealing. Spontaneous realignment of two single strands to reform a DNA double
helix that had been denatured.
Recapitulation Theory. A theory stating that ontogeny recapitulates phylogeny (the
developmental history of organisms repeats their evolutionary history).
Receptor Element. A controlling element that can insert into a gene (making it a mutant)
and can also excise (thus making the mutation unstable). However, both of these
functions are non-autonomous, being under the influence of the regulator element.
Recessive Allele. An allele whose phenotypic effect is not expressed in a heterozygote.
Recessive Phenotype. The parental phenotype that is not expressed in the hybrid
(heterozygote/cross/F 1 offspring) between two parents (homozygotes). It is the
phenotype of a homozygote for the recessive allele (dwarf strain of Mendel’s peas, for
example).
Recessivity. The failure of one allele to express itself phenotypically in heterozygous
state (heterozygote).
Reciprocal Crosses. Crosses in which sources of male and female gametes are reversed;
that is, A×B and B×A.
Reciprocal Recurrent Selection. A procedure for simultaneous improvement of two
genetically unrelated populations for both GCA and SCA. The procedure is very
similar to R.S. for GCA except that the two populations serve as the tester for each
other.
Reciprocal Translocation. A translocation in which part of one chromosome is
exchanged with a part of a nonhomologous chromosome.
Recombinant DNA. A novel DNA sequence formed by ligating together the pieces of
two nonhomologous DNA molecules with the use of restriction endonucleases. The
two DNA molecules may be from two separate species within a genus, family, and
kingdom or even from different kingdoms.
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Recombinant Frequency. A ratio of the number of recombinant cells (individuals) to the
total number of cells (individuals). The mean recombination frequency never exceeds
the limit of 50 per cent (0.5) as only two chromatids out of four take part in crossing
over at any one point of time.
Recombinant Inbred Lines (RIL). A set of inbred lines, each derived independently,
usually by selfing single plants within lines during a number of filial generations. In
plant breeding, individual RIL are often derived by single seed descent from a
backcross of the F 2 to one or the other parent. Each RIL is ultimately expected to have
a different pattern of alternative alleles of multiple loci within an otherwise unique
homozygous genetic background, allowing effects of individual alleles (or
combinations of alleles) to be evaluated in an otherwise homozygous background).
Recombinant. A cell or an individual with a genotype produced through recombination.
Recombination Gene. Any gene (allele) that controls the frequency of genetic
recombination. Such genes may be concerned with the structure of an enzyme or the
structure of a regulatory substance that influences genetic recombination.
Recombination. Formation of new combinations of genes as a result of segregation in
crosses between genetically different parents;.also the rearrangement of linked genes
due to crossing over.
Recombinational Repair. A mechanism for the repair of a DNA lesion through a
process, similar to recombination, which uses recombination enzymes.
Recombined Triticales. See Triticales.
Recon. Aregionofagenewithinwhichtherecanbenocrossing-over(recombination),
now known to be a nucleotide pair.
Recurrent Parent. The parent to which successive backcrosses are made in the back
cross breeding. It is usually an agronomically sound variety under cultivation. It is
usually composed of a number of closely related pure lines (putative isogenic lines)
that interact synergistically to give the recurrent parent its agricultural characteristics.
However, it is deficient in one or a few characters.
Recurrent Selection (for GCA). A form of recurrent selection in which a number of
desirable plants on the basis of performance of their testcrosses with a heterozygous
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tester are propagated from the selfed seed and intercrossed in all combinations to
establish a population for the next cycle of selection. The population so improved can
be used as a source population for extraction of superior inbred lines.
Recurrent Selection (for SCA). AsystemofbreedinginwhichanumberofS 1 lines
based on the testcross performance with a homozygous tester (inbred line) of original
selections (S 0 ) are selected and crossed in all combinations to form the new
population for the next round of selection. The inbred derived from such improved
population must be crossed with the homozygous tester for effective exploitation of
non-additive gene action.
Recurrent Selection. A cyclical scheme of plant selection designed to increase
systematically the frequency of desirable genes in a population primarily for
quantitatively inherited traits. The method involves repeated selection in each
generation among the progeny produced by intercrossing in all possible combinations
of the selected individuals (or their selfed progeny) of the previous generation. Since
intercrosses are made in each cycle of selection, appreciable heterozygosity (and
hence, genetic variability) is maintained. Thus opportunity for continuous selection is
enhanced because new recombinants are continually generated. Inbreeding depression
also remains within limits.
Rediscoverers of Mendel’s Principles. Carl Correns (of Germany), Hugo de Vries (of
Netherlands), and Tschermak (of Austria). However, some scientists credit principally
Carl Correns as the rediscoverer of Mendelian principles of heredity.
Reduction Division. The first nuclear division during meiosis that produces daughter
nuclei each having one-half as many centromeres as the parental nucleus.
Redundant DNA. Also called repetitive DNA; the DNA sequences that are present in
many copies per chromosome set.
Registered Seed. The progeny of foundation seed normally grown to produce certified
seed. This class of seed is not produced in India.
Regression Coefficient. A numerical measure of the rate of change of the dependent on
the independent variable. It measures the magnitude of change in a dependent variable
for a unit change in independent variable. It is mathematically the slope of the straight
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line that most closely relates two correlated variables. It is statistically designated by
‘r’ and its numerical values range from +∝ to -∝. It is expressed as a ratio of
covariance of the two variables and variance of the independent variable [b y/x =
σxy/σ 2 x]. It is a unit dependent measurement and is expressed in the unit of the
dependent variable. Further it is independent of change of origin, but is dependent
upon change of scale.
Regression. A statistical description of functional relationship between a dependent and
independent variables. It describes the effect of one or more characters (designated as
independent variables) on a single character (designated as the dependent variable) by
expressing the latter as a function of the former. It can be classified according to the
number of independent variables or the kind of functional relationship between the
dependent and the independent variables. Thus we may have simple or multiple
linear/non-linear regressions. It is a term originally coined by Galton for the tendency
of the quantitative traits of offspring to be closer to population mean than are their
parents’ traits. It arises from dominance, gene interaction, and nongenetic influences
on traits.
Regulatory Genes. Genes that are involved in turning on or off the transcription of
structural genes.
Regulatory Region. Upstream (5’) end of a gene to which bind various proteins that
cause transcription of the gene at the right time and place.
Regulon. A group of separately transcribed genes having operators with the same
specificity.
Remote Sensing. The process of sensing, identifying, and delineating various objects and
ground features from a distance without coming in direct contact (physical) with
them. The term was at first used by US navy in 1961, when a naval project on a study
of aerial photography was renamed remote sensing. Electromagnetic radiation is the
medium by which an object is sensored. The data gathered by remote-sensing
satellites can be utilised for agricultural crops, acreage and yield estimation, drought
and flood monitoring and assessment, flood control, land use, land cover, and
mapping for agro-climatic zone planning.
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Renaturation. The return to the native configuration of a denatured macromolecule (such
as DNA or protein). In case of DNA, annealing is the process, which brings back the
denatured DNA to its original state.
Renner Effect. Competition among the four genetically different megaspores (formed by
meiosis) as to which one will form the embryo.
Repair. Restoration of the original structure (biological activity) of damaged DNA by
any one of the recovery mechanisms. Repair includes both reversion and replacement
of an induced lesion.
Replicase. An enzyme catalysing the process of replication of nucleic acids.
Replication Fork. The point at which the two strands of DNA are separated to allow
replication of each strand.
Replication. In statistics, the process of repeating the same treatment to different plots. It
is essential for estimation of experimental error. In this process, the same treatment or
the same replica is planted twice, thrice or even four times. The process of replication
is something that amounts to sampling. In genetics, replication implies DNA synthesis
through which a new replica (copy) of the original DNA is produced.
Replicon. A chromosomal region under the influence of one adjacent replicationinitiation
locus. It is a unit of replication, represented by a sequentially replicating
linear or circular DNA structure.
Reporter Gene. A gene whose phenotypic expression is used to study tissue-specific
promoter and enhancer activities in transgenes. In essence, it reports the presence of
other gene or genes.
Repressor Protein. A molecule that binds to the operator and prevents transcription of an
operon.
Repressor. A protein that binds to a cis-acting element such as an operator or a silencer,
thereby preventing transcription from an adjacent promoter.
Reproduction. The propagation of an organism, a cell, or an organelle by one like itself.
Reproduction may occur by sexual means (sexual reproduction) or by asexual means
(asexual reproduction).
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Reproductive Cell. Any of the germ cells (usually gametes) and their immediate
predecessors from which they arise by division.
Reproductive Isolation. Isolation by various genetically controlled mechanisms, which
prevent gene exchange between two populations and preserve differences in the gene
pools of populations previously attained by selection and other evolutionary factors.
Reproductive Potential. An organism’s ability to produce surviving offspring (up to the
stage of sexual maturity).
Reproductive System. Several variables (organs) of an individual orchestrated to
perform reproduction.
Repulsion Conformation. A type of linkage conformation in which one dominant and
one recessive gene are linked on one chromosome and their counterparts occur in
other homologous chromosome (Ab/aB).
Resistance Breeding. A system of breeding that involves selection of desired resistant
types from a genetically variable population under artificial epiphytotic condition in
the breeding nursery. It has assumed a very important position in plant breeding
because: (a) it plays a significant role in increasing and stabilizing supplies of food,
fibre, vegetable oils, etc., (b) it adds little or nothing to the cost of production, (c) it
does not create environmental or health hazards, and (d) it is an initial factor in
integrated disease management. Resistance breeding differs from any other
conventional methods for two reasons: (a) resistance can be assayed only by diseasing
plants, i.e., employing another living and variable organism, and (b) resistance may
appear elusive, that is, the resistance may break down.
Resistance Erosion. The gradual loss of resistance of the host against a particular
pathogen or pest. A variety may lose the resistance gene either due to mutation,
segregation or natural out crossing with a susceptible variety. It is interesting to note
that environment also plays an important role in the expression of resistance gene(s).
The same genotype that shows resistance to a particular race of the pathogen in one
season succumbs to the same race in other season. For instance, tomato varieties
carrying cf 1 gene show resistance to Cladosporium fulvum in summer but become
susceptible in winter.
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Resistance Mosaics. A strategy of resistance management that involves the use of a
number of cultivars, each carrying a different resistance gene, simultaneously or in
succession (that is, in space and time) in a production area. It can provide a diverse
environment to pathogens that would decrease the erosion of host resistance. It may
be fruitful if resistance genes are known, the racial composition of the pathogen
population is monitored closely and the mosaic pattern of cultivar use is controlled. In
absence of the orchestrated pattern of cultivar use, the strategy is certain to fail.
Resistance. A heritable property of a host plant that lessens the effects of parasitism. It
implies that some degree of host-pathogen interaction is always evoked. Therefore, a
resistant plant may resist the establishment of a pathogen by reducing the amount of
initial inoculum or may interfere the growth and reproduction of the established
pathogen. Thus some symptoms of disease always appear contrary to immunity
wherein no symptoms develop at all.
Resolving Power. The ability to discriminate between two points. The human eye cannot
resolve two points separated by less than 100m. The light microscope introduces a
500-fold increase in resolution over the eye, and the electron microscope provides a
500-fold increase over the light microscope.
Resource Conservation Technology. A technology aiming at conserving available
resources to maximize the output and profit in a cropping system. It includes zero-till
(no-till), raised bed furrow system, laser levelling, innovative cropping systems and
management of crop residues. The technology is scale-neutral and can be used with
advantage by all farmers. It lowers production costs through omitting tillage
operations and saving one or two irrigations. It has potential to maintain and sustain
productivity. Scientists have asserted that conservation agriculture can raise crop
yields by 20 to 50 per cent.
Restitution Nucleus. A single nucleus with unreduced chromosome number (produced
as a consequence of failure of the first or second meiotic division) or with doubled
chromosome number (owing to failure of mitosis).
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Restoration. Mitigating the effects induced by any physical or chemical agent to a
biological system. The terms like recovery, reactivation are frequently used as
synonyms for restoration.
Restorer Gene. A gene that brings back the changes brought about by the cytoplasmic
male sterility.
Restorer Line. A line which restores pollen-producing ability in the hybrid after crossing
with A line by providing a copy of restorer gene; also called R line.
Restriction Endonucleases. Enzymes that recognise and cut specific nucleotide
sequences in DNA. Type II restriction endonucleases are used in the construction of
recombinant DNA molecules. These enzymes provide powerful tool for analysis of
DNA organization, gene structure and gene regulation.
Restriction Enzyme. An endonuclease that recognises specific target nucleotide
sequences in DNA and breaks the DNA chain at those points. A variety of such
enzymes is now known, and is used extensively in genetic engineering.
Restriction Fragment Length Polymorphism (RFLP). Variations occurring within a
species in the length of DNA fragments generated by a specific endonuclease. RFLPs
are useful as molecular markers.
Restriction Fragment. A fragment of DNA created by cleavage at specific sites by a
restriction endonuclease.
Restriction Map. A map of a chromosomal region showing the positions of target sites
of one or more restriction enzymes.
Restrictive Conditions. Environmental conditions under which a conditional mutant
shows the mutant phenotype.
Retrotransposon. A transposable element that utilises reverse transcriptase to transpose
through an RNA intermediate.
Retrovirus. An RNA virus that replicates by first being converted into double-stranded
DNA.
Reverse Genetics. The study of genetics that begins with a segment of DNA (genotype)
or a protein sequence to investigate its function (phenotype). It also determines how a
mutation at a particular site in a gene affects the phenotype. It is so called as it directly
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deals with and changes in the DNA unlike Classical/ Mendelian genetics, which uses
a phenotype to trace the genotype. (Mendelian genetics is called forward genetics).
Reverse Mutation. A heritable change in a mutant gene that restores its original
constitution (nucleotide sequence).
Reverse Transcriptase. An enzyme that catalyses the synthesis of a DNA strand using
RNA as its template.
Reversion. The production of a wild-type gene (phenotype) from a mutant one
(phenotype) by either genetic or non-genetic mechanisms.
RFLP Mapping. A technique in which DNA restriction fragment length polymorphisms
are used as reference loci for mapping known genes or other RFLP loci.
Rhizobium. A bacterium that resides insides the root nodules of legumes as a symbiont.
It is highly sensitive to higher O 2 concentration. Legume plants provide it shelter; in
return, it provides them high quality growth protein through fixing atmospheric
nitrogen.
Rho. A protein factor required to recognise certain transcription termination signals in E.
coli.
Ribonucleic Acid. See RNA.
Ribonucleotide. A nucleotide having ribose sugar.
Ribosome. A complex organelle that catalyses the translation of genetic message of
mRNA into an amino acid sequence of a polypeptide. It is composed of proteins and
rRNA.
Ribozymes. RNA molecules having catalytic activities, the self-splicing RNA molecules
in Tetrahymena, for instance.
RNA Polymerase. An enzyme that catalyses the synthesis of an RNA strand from a DNA
template. In eukaryotes, there are several classes of RNA polymerase; structural genes
for proteins are transcribed by RNA polymerase II. However, in prokaryotes, a single
class of RNA performs all the necessary functions.
RNA. Ribonucleic Acid; a constituent of all living cells, consisting of a long chain of
nucleotides with ribose (as a pentose sugar) and uracil (as a distinguishing pyrimidine
N-base). It sometimes takes double-stranded structure while performing protein
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synthesis. It is mainly of three kinds: mRNA, rRNA and tRNA. Thebasesequenceof
rnRNA determines the amino-acid sequence of polypeptide. The rRNA is the major
constituent of ribosomes, which act as sites of protein synthesis. The last one, tRNA,
identifies and carries a specific amino acid to the site of protein synthesis in response
to the codons of mRNA. Some RNAs also act as enzymes, called ribozymes. In
nature, it evolved prior to DNA due perhaps to its simple structure. RNA replicase
must have been the first simplest living molecule that catalysed its own replication. It
contained ribose sugar because it is more readily synthesized than deoxyribose of
DNA.
Robertsonian Translocation. A translocation that involves breakage at centromeres of
two non-homologous acrocentric chromosomes and rejoining of the larger parts of the
two to form a metacentric chromosome. The smaller chromosomal segments are
subsequently lost. Robertsonian translocation, which is also called centric fusion, has
been important in the phylogeny of many species in reducing chromosome number
(Drosophila melanogaster L., for example).
Rogue. A variation from the standard type of a variety or strain.
Roguing. The removal of undesirable individuals to purify the seed stock of a variety.
rRNA. Ribosomal RNA; a class of RNA molecules, coded in the nucleolar organiser,
which have an integral role in ribosome structure and function. However, its role in
ribosome structure and function is not fully understood at present.
Rule of Thumb. A principle of broad application which is not so much (100%) precise
and accurate. However, it is easily learned and easily applied procedure for finding
some value or making some determination with closer approximation. For instance,
given the probability of an infrequent desired event is p, the no. of individuals to be
sampled to give 95% chance of at least one occurrence of this event will be ~ 3/p.
Thus, if the probability of an event (aa in F 2 generation from an F 1 hybrid, Aa) is¼, it
can be shown that the actual number of individuals to be sampled is 11.9, whereas
3/p=3×4=12, a very close approximation.
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S. Svedberg unit; a unit of sedimentation velocity that is commonly used to describe
molecular units of various sizes as sedimentation velocity is proportional to the size.
S 0 . Symbol used to designate the original selfed plant.
S-allele. Any allele of those genes which control incompatibility in plants. The various
alleles of the incompatibility may be represented by S 1 , S 2 , S 3 , etc. S F -alleles belong
to the same allelic series but which control self-fertility.
SPhase.The part of interphase of the cell cycle in which DNA synthesis occurs.
S 1 , S 2 , S 3 … Symbols used to designate first, second, etc. generations after selfing an
ancestral plant (S 0t ).
Sample. A finite series of observations taken from a population. Since it is not feasible to
study all the individuals of a population owing to its large size and hypothetical
nature, samples are almost always taken to represent the entire population.
Sampling. The process of selecting a part of the population to represent it as a whole.
Sampling Distribution. A process through which different sample values are put
together for further study.
Sampling Error. Deviation of a sample value from the true value owing to the limited
size of sample.
Sampling Variance (of mean). The square of the standard deviation of the sample means
(that is, standard error).
Satellite. A terminal section of a chromosome that is separated from the main body of the
chromosome by a narrow constriction.
Satellite Chromosome. A chromosome that seems to be an addition to the normal
genome.
Satellite DNA. Any type of DNA having highly repetitive nucleotide sequence. It was
formerly defined as the DNA forming a satellite band after cesium chloride density
gradient centrifugation.
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Saturation Mutagenesis. Induction and recovery of large numbers of mutations in one
area of the genome or in one function, in the hope of identifying all the genes in that
area or affecting that function.
Savannas. Tropical/sub-tropical grasslands usually interrupted with scattered patches of
shrubs/trees. These occur in areas in which warm-rainy season is interrupted by a
long, cool season dry spell. This long dry spell favours large seeded annuals as well as
plants with rhizomes, tubers, stolons, bulbs, etc.
Science. The systematic and real knowledge about a thing. The word has been derived
from a Latin word – sciencia – that means learning or knowledge in its widest sense.
The science pursued for the sake of its own existence is called basic science.
However, the science that becomes a means of producing material necessities of a
society is referred to as applied science.
Second Division. The second of the two divisions of a normal meiosis; also called
equation division.
Second Filial Generation. The progeny generation obtained by selfing or crossing of F 1
generation plants. It is designated by F 2 . Subsequent generations obtained by selfing
this F 2 generation plants are called F 3 , F 4 , …., etc.
Secondary Centres of Origin. The areas having appreciable extent of genetic diversity
of a plant species. Notice that these areas are found far removed from the primary
centres of origin of crop plants. Now the concept of centres of origin has bee revised
(see centres of origin).
Secondary Gene Pool. Genetic resources that include biological species having some
barriers of crossability with the cultivated species (primary gene pool). Chromosome
pairing at meiosis is not normal. However, special techniques (e.g., embryo rescue)
can overcome barriers of crossability and lead to normal seed set.
Secondary Species. (See digenomic species).
Second Cycle selection. A method of improving inbred lines. It consists of crossing two
inbred lines that complement each other in desirable attributes accompanied by
selection for desirable recombinations in the segregating generations. Also called
pedigree selection.
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Second Site Mutation. The second mutation of a double mutation within a gene. In many
cases, the second-site mutation suppresses the first mutation resulting in wild-type
phenotype of the double mutant.
Secondary Structure (of protein). A spiral or zigzag arrangement of the polypeptide
chain.
Secondary Trisomics. Trisomics with a secondary or isochromosome as the additional
one (2n + one isochromosome). Total number of secondary trisomics is equal to the
diploid chromosome number of the species. Only a few trisomics has been reported in
barley.
Secondary Triticales. See Triticales.
Sector. An area of tissue with a distinct phenotype from that of the rest of the tissue.
Sectorial. Of a chimera in which the genetically distinct tissues are cross-sectionally
arranged as sectors of a circle.
Sedimentation. The sinking of a molecule under the opposing forces of gravitation and
buoyancy.
Seed. A mature ovule with normal coverings. A seed consists of the seed coat, embryo,
and in certain plants, an endosperm.
Seedling Resistance. A category of resistance that is expressed in the early stage of
plants and may persist even in the later growth stages.
Segment. A portion of a chromosome taken as a unit under given circumstances.
Segregation. Separation of paternal from maternal chromosomes at meiosis and
consequent separation of different alleles from each other leading to the possibility of
recombination in the offspring. It may also be the production of two separate
phenotypes, corresponding to two alleles of a gene, either in different individuals
(meiotic segregation) or in different tissues of the same individual (mitotic
segregation).
Selection. A process in which individuals with certain characteristics are favoured in
reproduction. It is the discrimination among individuals in the number of offspring
contributed to the next generation. It is thus a non-random process that causes
individuals with different genotypes to be represented unequally in subsequent
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generations. Natural selection operates only for traits that are necessarily related to the
survival and reproduction of the individuals. Artificial selection, which is practised by
plant breeders, on the other hand, may act mostly in contrary to the natural selection.
However, it is a purposeful process with goals set by breeders. In both evolution and
plant breeding, selection is a primary force that alters the genetic composition of the
population. It is also the result of the process, that is, a plant, line, or strain that
originated by a selection process is referred to as selection. Selection, whether natural
or artificial, acts through differential reproductive rates (fitness) of the genotypes.
Suppose a locus with two alleles in a diploid species. Three possible genotypes are
AA, Aa, and aa (and W AA , W Aa , W aa are their fitness, respectively). Frequencies of
alleles A and a are represented by p and q, respectively. After one generation of
selection, the frequency of A becomes p′. Thus p′ =p(W A /W), whereas W A is mean
fitness of A alleles (= pW AA +qW Aa ), and W is the mean fitness of the population (=
p 2 W AA + 2pqW Aa +q 2 W aa =pW A +qW a ). Therefore, if (W A /W) > 1.0, p′ is larger
than p and vice-versa. Thus, the change in the gene frequency after one generation of
selection can be given by p =(p′- p)=[pq(W A -W a )/W]. This change can be zero, if
either p or q is zero or there is no difference between mean fitness of the two alleles
(W A -W a =0). Selection increases the frequency of superior allele(s); therefore, it is a
process that increases mean fitness of the population.
Selection Coefficient. A quantitative measure of intensity of selection, designated by ‘s’.
It refers to proportional reduction in the gametic contribution of a particular genotype
compared to the standard one, usually most favoured. The contribution of the most fit
genotype is taken as unity and the contribution of the genotype selected against is “1-
s”.
Selection Differential. The average phenotypic superiority of the selected individuals
over the base population of their origin. The s.d., which is symbolised by S, is a
measure of the selection applied.
Selection Intensity. The percentage of the population selected.
Selection Limit. The point at which there is no selection response. The limit is attained
after the selection has exhausted entire genetic variability.
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Selection Pressure. The intensity of selection usually measured by the change of gene
frequency per generation under the influence of selection. Selection pressure is
intense if the intensity of selection is the highest.
Selection Progress. See selection response.
Selection Response. The difference between the mean of the progeny of the selected
individuals and the mean of the original population. Quantitatively, it is expressed as:
Rs = Sd×h 2 = k×σ P ×h 2 . Numerically, it is equal to the genetic gain. It is also called
selection progress.
Selective Advantage. The relative advantage of a genotype when it competes for survival
and reproduction against another in the same population. In artificial selection, s.a. is
conferred by the breeder on some genotypes because they contain some desired
characteristics. For example, modern semi-dwarf varieties of wheat and rice are poor
competitors in mixture, but have been propagated by careful nurturing since they are
high yielders in isolation.
Selective Neutrality. A situation in which different alleles of a gene are adaptively
neutral, that is, each allele confers equal fitness to the corresponding genotypes. For
example, under situation of selective neutrality, both homozygotes (AA and aa) and
the heterozygote (Aa) will have equal fitness. This condition gives rise to a passive or
neutral equilibrium instead of a stable one such that an allele frequency p is as good
as any other q. Therefore, the presence of a widespread polymorphism is not
necessarily the evidence for superior heterozygotes.
Selective System. An experimental technique that enhances the recovery of specific but
usually rare genotypes.
Selective Value. The relative survival value and the reproductive ability of a genotype in
a population in any given environment; also called adaptive value, fitness, biological
fitness or Darwinian fitness.
Self Assembly. The ability of certain multimeric biological structures to assemble from
their component parts through random movements of the molecules and formation of
weak chemical bonds between surfaces with complementary shapes.
Self-fertile. Capable of setting seed following fertilization after self-pollination.
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Self-Fertilization. Fusion of male and female gametes from the same individual.
Self-Fertility. Capability of producing seed upon self-fertilization.
Self-Incompatibility. Genetically controlled physiological hindrance to self-fruitfulness.
Self Pollination. The transfer of pollen grains from an anther of a flower to the stigma of
the same flower or of different flowers of the same plant or clone. Its genetic
consequence is the same if different plants with the same genetic make-up are crosspollinated
(e.g., cross pollinating members of a pure line, of a heterozygote, Aa, and
of a clone).
Self-sterile. Incapable to set seed due to failure either to produce functional gametes or to
complete fertilization after self-pollination.
Semiconservative Replication. The established model of DNA replication in which each
of the two parental strands acts as a template for the synthesis of a new strand so that
each daughter DNA molecule has one parental strand and one newly polymerised
strand.
Semigamy. An abnormal type of fertilisation in which a male gamete enters into the egg
cell but there is no karyogamy (no fusion of nuclei). Both sperm and egg nuclei divide
independently so that the embryo contains separate sectors of paternal and maternal
tissues. It can be utilised to produce haploids in cultivated cotton.
Semi-isolation. Incomplete isolation between two populations, allowing incidental gene
flow.
Semisterility. A condition in which plants are characterised by having reduced number of
viable gametes and hence reduced fertility. Plants heterozygotic for certain types of
chromosome aberrations (e.g., inversion/translocation heterozygotes) usually show
this condition. Nevertheless, plants show sterility for male gametes more frequently
than for female ones.
Sequence Tagged Site. A relatively small sequenced region of a cloned genomic
fragment that can be used by a computer to align the cloned fragment into a contig.
Sex. A complex developmental character that refers to the state of being male and female.
It can be the sum total of contrasting and complementary characteristics exhibited by
male and female (as in humans and other higher animals and a few higher plants), +
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or – (in case of physiological differences), donor or recipient (as in prokaryotes)
individuals or clones within the same species and their capacity to perform
amphimixis and genetic recombination through reproduction. The biological
divergence of sex may be inherent (that is, genetic/chromosomal) or impressed on it
by its environment.
Sex Cell. A gamete.
Sex Chromosome. A chromosome whose presence or absence is correlated with the sex
of the bearer owing to its role in the sex determination, for example, XX-XY system
in humans or WW-WZ system in birds.
Sex Controlled Traits. Hereditary traits that appear more frequently in one sex than in
the counterparts. For example, pattern baldness occurs more frequently in men than in
women. The gene is located in the autosomes. Its expression is influenced by the
specific hormone. Also called sex-influenced traits.
Sex Cycle. The alternation of karyogamy and meiosis in eukaryotes.
Sex Determination. Any process by which sex of an individual is established. The
mechanism or process may be chromosomal, genetic or environmental.
Sex Limited Traits. Traits that are limited to one sex only. However, genes conditioning
such traits are located on the autosomes of both sexes (e.g., genes influencing the
milk yield in dairy cattle).
Sex linkage. The location of a gene on a sex chromosome, for example, X-linkage.
Sex Linked Traits. Traits governed by the genes carried on the specific sex chromosome
(usually X-chromosome), colour blindness, for example.
Sex Mosaic. Gynander/gynandromorph.
Sex Ratio. The number of males per 1000 females at zygote formation (primary sex
ratio), at birth (secondary sex ratio), or at sexual maturity (tertiary sex ratio).
Sexual. Of all processes involving both meiosis and fertilization by means of which
sexual reproduction and genetic recombination are achieved.
Sexual Polyploidization (Mendiburu and Peloquin 1970s). A process of obtaining a
polyploid via fusion of unreduced female and male gametes. If the fusion occurs
between an unreduced gamete (2n) and a normal one, it is called unilateral sexual
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polyploidization. In case, both the fusing gametes are unreduced ones, the process is
termed bilateral sexual polyploidization. Both types have been reported in the
progeny derived from crosses between Solanum spp. Sexual polyploidization via 2n
gametes is a distinctly separable process from somatic doubling. It has the advantage
of providing for heterosis, variability, minimal inbreeding, high fertility and
maximum heterozygosity (new intra- and inter-locus interactions). For example,
tetraploids derived from diploids via somatic doubling with colchicine have the same
or low yield compared to their undoubled counterparts and were inferior to tetraploids
derived from bilateral sexual polyploidization in potato.
Shattering. Falling out and the loss of seeds from a plant before harvest or during
harvesting operations. Resistance to shattering is important to prevent the loss in
crops like soybeans, mungbeans, and many other crops.
Shift. A chromosomal structural change resulting in the transposition of a chromosome
segment to another place in the same chromosome. It may be homo- or
heterobranchial depending on whether transposition occurs within the same or
different arms of the chromosome.
Shuttle Breeding. A breeding approach that aims at speeding up advancement of
segregating materials by moving the germplasm rapidly from place to place. Although
the primary objective is to get two or more generations per year, a valuable side effect
results in terms of incorporation of insensitivity to photoperiod in the selected
materials. This system was initially followed in breeding for stem rust resistance. It
led to selection of photo-insensitive wheat genotypes besides permitting wide-spread
adoption of CIMMYT cultivars. Thus wide-testing strategy of CIMMYT led not only
to the development of many widely grown wheat varieties, but also led to the concept
of megaenvironment and the use of very wide testing in many other crop species.
Shuttle Vector. A vector (for instance, a plasmid) constructed in such a way that it can
replicate in at least two different host species, allowing a DNA segment to be tested
or manipulated in several cellular settings.
Shotgun Technique. Cloning a large number of different DNA fragments as a prelude to
selecting one particular clone type for intensive study.
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Sibs. Progeny of the same parents derived from different gametes. Also called full sibs.
Half-sibs are progeny with one parent in common. Also called siblings.
Sib Mating. The intercrossing of the progeny of the same parents, that is, brother-sister
mating. It is the closest form of inbreeding possible in self-incompatible species such
as alfalfa.
Sickle-cell Anaemia. A potentially lethal human disease caused by a mutation in a gene
that codes for the oxygen-transporting molecule, haemoglobin. Haemoglobin, which
is a conjugated protein and an integral part of red blood cells, is made up of four
polypeptide chains (2α and 2β chains). A single gene mutation causes alteration at 6 th
amino acid position in β-globin chain wherein amino acid valine is substituted for
glutamic acid [codon change: GAG (glutamic acid)→ GTG (valine)]. The altered
molecule causes red blood cells to be sickle-shaped. This trait is inherited as an
autosomal recessive.
Silencer. A cis-regulatory sequence that can reduce levels of transcription from an
adjacent promoter.
Silent mutation. Mutation in which the amino acid sequence and function of the protein
is unaltered. For example, a single gene mutation can alter the nucleotide sequence of
DNA in such a way that the altered codon encodes the same amino acid in a
polypeptide chain (CGU ↔ CGA; both coding for the same amino acid, arginine). As
amino acid sequence does not change, the mutation goes on unnoticed, that is,
silently.
Simple Recurrent Selection. A system of breeding in which the population for next
cycle of selection is synthesized from intercrosses made in all possible combinations
from a number of desirable genotypes (or their selfed progeny) of the original
population. Since testcrosses are not made, the effective use of simple recurrent
selection is restricted to characters with high heritability that can be judged visually or
by simple breeding tests (e.g. resistance to leaf blight in corn).
SINE. Short Interspersed Element; a type of small repetitive DNA sequence found
throughout the eukaryotic genome.
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Single Cross. An F 1 hybrid between two different inbreds. The number of possible
single crosses involving n inbreds will be [n.( n-1 ) / 2 ]. The current trend in breeding
hybrid maize is to develop single cross hybrid as it displays heterosis twice as much
as double cross hybrids.
Single Pod Descent Method. A method of handling segregating generations in which F 2
plants and their progenies are advanced rapidly in succeeding generations by seeds
from a single pod. Evaluation of progeny rows start in F 6 generation. (Also see single
seed descent method).
Single Seed Descent Method (Goulden 1939). A modification of bulk population
breeding in which F 3 and succeeding generations are advanced from single seeds of
each harvested plant of the previous generations followed by progeny evaluation from
F 6 /F 7 . Theobjectiveistomaintainthedescendants from a maximum number of F 2
plants during segregating generations. It is usually accepted that this method is quite
convenient, timesaving, and economical and is able to maintain considerable
variability. However, the inherent drawback attached to this method is that a number
of superior worthwhile genotype may be lost at any stage of advancement.
Single Test. Evaluation of genotypes performed only once. This does not provide an
accurate picture of the genetic variance as it contains various interaction variances in
addition. However, progressively more and more of the interaction variances can be
identified and removed as more and more locations and times are sampled.
Single Factor Experiments. Experiments with varying levels of a single factor while all
others are kept constant. For example, most crop variety trials are single-factor
experiments because the variety is a single factor and the factor levels (that is,
treatments) are the different genotypes/varieties. Only the variety differs from one
experimental plot to another while all management factors are applied uniformly to all
plots.
Sire. Themaleparentinmammals.
Sister Chromatids. Chromatids derived from the same chromosome through duplication.
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Sister Chromatid Exchange. An event similar to crossing-over that can take place
between sister chromatids at mitosis or meiosis. It has been detected in harlequin
chromosomes.
Site. The smallest unit in a gene capable of independent mutation. The entity is a single
nucleotide. Also called mutational site.
Site Directed Mutagenesis. A process of introducing specific mutations into the
sequences of a cloned gene in order to assay the resulting phenotype to make it
possible to ascertain the function of the gene. Site directed mutagenesis based on
polymerase chain reaction is the most popular.
Site Specific Recombination. Recombination between two specific sequences that need
not be homologous. It is mediated by a specific recombination system.
Skewness. The departure from symmetrical spread of the frequency curve around the
central value. Positive skewness enhances value of the constant of selection intensity
(> 2.06 at 5% SI) whereas the negative skewness retards it. Therefore, the data must
be tested for normal distribution while making prediction for genetic advance. In case
of skewness, genetic advance is overestimated (or underestimated).
Soma. The body of an organism aside from the germ line cells.
Somaclonal Variation (Larkin and Scowcroft 1983). Tissue culture induced variation
occurring within a clone of cells. Such variations may be developmental or heritable
(stable). If stable, it may provide for broadening the genetic base and thus obtaining
incremental improvement in the commercial cultivars, more particularly in clonally
propagated crops. In India, Pusa Jaikisan (a somaclone of Varuna variety of Indian
mustard) and B-13 (a somaclone of a medicinal plant, Citronella java) havealready
been released to farmers for general commercial cultivation.
Somatic. Referring to diploid body cells (2n), normally with one set of chromosomes
coming from the male parent and the other from the female one.
Somatic Cell. A “body cell” that is not destined to become a gamete. Its genes will not be
passedontofuturegenerations.
Somatic Cell Genetics. The genetics of somatic or body cells. It is the asexual genetics,
including the study of somatic mutation, assortment, crossing-over, and cell fusion.
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Somatic Mutation. A mutation in a somatic or body cell.
Somatic Embryo. The embryo of somatic origin. It differs from zygotic one in lacking a
well-defined hypocotyl. It also does not experience desiccation and dormancy; these
properties seem to be imposed by accumulation of abscisic acid that, in turn, is
presumably provided by the maternal tissues (in the zygotic embryo).
Somatic Embryogenesis. The genesis of embryos from somatic cells through tissue
culture technology. Such embryos are desiccated and encapsulated to form artificial or
somatic seeds.
Somatic Seed. A somatic embryo derived through tissue culture and encapsulated in a
suitable matrix (gel) with nutrients, growth regulators, and antibiotics necessary for
the development of a complete plant. It is also called synthetic or artificial seed. It can
be stored for several years and can be sown like natural ones.
Somatic Segregation. The segregation at mitosis of genes giving rise to variegation or
sectoring.
Somatoplastic Sterility. A condition of embryo abortion following self-fertilization due
to disturbances in embryo-endosperm relationship (e.g., alfalfa). This condition
obviously favours heterozygosity.
Sorts. The different botanical forms of a species.
SOS Repair. Save our Soul Repair; a process or mechanism of repairing damage to
DNA. It is an error-prone process whereby gross structural DNA damage is
circumvented by allowing replication to proceed past the damage through imprecise
polymerisation. A biological system resorts to this mechanism of DNA repair when
all other such systems fail. As it is an error-prone and imprecise mechanism,
phenotypic expression of repaired DNA may be altered.
Southern Blot. Transfer of electrophoretically separated fragments of DNA from a gel to
an absorbent sheet such as paper. This sheet is then immersed in a solution containing
a labelled probe that will bind to a fragment of interest.
Spacer. The nucleotide sequence of DNA found between genes (compare it with introns
or intervening sequences, which are present within a eukaryotic gene). It may be
transcribed, but is never translated. Its function is still unknown.
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Speciation. The formation of a new species including the development of species
diversity. It refers to the origin of a group of individuals capable of living in a new
way and at the same time acquiring some barriers to genetic exchange with the
species from which it arose. Once speciation gets completed, each newly formed
species will take an independent evolutionary course; inevitably the species will
become increasingly different as time passes. It occurs when gene flow within the
common gene pool is interrupted by an isolating mechanism. When isolation occurs
through geographical separation of populations descended from a common ancestor,
allopatric (different fatherland) speciation may result that is more common in birds. If
isolation occurs through ecological or genetic means within the same area, sympatric
(joint fatherland) speciation is a possibility as occurs more commonly in parasites. In
plants and in a few group of animals, a different form of speciation is effected through
polyploidy (instantaneous speciation), the doubling of chromosomes, because
polyploid individuals are immediately isolated reproductively from their parents.
Species. A natural biological unit tied together by the sharing of a common gene pool. In
sexually reproducing organisms, a species is a group of potentially interbreeding
populations that are reproductively isolated from any other such groups. Species are
often subdivided into subspecies, races, ecotypes, varieties, or other subspecific
categories.
Specific Combining Ability. The deviation in performance of a cross from average of
all crosses possible among n number of parents. For an accurate measure of SCA, the
tests must be conducted in more seasons and more locations compared to GCA.
Specific Locus Test. A test for detecting recessive mutations in diploids. Normal
individuals treated with a mutagen are mated to testers that are homozygous for the
recessive alleles at a number of specific loci. If markers are related to the seed,
recessive phenotype (in case of mutation) can be visible in the mating-generation
itself [white endosperm colour (ccc) in Indian corn; cc × CC (mutagenised)].
Otherwise, screening is done for recessive phenotypes in the next generation.
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Specific Resistance. Host plant resistance to specific race or biotype of a pathogen. Such
resistance reaction operates usually on a gene (for host resistance)-for-gene (pathogen
avirulence) basis.
Sperm. A male gamete. In animals and plants, it is called spermatozoon and
spermatozoid, respectively.
Spermatid. Any of the four haploid cells arising by meiosis from a single spermatocyte.
Each spermatid gives rise to a spermatozoon without further nuclear division.
Spermatocyte. Any of the sperm mother cells, derived from spermatogonium, which give
rise to spermatids.
Spike. An inflorescence with a more or less elongated axis, along which flowers are
sessile or nearly so.
Spikelet. A unit of inflorescence in the grasses, composed of the glumes, the rachilla, and
the florets.
Spindle. The set of microtubular fibres that seem to move eukaryotic chromosomes
through their attachment to centromere during cell division. Its main constituent is a
protein, called tubulin.
Spindle Poison. Any mitotic poison affecting the formation or function of the spindle and
blocking karyokinesis at metaphase. The prototype of spindle poison is colchicine.
Splicing. The reaction that removes introns and joins together exons to form functional
mRNA transcript in eukaryotes.
Split Plot Design. A design specifically suited for a two-factor experiment that has more
treatments than can be accommodated by a complete block design. In this design, a
factor whose precision of effect could be sacrificed is allotted to the main plot. Then
the main plot is sub-divided into sub-plots to which the second factor is assigned.
Thus each main plot becomes a block for the sub-plot treatments. Therefore,
measurement of the main effect of the sub-plot factor and its interaction with the
main-plot factor is more precise than that obtainable with a randomized complete
block design. Degree of precision, relative size of the main effects, and management
practices required by a factor are considered while assigning a factor to the main or
sub-plot.
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Spontaneous Mutation. A mutation occurring in the absence of mutagens, usually due to
errors in the normal functioning of cellular enzymes (which are normally required for
DNA replication).
Spore. A haploid cell destined to form or act either as a gamete or as the initial cell for a
new haploid individual. In plants and fungi, sexual spores are the haploid cells
produced by meiosis. In certain other fungi, asexual spores are somatic cells that may
act as gametes or initiate the development of a new haploid individual.
Sporocyte. A diploid cell undergoing meiosis to form haploid spores; a spore mother
cell.
Sporophyte. The diploid sexual-spore-producing generation or phase in the life cycle of
plants, that is, the stage in which meiosis occurs.
Sporophytic Incompatibility. A kind of mating wherein incompatibility reaction of the
pollen is governed by the genotype of the parent. The gene for incompatibility has
numerous alleles that may show dominance, individual action or competition in either
pollen or styles. This complex incompatibility reaction leads to (a) frequent reciprocal
differences, (b) occurrence of female sterility, and (c) homozygosity at the locus
concerned (for example, members of crucifereae).
Sport. In plant and animal breeding, an aberrant individual resulting from mutation. Bud
sport is a somatic mutation occurring in a bud.
Spurious Correlation. Associations between two variables, which are purely accidental
or arise due to chance factors. The two variables have no connection whatever.
(Population increase in India and England after 1947, for example).
Square. An unopened flower bud in cotton with its accompanying bracts.
SSLP. Short Sequence Length Polymorphism; the presence of different numbers of
short repetitive elements (mini- and microsatellite DNA) at one particular locus in
different homologous chromosomes. Heterozygotes for such SSLP represent useful
markers for genome mapping.
Stability. The consistency in the performance of a genotype(s) across environments with
respect to a characteristic(s). If genotype-environment interaction is zero, it means the
genotype is stable. Terms such as homeostasis and buffering have also been used to
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describe stability. The mean yield of a variety is the most common description of its
performance. Multiline varieties, bulk populations, composites, or F 3 /F 4 derived lines
varieties are usually more stable than pure line varieties. Stable genotypes, however,
usually have below average yields. Several models have been used to study stability
of varieties. For example, Finley and Wilkinson (1963) used regression as a parameter
of stability. However, in the Eberhart and Russel (1966) model (the most commonly
used model), regression is termed response index and mean square deviations from
regression is referred to as the parameter of stability.
Stability Analysis. An analysis of consistency in performance of genotypes over a wide
range of environments based on certain statistical parameters. If a cultivar shows
consistent performance over a set of environments, it is released for wider cultivation.
For stability analysis, a number of stability models such as Finley and Wilkinson
model (1963), Eberhart and Russell model (1966), Perkins and Jinks model (1968),
Freeman and Perkins model (1971), and the like is available.
Stabilizing Selection (Van der Plank 1963, ’68). The reduction in the fitness of the
pathogen races that carry unnecessary virulence genes. Thus stabilizing selection
should favour simple races, that is, races with least number of unnecessary virulence
genes. In nature, however, there are many cases that contradict the concept of
stabilizing selection. For example, Flor (1971) reported several races of pathogens
(Puccinia sp.) carrying virulent genes not needed to parasitise the host crop. Overall,
stabilizing selection probably is not an important factor in preventing the
development of complex races.
Stable Feedback System. A system that promotes retention of a large number of
favourably interacting genotypes in a heavily inbreeding population. This system is
necessary in such a population as it leads to continuous release of genetic variability
through chance crossing (among the existing genotypes) followed by segregation and
recombination.
Staggered Cuts. The cleavage of two opposite strands of duplex DNA at points near one
another.
Stamen. The pollen-bearing organ in the flower composed of an anther and a filament.
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Staminate flower. A flower bearing stamens but no pistil.
Standard Deviation (Fisher 1913). A measure of variability. This statistic ‘s’ [or
parameter ‘σ’] is defined as the square root of the average squared deviations of the
individual observations from their mean [di 2 /N] ½ . This statistic is used more
frequently in plant breeding experiments. It quantifies the variability of a character in
the same unit in which observations themselves are measured. Thus it is more useful
than variance, which is square of standard deviation.
Standard Error. A statistic, which is the estimate of the standard deviation (parameter).
It is the standard deviation of the sample means.
Standard Type. An arbitrarily specified gene or genotype taken as the basis of
comparison for genetic investigations. The standard genes are designated as ‘+’
irrespective of whether being dominant or recessive. However, the designated
standard types are supposed to be the most frequent types in nature. Also called wild
types.
Staple Length. Also called fibre length; the normal length of a typical portion of the
fibres of a cotton sample.
Statistic. Any function based on sample values. It is the estimate of a parameter made
from a sample. It strictly applies to a sample whereas a parameter to a population.
Statistical Distribution. The array of frequencies of different qualitative or quantitative
classes in a population.
Statistics. A branch of mathematics, which deals with collection, analysis and
interpretation of numerical data.
Stem Cell. The germ cell; an undifferentiated cell capable of giving rise to specialized
daughter cells.
Sterility. A condition in which plant fails to produce functional gametes. It may be
caused by chromosomal aberrations (structural or numerical), gene action or
cytoplasmic effects which may, in turn, lead to abortion or modification of entire
flower-buds, stamens, pistils or may disrupt the development of pollen, embryo sac,
embryo or endosperm.
Stigma. The portion of the pistil that receives the pollen.
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Stolon. A trailing stem, capable of forming roots and shoots from its nodes.
Stoma. A minute opening in the epidermis of a plant organ (as in leaf) through which
gaseous exchanges occur.
Strain. A group of similar individuals from a common origin. It is generally a more
narrowly defined group than a variety. It may also be the progeny of a single plant
irrespective of the mode of pollination. It can also be a pure-breeding lineage, usually
of haploid organisms, bacteria, or viruses.
Strain Building. A broad term used by early forage breeders to include any system of
mating by which a strain is built up from crossing carefully selected plants. Thus it
includes a number of breeding procedures varying from simple mass selection to
methods that resemble recurrent selection.
Strigol. A stimulant produced by roots of host species (sorghum, sugar cane, bajra,
maize, etc.) of Striga (S. asiatica, S. densiflora, and other species). Strigol produced
by such host plants stimulates germination of seeds of Striga spp. This obligate
parasite, after germination, grows towards the host roots, penetrates, and thus uses
host photosynthates. However, host varieties differ in strigol production. Host
cultivars that do not produce this stimulant can avoid Striga establishment. Host
varieties that have hard roots (due to highly sclenchymatised endodermal tissues
along with deposition of silicon) are also likely to be protected. Certain Striga
resistant sorghum varieties have been found to possess phenolic acids in their roots.
Strip Plot Design. An experimental design wherein each block is divided into vertical
and horizontal strips. The design is specifically suited for a two-factor experiment in
which the prime object is to attain a higher degree of precision for measurement of
interaction effects of two factors than that for measuring the main effect of either one
of the two factors. The vertical and horizontal strips are always perpendicular to each
other; however, there is no relationship between their sizes, unlike the case of mainplot
and sub-plot of the split-plot design. The interaction plot is, of course, the
smallest. Thus, in this design, the degrees of precision associated with the main
effects of both factors are sacrificed in order to improve the precision of the
interaction effect.
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Structural Gene. A gene encoding amino acid sequence of a protein.
Structural Genomics. Characterising and locating the entire set of genes in a genome.
Structural Heterozygosity. Heterozygosity for a chromosomal structural change.
Structural heterozygotes are usually characterized by a reduction in fertility due to
production of genotypically defective gametes and unbalanced zygotes.
Style. The stalk connecting the ovary and the stigma.
Subsampling. The process of selecting a random sample at successive stages. The device
is extensively used in sampling on account of the ease in selection and economy of
labour (as in wheat, for example).
Subsistence Agriculture. Agricultural activities performed by resource poor farmers just
to produce food for themselves, usually in harsh, erratic and unpredictable climatic
conditions. In subsistence agriculture, yield at farmers’ fields is many fold less than
what is realized in well-managed agricultural fields; low level of yield results from
low or negligible use of inputs as well as less than ideal agricultural practices.
Resource poor farmers manage subsistence agriculture through strategies based on
spreading of risk through interspecific (inter- or mixed cropping) and intraspecific
(heterogeneous cultivars) diversity.
Subspecies. An aggregate of breeding populations of a given species that inhabit a
geographical subdivision of the species range. It differs from any other such groups
both taxonomically and with respect to certain gene pool characteristics.
Subvital. Of a gene or genotype that reduces the viability of its carriers compared to the
standard type.
Subvital Gene. A gene that causes the death of some proportion (but not all) of the
individuals that express it. It possibly reduces the fitness of the individual carrying
and expressing it.
Sum Rule. A probability rule. The probability of either one of two mutually exclusive
events occurring is the sum of their individual probabilities. For example, with two
dice, probability of either two-fours or two-fives is 1/18 (1/36 + 1/36 = 1/18).
Supercoil. A closed and double-stranded DNA molecule that is twisted on itself.
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Supergene. A cluster of mutually cooperating genes; a group of genes held together
mechanically by various mechanisms and inherited as a single unit. If co-adapted,
such genes, even though not necessarily functionally related, may cooperate to
produce some adaptive characteristic.
Superinfection. Phage infection of a cell that already harbours a prophage.
Supersuppressor. A mutation that can suppress a variety of other mutations. It is
typically a nonsense suppressor.
Super-vital. A gene or genotype which, when expressed, increases the viability of its
carrier compared to the standard type (heterotic effects, for example).
Suppressor. A secondary mutation that can cancel the effect of a primary mutation,
resulting in the wild-type phenotype.
Suppressor Mutation. A mutation that cancels the effects of another mutation. A
suppressor maps at a different site from the mutation it counteracts, either within the
samegeneoratamoredistantlocus. Noticethat different suppressors act in different
ways.
Sustainability. Improving the quality of human life while living within the carrying
capacity of supporting ecosystem.
Sustainable Agriculture. A form of farming which produces sufficient food to meet the
needs of the present generation without eroding the ecological assets and the
productivity of life-supporting system of the future generations.
Susceptible. The characteristic of a host plant such that it is incapable of suppressing or
retarding an injurious pathogen or other factor.
Susceptibility. The inability or incapacity of plants to interfere the normal growth and
development of pests or pathogens.
Sweet Corn. Any type of corn (Zea mays saccharata) that is consumed in immature
stage. It contains relatively high sucrose content (e.g., Madhuri).
Symbiont Hypothesis. A hypothesis that mitochondria and chloroplasts might have
originated as prokaryotic intracellular parasites that established a symbiotic
relationship with the eukaryotic cell. Mitochondria could be the result of a bacterium
parasite, and chloroplasts, the result of a blue green alga (having chlorophyll). In
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evolutionary terms, it is possible that a symbiotic relationship could have evolved into
the present situation, in which these organelles have only a certain degree of
autonomy.
Symbiosis. A kind of inter-species interaction in which both the interacting partners
benefit from each other. Symbiosis literally means living together. Contrary to
protocooperation, this kind of interaction is obligatory, Rhizobium-legume
association, for example. Instances of mutualism (symbiosis) are more likely to
develop between organisms with widely different requirements whereas organisms
with similar needs are more likely to get involved in competition. Each step of
symbiosis is controlled and/or influenced genetically.
Sympatric. Of two or more populations occupying the same geographical area. Such
populations may remain completely separated by different habitat preferences, or by
different habits.
Sympatric Species. Species formation in the absence of geographical isolation by
acquiring mechanisms of reproductive isolation within a panmictic unit.
Syn-0, Syn-I, Syn-II. Symbols for designating the original synthetic population, first
synthetic generation (progeny of Syn-0), and second synthetic generation (progeny of
syn-I)
Syn-II Generation. The panmictic generation following synthesis by crossing a number
of lines having proven combining ability (analogous to F 2 generation).
Synapsis. Close-pairing (point-to-point pairing) between homologous chromosomes at
zygotene sub-stage during meiosis I.
Synaptic Genes. Genes influencing the extent of meiotic pairing.
Synaptonemal Complex. A complex structure that unites homologs during prophase of
meiosis-I. A protein plus a small amount of DNA constitutes the complex structure.
Syncytium. A single cell with many nuclei.
Syndrome. A set of specific characteristics (symptoms) of an abnormal genetic
condition. A genetic syndrome may be transmitted to the offspring as a unit.
Syngamy. Sexual reproduction; the union of male and female gametes leading eventually
to zygote formation.
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Synonymous Substitution. Mutational replacement of an amino acid with one of similar
chemical properties.
Synteny. The description of DNA segments in which the gene order is identical in
different related species. Sufficient mapping of the same genes from mouse and
humans has been done to observe that many blocks of closely linked genes in humans
are in closely linked blocks in the mouse as well. Such blocks of genes are said to be
syntenic. A synteny map shows how the chromosomal locations of the block in one
species relate to the comparable blocks in another. The study of synteny shows that
chromosomal rearrangements have been instrumental in evolution at the chromosome
level.
Synthetic Seed. See somatic seed.
Synthetic Variety. A variety maintained by open-pollination in isolation following its
synthesis by crossing in all possible combinations a number of genotypes having
proven general combining ability. The farmers are usually given this open-pollinated
seed (Syn II generation), which may be used for 3/4 generations without deterioration
in performance.
System. The regularly interacting and interdependent components forming a unified
whole; a set of mutual relationships constituting an identifiable entity, real or
postulational.
Systemic Pressure. Any of the non-random evolutionary pressure (selection, mutation,
migration, etc.).
Systematic Inbreeding. Deliberate mating between relatives generation after generation.
For example, in experimental genetics (especially in plant breeding), systematic
selfing, full-sib, parent-offspring, or any other form of mating between relatives may
be used to increase homozygosity.
SWOT Analysis. An analysis vis-à-vis strength, weaknesses, opportunities, and threats
of a breeding programme.
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Tagging. TheuseofapieceofforeignDNAoratransposontolabelagenesothata
clone of that gene can be identified readily in a library.
Taichung Native-1. A variety of rice introduced from Taiwan in 1966. It heralded the
beginning of cultivation of photoinsensitive rice in India. It was developed from a
cross between Tsai-yuan-chung and Dee-geo-woo-gen.
Tandem Duplication. Adjacent identical chromosome segments (abcdbcdef). Whether
tandem or reverse tandem, duplication supplies additional genetic material capable of
evolving new functions.
Tandem Selection. Successive selection of different types one after another through one
or more cycles. For example, screen a population intensively for resistance to pest A
through one or more cycles of selection, then repeat the same for pest B, and then for
pest C.
Tautomeric Shift. The spontaneous isomerization of a nitrogen base to an alternative
hydrogen-bonding structure/ condition, possibly resulting in a mutation.
Taxon. A group of similar individuals sharing a set of characters that are considered
sufficiently distinctive to be worthy of a formal name.
T-Chromosome. A monocentric chromosome whose terminal segments (T-ends) show
neocentric activity at meiosis.
T-DNA. A part of the Ti plasmid that is inserted into the genome of the host plant cell.
Technology. The application of art and science to the welfare of mankind. In its broadest
sense, it is the applied aspect of art and science collectively. As plant breeding is
concerned with the art and science of improving the heredity of crop plants in relation
to their economic use, it is essentially a technology.
Technology Mission. A technology oriented mission that was launched by the
Government of India during 1986. Initially it included programmes pertaining to
improvement in production of oilseeds, availability of drinking water, and child
immunisation. Later on, several other programmes such as literacy,
telecommunication, dairy development, and enhancement of pulses and sugar cane
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production were also added to the TM. The mission ended by the end of ninth fiveyear
plan.
Telocentric Chromosome. A chromosome having the centromere (placed) at one end.
Telomerase. An enzyme that acts from telomeric end; an enzyme that adds repetitive
units to the ends of linear chromosomes to prevent shortening after replication, by
using a special small RNA as a template. By way of its action, it shortens the length
of a chromosome. Cancerous cells do not have this enzyme.
Telomere. The tip (or end) of a chromosome. It consists of repetitive units of nucleotides,
which possibly confer non-sticky property to the telomere.
Telophase. The last stage in cell division during which daughter chromosomes reach
towards respective poles and daughter nuclei re-form.
Telotrisomics. An otherwise diploid individual carrying a telocentric chromosome as the
extra one (2n + telocentric chromosome). The total number of telotrisomics amounts
to 2n.
Temperate Phage. A phage that can become a prophage.
Temperature-sensitive Mutation. A conditional mutation that produces the mutant
phenotype in one temperature range and the wild-type phenotype in another
temperature range.
Template. Amolecularmold that shapes the structure or sequence of another molecule.
For instance, the nucleotide sequence of DNA acts as a template to control the
nucleotide sequence of RNA during transcription.
Teosinte Theory (Beadle 1939). An accepted theory for evolution of maize. According
to it, modern maize was derived directly by human selection from teosinte (Zea
mexicana L.). Several scientists and findings in the course of time have substantiated
this old theory.
Teratogen. An agent that interferes with normal development.
Terminalization. A process by which chiasma move towards either end of the paired
homologues.
Terminator Codon. A chain terminating stop codon.
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Terminator Technology. A technology that ensures that the plants grown from
genetically engineered seeds will produce only sterile seeds of their own. This is
achieved by selectively programming a plant’s DNA to kill its own embryo. That the
patent that was initially granted to Delta and Pine Land Company (DPL), which was
later on owned by Monsanto, is innocuously titled “Control of Plant Gene
Expression”. However, it has been globally derided as the Terminator Technology.
[The term has actually been coined by Rural Advancement Foundation International
(RAFI), which has been spearheading an international campaign against it]. TT is
biotechnology at its sophisticated best. It uses three constructs of DNA that ensure
that the transgenic plants become terminators that abort their own embryos. The three
constructs are something akin to promotor, inducer and repressor. The technology has
several drawbacks: (1) it breaks plant-to-seed-to-plant to seed cycle, the cycle that
supports most life on the earth, (2) it gives multinational companies an unprecedented
and extremely dangerous capacity to control world’s food supply, (3) the terminator
gene could be transmitted to other cultivated species for some extent of outcrossing
takes place in almost all species, (4) the planting of such strains owing to their
seeming superiority may wipe out biodiversity, (5) the monoculture of such strains
will enhance their vulnerability to insect pests, (6) the ecologists fear that the use of
three gene constructs will adversely affect soil microflora and fauna, (7) the
nutritional aspects of such seeds are also likely to be affected, and (8) the fallout of
the technology will directly affect 90% of the farmers who operate in the traditional
style. Because of these and other limitations, the technology has been banned.
Tertiary Gene Pool. Genetic resources of a crop species that includes distantly-related
wild species compared to secondary gene pool. Hybrids tend to be sterile or lethal due
to abnormality in development of embryo. Successful hybrid embryos can be
produced through techniques of embryo culture, tissue culture and chromosome
doubling or bridge species.
Tertiary Structure (of protein). The folding or coiling of the secondary structure to form
a globular molecule.
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Tertiary Trisomics. Trisomics carrying an extra tertiary or translocated chromosome.
Considerable number of tertiary trisomics was developed by Ramage and coworkers
(1961) in barley. Total number of TTs may be infinite.
Test Cross. A cross of an F 1 hybrid to its corresponding recessive (homozygous) parent
to test for homozygosity or linkage. In plant breeding, it is the cross of an individual
of unknown genotype to an individual known to carry only recessive alleles of loci in
question for the purpose of determining the genotype of the unknown parent. The
term is also extended (in plant breeding) to include a cross of an individual or an
inbred line with an appropriate tester to evaluate combining ability of that individual
or inbred line.
Test of Significance. A statistical test designed to distinguish differences due to sampling
error from differences due to discrepancy between observation and hypothesis. The
test of significance is necessary in case of biological experiments because these are
affected by considerable amount of uncontrollable variation. But in sciences like
physics and chemistry, uncontrolled variation is so small compared to the magnitude
of the observation that the mean of two or three repetitions of an observation comes to
the real one.
Tester. An inbred line, single cross, three-way cross, double cross or an open-pollinated
variety used to test combining ability. A desirable tester is one, which provides
maximum information about the performance of the line(s) likely to be used in hybrid
breeding. It should also be that is simple to use. The choice of tester is decided
according to the desired objectives; if the object is to determine general combining
ability, an open-pollinated variety is obvious choice. However, if the objective is to
find a replacement for an existing inbred line in a single cross, the most appropriate
tester will be the second inbred line of that single cross. In genetics, an individual
homozygous for one or more recessive alleles thatisusedinatestcrossiscalleda
tester.
Tetrad Analysis. The use of tetrad (products of meiosis) to study the behaviour of
chromosomes and genes in meiosis.
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Tetrad. A group of four. It may be four homologous chromatids in a bundle visible
during meiotic prophase and metaphase. It may also be the four haploid product cells
of a single meiosis. Dyad and octad mean a group of two and eight cells or
chromosomes, respectively.
Tetramer. A protein consisting of four polypeptide subunits, for example, haemoglobin.
Tetraploid. A cell with four basic (x) sets of chromosomes. An organism composed of
such cells is also referred to as a tetraploid. If all the sets are identical, the individual
is said to be an autotetraploid (AAAA). Allotetraploids (AABB) have dissimilar
genomes or sets of chromosomes. They are also sometimes called amphidiploids.
Tetrasomic. An otherwise diploid cell having a particular chromosome present four
times, thus having 2n+2 chromosomes; also the organism composed of such cells.
Tetrasomics often behave more regularly than the other aneuploids with odd
chromosome number. The pairing possibilities among the four homologs and gametic
output are almost similar to those of tetraploids (4x).
Textbook. A book giving a systematic presentation of the principles and vocabulary of a
particular subject.
Three-line System. A widely used system for producing hybrid varieties involving A, B,
and R lines.
Three-point Testcross. A testcross in which the tester parent has three homozygous gene
pairs, for example a cross between AaBbCc × aabbcc.
Three-way Cross. A cross between a single cross and an inbred line [(A × B) ×C]. The
number of possible three-way hybrids involving n inbreds will be [n. (n-1). (n-2) / 2].
Threshold Effect. A term usually applied to traits with a polygenic basis that develop
when the dosage of contributory alleles exceed a critical value in particular
environments (sometimes used to explain all-or-none phenomena based on
polygenically inherited characters such as resistance vs. susceptibility to some
diseases); the environmental condition which obscures expression of a trait. It allows
character expression below/above a particular environmental condition. It is discerned
that a character develops through a series of physico-chemical reactions, which in turn
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is dependent upon genetically controlled threshold - the point at which a stimulus
(gene) produces a response (character), e.g., temperature sensitive albinism.
Thymidine. The nucleoside having thymine as its base.
Thymine Dimer. A pair of chemically bonded adjacent thymine bases in DNA. If
present, this lesion is repaired by cellular processes. However, errors may be left
during DNA repair that ultimately results in mutation.
Thymine. A pyrimidine base that pairs with adenine in a DNA double helix. It has a
single ring structure compared to its pairing partner, which has two.
Ti Plasmid. Tumour Inducing Plasmid; a vector for gene transfer to plant cells. It is a
circular plasmid that enables the bacterium to infect plant cells. It is contained outside
genome of Agrobacterium tumefaciens and causes crown gall of plants. It has a vir
region (responsible for virulence towards the host) and a T-DNA region that is
transferred to the host along with the gene intended to be transferred. It has selectable
marker genes that help in selecting transformed cells.
Tift 23A (Burton 1965). A cytoplasmic male sterile line containing A 1 cytoplasm. The
male-sterile condition was found due to homozygous recessive alleles of a nuclear
gene pair ‘ms 1 ms 1 ’, besides sterile cytoplasm. It was used previously as a female
parent to develop hybrid bajra (HB series). Its use stopped after 1970 when it gave
way to downy mildew.
Tissue. A mass of cells woven together with their intercellular substances.
Tissue Culture. In vitro culture of cells, tissues, organs, etc on an artificial medium
under aseptic condition.
Tissue Specific Expression. Theexpressionofageneinahighereukaryoteinaspecific
and reproducible subset of tissues and cells during development.
Tolerance (Insect Pests). The ability of the host to grow and reproduce or to repair insect
injury to a marked degree in spite of supporting an insect population nearly equal to
that damaging a susceptible host. Tolerance, which is measured through yield
depression being minor, is polygenically determined. A tolerant and non-tolerant
cultivar would be indistinguishable when rated for the number of insects present, but
they would less affect the tolerant one. Tolerance differs from non-preference and
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antibiosis in its mechanism. The latter two require an active insect response or lack of
response. However, the former (tolerance) response requires a thorough
understanding of the ways in which plants are injured by insects as well as the ways in
which plants may repair the damage. Tolerance is more subject to variation as result
of environmental fluctuations more than non-preference and antibiosis. The age or
size of the plant and size of the insect population strongly influence the degree of
tolerance. Since tolerance does not involve prevention of insect infestation or injury to
the plant, some researchers prefer not to include it as a type of resistance.
Tolerance (Disease). An inherent or acquired ability of the host to endure a disease or
parasite. However, Virologists use the term to imply lack of symptom expression.
Tolerant cultivars show few or no disease symptoms although the virus is present.
Tolerance, like avoidance or resistance, helps the host to cope with the parasite. It
enables the host to suffer low biological damage compared to susceptible ones. To
evaluate tolerance, it is imperative to compare the damage inflicted by equal amounts
of parasite at the same developmental stage of each host genotypes. However, it is
difficult to measure because it is confounded with partial resistance and disease
escape (because of early maturity). A tolerant genotype should be biologically more
efficient and productive. When the quality of product depends on the absence of
disease symptoms, tolerance would be of little worth. For example, tolerance to smut
pathogens in cereals is of no value because they attack kernels.
Tonoplast. A unit membrane bordering the vacuole of plant cells. It has characteristics
similar to those of plasma membrane excepting greater permeability barrier.
Top Cross Test. A test for GCA of selected individuals/inbred lines or clones by
evaluating their progenies derived from crossing them with a single tester variety
(usually an open-pollinated variety).
Top Cross (Davis 1927). In corn, an inbred-variety cross; a cross between a selection,
line, clone, etc., and a common pollen parent which may be a variety, single cross, or
a number of elite inbred line. The common pollen parent is called the top cross or
tester parent. It is used to test general combining ability of inbreds or selections. Top
crosses to an open-pollinated variety, do in fact, reduce the burdensome labour of
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making [n.(n-1)/2] single crosses and their evaluation for estimating GCA. However,
single crosses are more efficient than top crosses in estimating GCA, and estimates of
additive effects of GCA should be based on as many single cross combinations as
possible.
Topography (of a gene). The differences in the properties of the sub-elements of a gene.
Topoisomerase. An enzyme that can cut and reform polynucleotide backbones in DNA
to allow it to assume a more relaxed configuration.
Topology (of a gene). The manner in which the component parts of a gene are
interconnected.
Totipotency. The ability of a cell or a tissue to develop into a complete adult individual
after proceeding through all the stages of development.
Totipotent. A cell having the property of totipotency. It is also the state of a cell lineage
that is able to give rise to all possible cell fates found within a given organism.
TPS Technology. A technology related to the production of botanical seeds through
sexual reproduction in cultivated potatoes. The technology offers low cost on seed,
ease in storage and transportation and lower incidence of disease and insect pests. It is
an alternative technology for crop production in areas where quality seed tubers
cannot be produced due to unfavourable agroclimate and transportation of seed
potatoes from distant places is very expensive. It is especially suited to small and
marginal farmers of Bihar, Orissa, Maharashtra, Karnataka, and North-Eastern States.
Thus far three TPS hybrids have been released (TSS C-3, HPS I/13 and 92 PT-27) for
general cultivation. In a clonally propagated crop like potato, F 1 generation itself is a
segregating one, individuals of such hybrids are heterogeneous.
trans Conformation. A conformation of having the arrangement a 1 +/+a 2 in a
heterozygote with two mutant sites within a gene or gene cluster.
trans-acting Factor. A diffusible regulatory molecule (almost always a protein) that
binds to a specific cis-acting element.
Transcription Factor. A protein that binds to a cis-regulatory element (for example, an
enhancer) and thereby, directly or indirectly, affects the initiation of transcription.
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Transcription. The synthesis of a RNA molecule using one of the two strands of DNA as
a template.
Transduction. A mechanism of unidirectional recombination in bacteria. It refers to the
movement of genes from a bacterial donor to a bacterial recipient with the use of a
phage as the vector.
Transfection. The process by which exogenous DNA in solution is introduced into
cultured cells.
Transformation. The directed modification of a genome by the external application of
DNA from a cell of different genotype. It may also be the conversion of normal higher
eukaryotic cells in tissue culture into a cancer-like state of uncontrolled division.
Griffitth (1928) initially discovered this mechanism in Diplococcus pneumoniae and
as such called it Griffith effect. In his experiment, transfer of genetic material from
one bacterium to another occurred without involving any other intermediate organism
(unlike transduction). A bacterial cell incorporating in itself genetic material from
medium is also referred to as transformation. Now-a-day, the term is extended to
include collectively introduction, integration, and expression of foreign genes in the
host.
Transformational Theory. An obsolete and disproved theory to account for the process
of evolution. The proponents including Jean-Baptiste Lamarck (1744-1829)
postulated the same transformation of quality for each individual organism within a
species, much as every individual in its lifetime changes from an infant to an adult.
According to this view, these individual changes are passed on to the offspring.
Transgene. A gene that has been modified by externally applied recombinant DNA
techniques and reintroduced into the genome by germ-line transformation.
Transgenic Plants. Plants that carry additional stably integrated and expressed foreign
gene(s) usually transformed from unrelated organisms. The whole process of
introduction, integration and expression of foreign gene(s) in the host is referred to as
transformation.
Transgressive Segregation. Appearance in segregating generations (F 2 , or F 1 in case of a
cross between clones) of individuals falling outside the parental range with respect to
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some characters. For instance, if we cross aaBB and AAbb, we are likely to get AABB
and aabb in F 2 generations. These genotypes may have enhanced or retarded
phenotypic expressions compared to their parents.
Transient Diploid. The stage in the life cycle of predominantly haploid fungi (and algae)
during which meiosis occurs.
Transition. A type of nucleotide-pair substitution involving the replacement of a purine
with another purine (adenine with guanine or vice-versa) or of a pyrimidine with
another pyrimidine (cytosine with thymine or vice-versa). This substitution may occur
due to a spontaneous error or mispairing between base pairs under the influence of a
physical or chemical mutagen. It ultimately results in mutation (GC → AT transition,
for example).
Translation. The ribosome-mediated production of a polypeptide whose amino acid
sequence is derived from the codon sequence of an mRNA molecule.
Translocation. The change in the position of a segment of a chromosome to another
location in the same or a different chromosome; the relocation of chromosome
segment in a different position in the genome. If in the same chromosome, it is called
simple translocation.
Transmission Genetics. The study of the mechanisms involved in the passage of a gene
(and thus a trait) from one generation to the next.
Transpiration Efficiency. The ratio of growth (net production) and water transpired
usually expressed as grams of dry matter produced per 1000 grams of water
transpired. Most species of agricultural crops have transpiration efficiency of 2 or
less. However, drought resistant crops, such as sorghum/ millets may have
efficiencies of 4.
Transposable Genetic Element. A general term for any genetic unit that can insert into a
chromosome, exit, and relocate. It includes insertion sequences, transposons, some
phages, and controlling elements.
Transposition. The transfer of a chromosome segment to another position owing to
intra- or interchromosome structural changes. See translocation/ shift.
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Transposon. A segment of genetic material that is capable of changing its location in the
genome, especially when it contains genetic material controlling functions other than
those needed for its insertion. It is a mobile piece of DNA that is flanked by terminal
repeat sequences and typically bears genes coding for transposition functions.
Transversion. A type of nucleotide-pair substitution in which a pyrimidine replaces a
purine or vice versa (thymine for guanine or adenine for cytosine), GC → TA
transversion, for example.
Treatments. The objects of comparison in an agricultural trial. Also called entries.
Triallel. A cross product of three parents, (A × B) ×C; also called three-way cross. The
number of three-way crosses or triallels possible among n inbred lines is given by a
formula: (½) × n (n-1)(n-2).
Triallel Analysis (Rawlings and Cockerham 1962). An analysis of triallels or three-way
crosses. In addition to information provided by the diallel analysis, it gives out
information regarding epistatic component of variances and order effects.
Trioecious. Of plants having male, female, and hermaphroditic flowers on different
individuals.
Tripartite Hypothesis (Mangelsdorf and Reeves 1939). A hypothesis proposed to
account for evolution of maize. According to this hypothesis: (a) wild maize was a
form of pod corn native to the low lands of South America, (b) teosinte originated
from a cross between cultivated maize and tripsacum, and (c) modern varieties of
maize arose from crosses between maize and tripsacum or teosinte. However, recent
advances in modern genetics and cytogenetics do not substantiate any of these points.
Triplet. The three-nucleotide (base) pairs that constitute a codon.
Triploid. A cell having three chromosome sets (3x) or an organism composed of such
cells. Triploids are usually autopolyploids. They are characteristically sterile due to
pairing irregularities during meiosis. Although trivalent can form, bivalents usually
occur leaving one homologue of the third set unpaired. The asynaptic chromosome
can randomly go to either of the poles. This happens for every chromosome
threesome; thus probability of obtaining functional gametes (2x, x) gets highly
reduced as the number of chromosomes in the basic set increases. The probability that
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a triploid will produce functional gametes (2x, x) is: (½) x-1 , where x is the no. of
chromosomes in basic set. For example, the triploid banana contains 11 chromosomes
in a basic set; hence, the probability of formation of formation of functional gametes
is: (½) 10 . Thus the probability that a triploid will form a viable seed is the product of
probabilities of functional male and female gametes, that is, (½) 11-1 × (½) 11-1 =
(½) 20 = 0.0001%. Seedlessness of triploidy has also been exploited in watermelons.
Tripping. A mechanism of pollen dispersal by an explosive action usually effected by
bees as in alfalfa. In the lucerne flowers, stamens and stigma are enclosed and held
under considerable tension by keel. When this tension is released by mechanical
pressure supplied by honeybees, pollen is dispersed. Lucerne flowers are usually
tripped by bees, although automatic tripping by wind, rain or heat may occur
occasionally. A flower may be tripped artificially through application of light pressure
on keel by the point of a pencil.
Triple Test Cross Design (Kearsey and Jinks 1968). A mating design that involves
crossing of randomly selected each F 2 individual to the F 1 individual in addition to its
inbred parents. Thus it is an extension of NCD-III, which excludes crossing with the
F 1 ’s. It is the most informative mating design for the random mating population. It
gives a test of significance for the presence of epistasis (that cannot be obtained with
NCD-III).
Trisomic. An organism diploid except for one kind of chromosome which is present in
triplicate, hence, having 2n + 1 chromosomes. The cell of the organism may be
characterized by the presence of a trivalent during meiotic prophase I. Trisomics tend
to be somewhat more stable genetically than monosomics (see aneuploids).
Nonetheless, the possibility that the unpaired chromosome (which may be the case)
may lag at meiotic anaphase cannot be ruled out. The gametic output of a trisomic
under the conditions of various pairing possibilities can be predicted just like triploids
(3x).
Triticale (Rimpau 1888). A new crop (man-made cereal) that was obtained by combining
the genomes of wheat and rye (Secale cereale). The principal objective in
synthesising triticale (X Triticosecale Wittmack) was to combine the quality of wheat
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and ruggedness of rye. The hexaploid triticale (obtained from tetraploid wheat, T.
turgidum)) has better agronomic traits than the octaploid form (obtained from bread
wheat, T. aestivum) and is generally favoured for this reason. Tetraploid forms
(synthesised from diploid wheat, T. monococcum) are more of a botanical curiosity
and have not been considered as agronomic crops. Plants of triticale are similar to
those of wheat, except for having large spikes and kernels, and greater vigour or
growth. Low fertility and shrivelled grains are some objectionable features. It is more
drought tolerant than wheat. Primary triticales are obtained directly from wheat-rye
cross. Unstable aneuploid forms characterize hexaploid and octaploid triticales,
necessitating selection among themselves for genetic stability. Recombined triticales
are obtained by crossing primary triticales among themselves accompanied by
selection for a superior type. Secondary triticales are obtained by crossing primary
triticales with hexaploid wheat or octaploid triticales followed by selection for
superior segregates in the ensuing generations. Earlier it was reported that grain of
triticale contained a higher protein percentage than wheat, but as yield and kernel
quality of the triticales were improved through hybridisation with wheat and among
themselves accompanied by selection, the differences in protein content between
triticale and wheat were diminished. A natural cross of wheat and triticale at
CIMMYT produced a superior genetic combination, which was named Armadillo. In
Armadillo, 2D from hexaploid wheat had been substituted for the 2R of the rye.
Additional substitutions of R chromosomes may be performed, however, as the
number of wheat chromosomes replacing rye chromosomes increase, the triticale
strains become more like wheats. The triticale characteristics are mainly due to 3R
and 5R rye chromosomes. Secondary triticales are released as triticale cultivars in
different parts of the world (such as TL 419, IL 2129, and the like).
Tritium. A radioactive isotope of hydrogen.
Tritordeum. A synthetic cereal obtained by crossing Triticum aestivum and Hordeum
chilense.
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Trivalent. A multivalent consisting of three meiotically paired chromosomes. Trivalents
may be seen by performing meiotic analysis in trisomics, triploids, or any other
polyploids.
tRNA. transfer Ribonucleic Acid. A class of small RNA molecules that bring specific
amino acids to the site of protein synthesis (ribosome). The specificity of amino acid
is determined by matching of the anticodon of tRNA with the codon of the mRNA
being translated.
True Breeding. Of genotypes which do not segregate because they are homozygous at
the locus (loci) in question.
Truncated Selection. A form of selection in which plants or lines performing above or
below a certain value (truncation point) are retained as parents for the next generation.
The rest are eliminated from breeder’s population. The phenotypic value (truncation
point) may be a fixed value over successive generations, that is, selection is by
constant truncation. In this case, selection intensity decreases with time, as more and
more of the population exceeds the fixed truncation point. However, an usual practice
is to alter the truncation point in each generation of selection – that is – selection is by
proportional truncation. In this case, selection intensity is constant, but the truncation
point moves upward as the population distribution moves.
t-Test. A statistical test designed to test significance of difference between two sample
means (of small size). It is only an approximate test, influenced by errors in
estimation of the population standard deviation due to small size.
Tube Nucleus. The nucleus of the pollen tube which controls the growth and behaviour
of the pollen tube in higher plants.
Twin Species. Two species which are phenotypically very similar but separated from
each other by reproductive isolation.
Twin Spot. A pair of mutant sectors within wild-type tissue, produced by a mitotic
crossover in an individual of appropriate heterozygous genotype.
Two-line System. A system for producing hybrid rice involving only two lines (A and R
lines). Since male-sterility of A line is mediated by photoperiod (or temperature), it
can be maintained easily under antagonistic condition. (Also see PSGMS).
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Two Stage Testing (Hanson and Brim 1963). A scheme for evaluation of advance
generation homozygous lines (F 6 /F 7 ) which involves testing of all lines in one or two
environments and selecting better performing lines for retesting in a number of
environments. The two stage-testing programme is more efficient than testing all the
lines in one or two environments. The use of two locations for the first stage test
provides an insurance against abnormal conditions or seasonal hazards.
Two-factor Cross. A genetic recombination experiment involving two genetic markers
(a+b+/ab).
Two-factor Experiments. Biological experiments that deal with two variables while
keeping constant all other factors. Thus when response to the factor of interest is
expected to differ under different levels of the other factors, factorial experiments are
designed to handle simultaneously two or more factors.
Type Number. The most frequently occurring chromosome number in a particular
taxonomic group (modal number). The term is often used with the implication of its
being the ancestral number of the group in question. Other numbers are then assumed
to be derived from the type number.
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Underdominance. A situation wherein the hybrid falls outside the limit of the parent
with lower phenotypic value, for example, early-maturing hybrids derived from a
cross between parents having medium maturity.
Unequal Crossover. A crossover between homologs that are not perfectly aligned
(absence of point-to-point pairing).
Uniformity Trial. A trial that involves planting an experimental site with a single crop
variety and applying all cultural and management practices as uniformly as possible.
It consists of measuring differences among several plot means treated alike. In such a
trial, all sources of variability except that due to native soil differences are kept
constant. It facilitates measurement of soil heterogeneity, and thus provides
information concerning presence of fertility gradient in an experimental field. Soil
heterogeneity can be minimized through proper choice of plot size and shape, block
size and shape, increase in the number of replications, etc.
Unilateral Incompatibility (Anderson and de Winton 1931). A unique phenomenon in
which one of the reciprocal cross combinations succeeds whereas the other fails. It is
very closely related to self-incompatibility, for it occurs in matings of self-fertile to
self-incompatible species and is controlled by the same physiological mechanism
(inhibition of pollen tube growth). Although it occurs most frequently between
species, it may be found within species and the term unilateral interspecific
incompatibility describes the same mechanism. It was first described in a cross of
self-fertile Nicotiana langsdorfii with self-incompatible N. alata. UI may be attributed
to the involvement of S-locus, S-allele specific ribonucleases, non-S-RNase factors
and HT proteins. It can be overcome by the use of mixed and mentor pollen,
manipulation of environmental conditions and of style and ovary, chemical treatment,
etc. There has been observed a clear linkage between SI and UI, which offers
opportunities to unravel the molecular mechanisms of pollen rejection.
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Unineme Theory. A theory that states that each chromatid represents a single linear
DNA molecule with its associated proteins.
Uniparental Inheritance. A kind of inheritance in which the transmission of certain
phenotypes occurs from one parental type to all the progeny. Such inheritance is
generally produced by organelle genes (mit-genes, plastomes).
Uniparental. Having only a single parent – parthenogenetic.
Unisexual. Of organisms having separate sexes (dioecious) and producing only one kind
of gametes. Such organisms are thus dimorphic.
Unit Character. A character difference which is transmitted as a unit in heredity; a
character difference controlled by one pair of alleles.
Unit Membrane (Robertson 1959). The basic unit of the membrane system of any cell.
The plasma membrane represents a single unit membrane. All other membranous
structures of the cell are also unit membrane.
Univalent. An unpaired chromosome in meiosis; also called an asynaptic chromosome.
Univalents either lack a homologue or result from asynapsis (due to asynaptic genes
or influence of environmental factors). Bivalents, trivalents, quadrivalents, etc. are
associations of 2, 3, 4, etc. homologous chromosomes held together by chiasmata.
Unpredictable Legume. A legume yield potential of which cannot be assessed visually
owing to underground formation of economic products. Peanut sometimes is called
unpredictable legume; however, there are other legumes also (yam bean, for example)
which form underground economic parts.
Unstable Mutation. A mutation having high frequency of reversion. Such mutation may
be caused by the insertion of a controlling element, whose subsequent exit produces a
reversion.
UPOV (1961). Union Pour La Protection Des Obtention Vegetables. It is an acronym
derived from the fresh translation of above words meaning “International Union for
the Protection of New Varieties of Plants”. The objective of the UPOV convention
(also called Paris convention, 1961) was to allow the breeder to acquire the sole right
to commercialise the newly developed variety and/or multiply and commercialise
under license. Many countries in the developed world have adhered to the UPOV
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convention, and a number of other countries have laws relating to plant variety
protection at an advanced stage. The system has worked well and led to significant
increase in plant breeding and seed industry investment in UPOV member states.
Uracil. A pyrimidine base that appears in RNA instead of thymine found in DNA.
URF. Unassigned Reading Frame; an open reading frame (ORF) whose function is still
undetermined. It is a sequence of nucleotides with proper start and stop codons but
with no known function. URFs occurring within introns appear to be involved in
specifying proteins important in splicing out of the introns themselves at the RNA
level.
Uridine. The nucleoside having uracil as its base.
U’s Triangle (U 1935). A triangle that depicts three basic diploid species (one each at
each end) and three amphidiploid species (one each at the middle of each side) of
Brassicas showing phylogenetic relationship of each alloploid species with the
parental diploid species. ThethreebasicspeciesareB. nigra (n =8), B. oleracea (n =
9), and B. campestris (n = 10). It is obvious that haploid number increases or
decreases by one chromosome at a time. Such changes in chromosome number could
have been brought about in nature by duplication or loss of kinetochore, combined
with a series of translocations.
****
Vacuole. Any of the large, transparent vesicles limited by a unit membrane in mature
plant cells. Vacuoles are filled with a dilute solution, called cell sap. In meristematic
tissues, vacuoles are numerous and small.
Variability. The state of being variable with respect to form, function, and other traits.
Variable. A property that may have different values in various cases.
Variance. The square of the standard deviation; a unified measure of pooled variation of
treatments, expressed in square of unit. It is the average squared deviation of a
population of observations from their mean [S 2 = di 2 /N]. In fact, most methods of
statistical analysis have been evolved round the square of the standard deviation or the
variance. It has been shown that the variance is the most informative among measures
of dispersion for populations commonly met with. The variance resulting from
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genetic and environmental (non-genetic) causes is called genetic and environmental
variance, respectively. The total variance, that is, genetic plus environmental is
referred to as phenotypic variance. In principle, variance should not be negative.
However, components of genetic variance sometimes turn out to be negative due to
sampling error, poor plot techniques, large experimental error, and the like.
Variant. An individual organism that is recognisably different from an arbitrary standard
type in that species. The basis of the difference may be genetic or non-genetic. It may
also be defined as a putative mutant yet to be tested for heritable nature of the trait
(for which it is a “mutant”).
Variate. A single observation or measurement. In a data set comprising x 1 , x 2 , x 3 , …, x n ,
each observation is a variate, for example. It is a quantity that varies from individual
to individual and aggregate of individual values is called a population.
Variation. The occurrence of differences among individuals due to differences in their
genetic composition and/or environment to which they are exposed. The variation,
which is totally environmental, cannot be exploited in plant breeding. Thus from the
standpoint of practical utility, the variation must be at least partly genetic in origin.
Variational Theory (Darwin 1859). An accepted theory to explain the process of
evolution. In this theory, variation exists among organisms within a species.
Evolution of the species as a whole results from differential rates of reproduction of
the various types, so that the relative frequencies of the types change over time (over
generations). Therefore, evolution in this view, is a sorting process rather than a
transformational one.
Variegation. The occurrence within a tissue of sectors with differing phenotypes.
Variety. A group of individuals within a species, which are distinct in form or function
from other similar arrays of individuals. The term may be used interchangeably for
cultivar. However, the term “cultivar” is commonly used in scientific literature and
must be preferred to variety as the latter is also used in the taxonomy. The distinction
of being named and made commercially available serves to set apart “variety” from
the experimental strain or line. It must have three properties: distinctiveness,
uniformity and stability (DUS). These requirements are clearly satisfied in the
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autogamous crops. In cross-pollinated crops, where individual plants heterozygous for
many traits interpollinate, the range of purity within a variety may be quite wide and
may vary from one generation to the next. Precisely for these reasons, “variety” is less
of distinct entity in the cross-pollinated crops than in the self-fertilized ones.
Variety Blend. A mechanical mixture of the seed of two or more varieties. In certain
environments, it may yield more than the average of component varieties (lines) as it
will have buffering effect against genotype-environment interaction, and will be more
stable over locations and years than a single variety. The superiority of a variety blend
is adversely affected with the increase in the number of component lines like a
multiline variety. To maintain stability of a variety blend, it should be reconstituted
regularly.
Vector. In genetic engineering or cloning, the plasmid or phage chromosome used to
carry the cloned DNA segment. However, in resistance breeding, vector is a diseasespreading
(transmitting) agent. For example, white fly (Bemisia tabaci) spreadsthe
inoculum of YMV; thus it is called the vector of yellow mosaic virus.
Verminator Tecnology. A technology that uses rodent fat genes to kill the embryo and
uses more sophisticated techniques to the same effect as in terminator technology.
(Also see Terminator Technology).
Vernalisation. The exposure to low temperatures to obtain flowering. Vernalisation of
the winter wheat may be accomplished by keeping germinated seeds or seedlings at
temperatures of 1-3°C for a period of 4-6 weeks.
Vertical Resistance (Van der Plank 1963). A type of resistance which is effective
against certain races of a pathogen only. It is called so for it provides complete
resistance to the concerned race(s). The gene(s) that confers such resistance usually
has a large and easily identifiable effect (e.g., Sr genes, Lr genes, and Pm genes in
wheat for resistance to stem rust). Such genes are assumed to operate on a gene-forgene
basis, and also by virtue of their large effects, they differentiate races carrying
different virulent genes, that is, differential interactions are obvious. The reliability of
VR is the minimum. It is so because the races, previously very less, will dominate or
the race against which the resistance is effective may produce a new race by sexual
251

reproduction or by mutation. If gene-for-gene relationships are common in host
specialized pathogen systems, VR and HR are extremes of continuum. With few
genes (each with large effects) operating, differential interactions are easily
discernible and the result is VR. With more genes (each with relatively small effects)
operating, differential interactions are less easy to discern, and the result is a mixture
of HR and VR. With many genes, each with still smaller effects, differential
interactions cannot be recognized, and the resistance is predominantly HR.
Vertifolia Effect (Van der Plank 1963, ’68). The loss of polygenic, partial resistance in
the process of breeding for vertical resistance. Van der Plank demonstrated it by
comparing potato cultivars with and without race-specific (R) genes for resistance to
Phytopthora infestans with races virulent on the R genes. The epidemic built up faster
on cultivars with race-specific than on those with race-nonspecific resistance.
Vertifolia effect, although occurring in some cases, is not a universal phenomenon.
For example, Parlevliet and Kupier (1977) reported that ‘Cebaka Capa’ barley carried
both race-specific resistance gene Pa 7 and a high level of polygenic, partial
resistance to leaf rust. Probably, the vertifolia effect does not occur universally for
two reasons: (1) very susceptible genotypes are constantly removed from breeding
nurseries, and in fact, parents used in crosses likely have some genes for partial
resistance, and (2) major race-specific genes often have an incomplete expression in
the field with their expression becoming more complete when backstopped by genes
for partial resistance.
Viability. The number of an individual’s offspring that survive to maturity and produce at
least one offspring each (in the broad sense); the probability that a fertilized egg will
survive and develop into an adult organism.
Vilmorin Principle. The selection of a line/individual from a population on the basis of
progeny test. It has come to be called so after the name of a French scientist who
successfully practised it.
Virion. The ultimate stage of virus development representing the static, inert form of a
virus. It is the extracellular state of a virus in which no reproduction and no
metabolism take place.
252

Viroids. Very small particles consisting of genetic material alone (RNA) and lacking
enclosing membrane (of protein). It causes certain diseases in plants, potato spindle
tuber (the simplest infectitious agent), for example. Viroids are simpler than viruses.
Virulence. Capacity of a pathogen to incite a disease. A measure of degree of
pathogenicity of a pathogen. It is used to denote qualitative rather than quantitative
differences in the pathogenicity of races of pathogens. According to Van der Plank
(1975), virulence and avirulence in the pathogen are the counterparts of vertical
susceptibility and vertical resistance in the host, respectively. Virulence involves gene
diversity, probably largely through mutation.
Virulent Phage. A phage that cannot become a prophage. Such a phage after infection
always leads to lysis of the host cell.
Virus. Any of the infectitious, subcellular, and ultramicroscopic particles representing
potentially pathogenic agents. A virus consists of a nucleic acid (DNA/RNA) plus
protein (capsid). These are independent genetic systems (endowed with genetic
continuity and mutability) which possess their own evolutionary history. Viruses have
only their own genetic material; they lack in a cell membrane, a metabolic machinery,
and a biosynthetic machinery for the synthesis of proteins. Therefore, they are not
considered to be living organisms. They use the synthetic machinery of host cells for
their multiplication. Major groups of viruses are bacteriophages (bacteria “eater”),
animal viruses (DNA + protein), and plant viruses (RNA + protein). Viruses may be
as small as X174 (having only 12 capsomeres), and as large as adeno virus (252
capsomeres).
Virus ( of computer). A software (computer programme) written to destroy or corrupt
data in a computer. It is transferred from one computer to another through floppy
disks and compact disks (CD’s). When an infected floppy disk/CD is used, the virus
enters the random access memory (RAM) directly or by first entering the hard disk. It
either deletes the data or disables the computer hardware and prevents it from
working. Some common viruses are Yankee Doodle, April the first, July the 4 th , etc.
253

Vitality Mutation. Any mutation the effect of which is difficult to determine but which
changes the viability of the carrier genotype if present in an effective dosage. Whether
semilethal, sub-vital, or super-vital, all these mutations fall in this category.
VNTR. Variable Number Tandem Repeat. A chromosomal locus at which a particular
repetitive sequence is present in different numbers in different individuals or in the
two different homologs in one diploid individual.
254

Wallace Effect. The process of selection for reproductive isolation.
Waterlogging Stress. A stress imposed to plants by continued stagnation of water. Some
crop plants, for example rice, tolerate waterlogging stress by virtue of their special
characters (presence of aerenchymatous cells). Other crop plants are prone to
waterlogging stress especially at seedling stage. Under waterlogged condition oxygen
diffusion rates (ODR) in flooded soil is about 100 times lower than air, and respiration of
plant roots, soil micro-flora and fauna leads to rapid exhaustion of soil oxygen, and
thereby causing anaerobiosis. Oxygen deprivation, either completely (anoxia) or partially
(hypoxia) is detrimental to most species of higher plants as it disturbs the respiratory
cycle of plant by changing it from aerobic (kreb cycle = 38 ATP) to anaerobic
(glycolysis=2 ATP) cycle. However, proximate causes of plant injury can be oxygen
deficit or mineral nutrient imbalances, a decrease in cytokinins or other hormones
released from the roots, a decrease in available soil nitrogen and/or nitrogen uptake, an
increase in toxic compounds in soil such as methane, ethylene, ferrous ions or
manganese,an increase in toxic compounds (in the plant) such as ethanol or ethylene, and
an increase in disease causing organisms. Empirical evidences suggest that waterlogging,
in general, causes rapid senescence and drooping of the shoot tips of plants. In pigeonpea,
it reduces plant height and delays flowering in surviving plants, resulting in reduction in
the number of pods, seeds/pod and seed yield. It has been observed that seed coat
thickness, aerenchymatous cells, lenticels and adventitious roots also affect tolerance to
waterlogging in pigeonpea. However, these traits need to be confirmed and re-validated
before these can be used as selection criteria in crop plants.
Western Blot. Membrane carrying an imprint of proteins separated by electrophoresis. It
can be probed with a labelled antibody to detect a specific protein.
Wide Cross. A cross between cultivated species and its wild relatives. The wild relatives
may be from primary, secondary or tertiary gene pools. If the wild relatives belong to
secondary or tertiary gene pools (that is, they are distantly related to the cultivated
ones) making the gene transfer possible only through biotechnological interventions,
255

thecrossmaybereferredtoasdistant cross. Thomas Fairchild (1717) made the first
recorded interspecific hybrid between carnation and sweet william. Such hybrids may
be characterized by their sterility, low yield or other poor quality traits, which may in
turn be ascribed to meiotic instability, aneuploidy, unbalanced gametes, and the like.
(Also see wide hybridization).
Wide Hybridization. Hybridization between cultivated types and their wild relatives.
Broadly defined, it refers to transfer or introgression of genes or gene combination
from non-adapted sources into breeding materials. Many problems are encountered
while attempting to wide hybridisation, which are collectively called barriers to wide
hybridisation. In segregating generations, variance is often reduced. However, recent
findings that wild relatives may contain useful genes related to yield has created a
renewed interest amongst plant breeders in wide hybridisation.
Wild Type. The genotype or phenotype that is found most frequently in nature or in the
standard laboratory stock for a given organism.
Winter Injury. The damage to crop plants caused by low temperature and heaving. Plant
genotypes differ in their tolerance to winter injury. The extent of winter injury may be
influenced by the stage of development of crop plants, spacing of the plants, soil
texture and moisture contents, wind, fertilizer treatments, previous hardening,
vegetative or snow cover, and plant disease. The most common measure to estimate
winter hardiness is survival percentage. However, as a final measure of hardiness, the
effect of the winter injury in reducing yield should always be considered.
Wobble. The ability of certain bases at the third position of an anticodon in tRNA to form
hydrogen bonds in various ways, and thereby causing alignment with several possible
codons at the third position of an mRNA.
World Collection. See germplasm collection.
WTO (1994). World Trade Organization (replaced GATT after 47 years); an
international organization to promote further liberalization of world trade of goods,
integration of agriculture and textiles as well as TRIPS. It has quasi-judicial rights
that GATT had been deficient in. Now it has become third economic pillar of
worldwide dimension along with World Bank and IMF.
256

x. Basic number of chromosomes in a polyploid series; the smallest number of
chromosomes in a chromosome set.
X 1 , X 2 , X 3 . Symbols used to denote first, second, third… generations from an irradiated
ancestral plant (X 0 ).
X Chromosome Inactivation. The process by which the genes of an X chromosome in a
mammal can be completely repressed as a part of the dosage compensation
mechanism. Such an inactivated chromosome is referred to as a Barr body.
X Linkage. The association of genes present on an X chromosome (but not on the Y).
X–and–Y Linkage. The inheritance pattern of genes found on both the X and Y
chromosomes.
X-ray Crystallography. A technique for deducing molecular structure by aiming a beam
of x-rays at a crystal of the test compound and measuring the scatter of rays.
X-ray Diffraction. Scattering of x-rays by the atoms of a crystal in a manner such that
the resulting diffraction pattern provides information about the structure and/or
identity of the substance.
Xenia. Effect of pollen on the embryo and endosperm.
***
Y Linkage. The inheritance pattern of genes found on the Y chromosome. Thus far, only
a few genes have been reported to have their association on the Y chromosome.
Yield. A measure of the relative reproductive capacity of an individual or a genotype.
ZTest.A statistical test of significance for difference between means of large samples
(>30). It is supposed that z-test is not affected by sampling error that must remain in
the estimation of population standard deviation due to small size of samples in t-test.
Zein. The main protein of Indian corn present in the endosperm portion of the seed.
Zero Selection. Highest selection pressure such that zero per cent individual is selected,
that is, 100 per cent individuals are rejected (selection intensity is most stringent, that
is zero).
257

Zygote. A fertilized egg cell; a cell formed by the union of two gametes (an egg and a
sperm) and the individual developing from this cell. The unique diploid cell will
divide mitotically to create a differentiated diploid organism. Such individuals contain
two complete but identical genomes. Hence, it is also called holozygote.
Zygotene. A sub-stage in meiotic prophase when the thread-like homologous
chromosomes pair.
Zymogen. The inactive precursor form of an enzyme. Zymogens are typically activated by
proteolytic cleavage.
258

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