Principles of Plant Genetics and Breeding

First cell division
Second cell division
Interphase
Anaphase 2
Centromeres divide; sister
chromatids move to opposite poles
Figure 3.3 Diagrammatic presentation of meiosis in a cell
with a diploid number of 4. The process has two distinct
cell divisions. Prophase I consists of five distinguishable
stages; the most genetically significant event of crossing
over occurs in the fourth stage, diplonema.
PLANT CELLULAR ORGANIZATION AND GENETIC STRUCTURE 39
Prophase 1
Divisible into 5 substages;
leptonema, zygonema,
pachynema, diplonema,
diakinesis
Homologous chromosomes
pair to form bivalents; each
cromosome duplicates; sister
chromatids pair; chiasmata
form; crossing over occurs
Metaphase 1
Homologous chromosomes
align at the equitorial plate;
nuclear membrane
disappears
Anaphase 1
Homologous chromosome
pairs separate, sister
chromatids remain
together
Telophase 1
Two daughter cells form;
nuclear membrane forms;
each cell contains only
one chromosome of the
homologous pair
Prophase 2
Short stage; DNA does not
duplicate
Metaphase 2
Nuclear membrane
disappears; chromosomes
align at the equitorial plate
Telophase 2
Nuclear membrane
forms; four haploid
daughter cells are
produced following
cytokinesis
the ploidy level of the species. By reducing the diploid
number to a haploid number before fertilization, the
diploid number is restored thereafter.
Mendelian concepts in plant breeding
As previously stated, genetics is the principal science
that underlies plant breeding. Gregor Mendel made
significant contributions to the development of the
discipline of genetics, albeit in absentia. He derived
several postulates or principles of inheritance, which are
often couched as Mendel’s laws of inheritance.
Mendelian postulates
Because plant breeders transfer genes from one source
to another, an understanding of transmission genetics
is crucial to a successful breeding effort. The method of
breeding used depends upon the heredity of the trait
being manipulated, among other factors. According to
Mendel’s results from his hybridization studies in pea,
traits are controlled by heritable factors that are passed
from parents to offspring, through the reproductive
cells. Each of these unit factors occurs in pairs in each
cell (except reproductive cells or gametes).
In his experiments, Mendel discovered that in a cross
between parents displaying two contrasting traits, the
hybrid (F1 ) expressed one of the traits to the exclusion
of the other. He called the expressed trait dominant
and the suppressed trait recessive. This is the phenomenon
of dominance and recessivity. When the
hybrid seed was planted and self-pollinated, he observed
that both traits appeared in the second generation (F2 )
(i.e., the recessive trait reappeared), in a ratio of 3 : 1
dominant : recessive individuals (Figure 3.4). Mendel
concluded that the two factors that control each trait do
not blend but remain distant throughout the life of the
individual and segregate in the formation of gametes.
This is called the law of segregation. In further studies
in which he considered two characters simultaneously,
he observed that the genes for different characters are
inherited independently of each other. This is called the
law of independent assortment. In summary, the two
key laws are as follows:
Law I Law of segregation: paired factors segregate
during the formation of gametes in a random
fashion such that each gamete receives one
form or the other.
Law II Law of independent assortment: when two
or more pairs of traits are considered simultaneously,
the factors for each pair of traits
assort independently to the gametes.
Mendel’s pairs of factors are now known as genes,
while each factor of a pair (e.g., HH or hh) is called an
allele (i.e., the alternative form of a gene: H or h). The
specific location on the chromosome where a gene
resides is called a gene locus or simply a locus (loci for
plural).