Principles of Plant Genetics and Breeding

Individual expresses
gene of interest
been developed. However, the number of petals of
the double flower is influenced by temperature. When
grown at between 1.5 and 10°C, the double flower
characteristic is lost or diminished, and the flowers
produce fewer petals. This gene interaction is called
variable gene expressivity, and describes the range of
expression of the genotype of interest (Figure 3.11b).
The effect of genetic background on the expression of
a phenotype is often difficult to assess. The expression of
other genes in a genome may affect the phenotype
observed, a phenomenon called genetic suppression.
Suppression of genes is known to modify the effect of
primary genes. Sometimes, relocation of a gene in the
genome can influence the expression of the gene, a
phenomenon called position effect. This may occur
when chromosomal mutations such as translocations
and inversions occur (a region of the chromosome is
relocated to another part of the chromosome).
Nucleic acids: structure and function
Nucleic acids are polymers of nucleotides. There are
two kinds of nucleic acids: deoxyribonucleic acid
(DNA) and ribonucleic acid (RNA). A nucleotide
consists of three basic components: pentose sugar,
nitrogenous base, and a phosphate group. The sugar is
a cyclic five-carbon sugar and is ribose in RNA and
deoxyribose in DNA. Similarly, there are two kinds of
bases: purines and pyrimidines. There are two purines,
adenine (A) and guanine (G), and three pyrimidines,
cytosine (C), thymine (T), and uracil (U). Thymine
occurs only in DNA, while uracil occurs only in RNA.
The letters A, C, T, G, are casually referred to as the
alphabets of life.
PLANT CELLULAR ORGANIZATION AND GENETIC STRUCTURE 47
Individual does not
express gene of interest
(a) (b)
Figure 3.11 Diagrammatic presentation of (a) penetrance and (b) expressivity.
N
N
Individual expresses gene of interest
to high degree
Individual expresses gene of interest
to a moderate degree
Individual expresses gene of interest
to a low degree
Adenine Phosphate
N
N
Figure 3.12 The basic chemical structure of a nucleotide
molecule, showing its three constituents: a sugar, a
nitrogeneous base, and a phosphate group.
O
H OH
Deoxyribose
When a base is linked to a sugar, the product is called
a nucleoside. A nucleoside linked to a phosphate forms
a nucleotide (Figure 3.12). Two nucleotides may be
linked by a phosphodiester group to form a dinucleotide.
Shorter chains (consisting of less than 20
nucleotides) are called oligonucleotides while longer
chains are called polynucleotides. A single nucleoside
is also called nucleoside monophosphate (NMP), while
two nucleosides form a nucleoside diphosphate (NDP).
Triphosphates are important in cellular bioenergetics,
especially adenosine triphosphate (ATP) and guanosine
triphosphate (GTP). When these compounds are
hydrolyzed, inorganic phosphate is produced, accompanied
by the release of energy (e.g., ATP → ADP +
energy).
H
OH
CH2O P O
OH