Principles of Plant Genetics and Breeding

It should be pointed out that recombination only
includes genes that are already present in the parents.
Consequently, if there is no genetic linkage, the new
gene recombination can be predicted. Where linkage is
present, knowledge of the distance between gene loci
on the chromosomes is needed for estimating their
frequencies. As previously discussed in Chapter 3, additional
variability for recombination may be observed
where intra-allelic and interallelic interactions (epistasis)
occur. This phenomenon results in new traits that were
not found in the parents. Another source of genetic variability
is the phenomenon of gene transgression, which
causes some individuals in a segregating population
from a cross to express the trait of interest outside the
boundaries of the parents (e.g., taller than the taller
parent, or shorter than the shorter parent). These new
genotypes are called transgressive segregates. The discussion
so far has assumed diploidy in the parents.
However, in species of higher ploidy levels (e.g.,
tetraploid, hexaploid), it is not difficult to see how
additional genetic variability could result where allelic
interactions occur.
One of the tools of plant breeding is hybridization
(crossing of divergent parents), whereby breeders
selectively mate plants to allow their genomes to be
reshuffled into new combinations to generate variability
in which selection can be practiced. By carefully selecting
the parents to be mated, the breeder has some
control over the nature of the genetic variability to be
generated. Breeding methods that include repeated
hybridization (e.g., reciprocal selection, recurrent selection)
offer more opportunities for recombination to occur.
The speed and efficiency with which a breeder can
identify (by selecting among hybrids and their progeny)
desirable combinations, is contingent upon the number
of genes and linkage relationships that are involved.
Because linkage is likely to exist, the plant breeder is
more likely to make rapid progress with recombination
by selecting plant genotypes with high chiasma
frequency (albeit unconsciously). It follows then that
the cultivar developed with the desired recombination
would also have higher chiasma frequency than the parents
used in the breeding program.
Ploidy modifications
New variability may arise naturally through modifications
in chromosome number as a result of hybridization
(between unidentical genotypes) or abnormalities
in the nuclear division processes (spindle malfunction).
Failure of the spindle mechanism, during karyokinesis or
VARIATION 81
Normal
spindle operation
Normal cell
Normal cell division
Abnormal spindle
operation
Abnormal cell division
Figure 5.4 Failure of the genetic spindle mechanism may
occur naturally or be artificially induced by plant breeders
(using colchicine), resulting in cell division products that
inherit abnormal chromosomes numbers. Plant breeders
deliberately manipulate the ploidy of cells to create polyploids.
even prior to that, can lead to errors in chromosome
numbers transmitted to cells, such as polyploidy (individuals
with multiples of the basic set of chromosomes
for the species in their cells) (Figure 5.4). Sometimes,
instead of variations involving complete sets of chromosomes,
plants may be produced with multiples of only
certain chromosomes or deficiencies of others (called
aneuploidy). Sometimes, plants are produced with half
the number of chromosomes in the somatic cells (called
haploids). Like genetic recombination, plant breeders
are able to induce various kinds of chromosome
modification to generate variability for breeding. The
subject is discussed in detail in Chapter 13.
Mutation
Mutation is the ultimate source of biological variation.
Mutations are important in biological evolution as
sources of heritable variation. They arise spontaneously
in nature as a result of errors in cellular processes such
as DNA replication (or duplication), and by chromosomal
aberrations (deletion, duplication, inversion,
translocation). The molecular basis of mutation may be
described by mechanisms such as: (i) modification of
the structure of DNA or a component base of DNA;
(ii) substitution of one base for a different base; (iii)
deletion or addition of one base in one DNA strand;
(iv) deletion or addition in one or more base pairs in