Principles of Plant Genetics and Breeding

170 CHAPTER 10
genes are so tightly linked they are resistant to the effect
of recombination. Gene transfer by hybridization is subject
to the phenomenon of linkage drag, the unplanned
transfer of other genes associated with those targeted. If
a desired gene is strongly linked with other undesirable
genes, a cross to transfer the desired gene will invariably
be accompanied by the linked undesirable genes.
Types of populations generated
through hybridization
A breeding program starts with an initial population that
is obtained from previous programs, existing variable
populations (e.g., landraces), or is created through a
planned cross. Hybridization may be used to generate a
wide variety of populations in plant breeding, ranging
from the very basic two-parent cross (single cross) to very
complex populations in which hundreds of parents could
be involved. Simple crosses are the most widely used in
breeding. Commercial hybrids are mostly produced by
single crosses. Complex crosses are important in breeding
programs where the goal is population improvement.
Hybridization may be used to introgress new alleles
from wild relatives into breeding lines. Because the
initial population is critical to the success of the breeding
program, it cannot be emphasized enough that it be
generated with much planning and thoughtfulness.
Various mating designs and arrangements are used by
breeders and geneticists to generate plant populations.
These designs require some type of cross to be made.
Factors that affect the choice of a mating design, as
outlined by C. Stuber include: (i) the predominate type
of pollination (self- or cross-pollinated); (ii) the type
of crossing used (artificial or natural); (iii) the type of
pollen dissemination (wind or insect); (iv) the presence
of a male-sterility system; (v) the purpose of the project
(for breeding or genetic studies); and (vi) the size of the
population required. In addition, the breeder should be
familiar with how to analyze and interpret or use the
data to be generated from the mating.
The primary purpose of crossing is to expand genetic
variability by combining genes from the parents involved
in the cross to produce offspring that contain genes they
never had before. Sometimes, multiple crosses are conducted
to generate the variability in the base population
to begin the selection process in the program. Based
on how the crosses are made and their effects on the
genetic structure of the plants or the population,
methods of crossing may be described as either divergent
or convergent.
Divergent crossing
Genetically divergent parents are crossed for recombination
of their desirable genes. To optimize results, parents
should be carefully selected to have the maximum
number of positive traits and a minimum number of
negative traits (i.e., elite × elite cross). This way, recombinants
that possess both sets of desirable traits will
occur in significant numbers in the F2 . The F1 contains
the maximum number of desirable genes from both
parents. There are several ways to conduct divergent
crosses (Figure 10.1a).
1 Single cross. If two elite lines are available that
together possess adequate traits, one cross [single
cross (A × B)] may be all that is needed in the breeding
program.
2 Three-way cross. Sometimes, desirable traits occur
in several cultivars or elite germplasm. In this case,
multiple crosses may be required in order to have the
opportunity of obtaining recombinants that consist
of all the desirable traits. The method of three-way
crosses [(A × B) × C] may be used. If a three-way
cross product will be the cultivar, it is important that
the third parent (C) be adapted to the region of
intended use.
3 Double cross. A double cross is a cross of two
single crosses [(A × B) × (C × D)]. The method of
successive crosses is time-consuming. Further, complex
crosses such as double crosses have a low frequency
of yielding recombinants in the F 2 that possess
a significant number of desirable parental genes.
When this method is selected, the targeted desirable
traits should be small (about 10). The double-cross
hybrid is more genetically broad-based than the
single-cross hybrid but is more time-consuming to
make.
4 Diallel cross. A diallel cross is one in which each
parent is crossed with every other parent in the set
(complete diallel), yielding n − (n − 1)/2 different
combinations (where n is the number of entries).
This method entails making a large number of
crosses. Sometimes, a partial diallel is used in which
only certain parent combinations are made. The
method is tedious to apply to self-pollinated species.
Generally, it is a crossing method for genetic studies.
Convergent crossing
These are conservative methods of crossing plants. The
primary goal of convergent crossing is to incorporate a
specific trait into an existing cultivar without losing any