Principles of Plant Genetics and Breeding

84 CHAPTER 5
100%
50%
(a)
100%
50%
(b)
100%
50%
(c)
Figure 5.6 Qualitative variation produces discrete
measurements that can be placed into distinct categories:
(a) parental phenotype, (b) dominant phenotype in F 1 ,
and (c) 3 : 1 phenotypic ratio in F 2 .
a large segregating population, especially if several loci
are segregating, to increase the chance of finding the
desired recessive recombinants. For example, if two loci
are segregating, a cross between AA and aa would produce
25% homozygous recessive individuals in the F 2
AA × aa
Aa × Aa
AA Aa aa
Parents
F 1
F 2
Distinguishable
(a) (b)
Indistinguishable
Distinguishable
(1AA :2Aa :1aa). A minimum of 16 plants would be
needed in the F 2 stage to include the desired recombinant.
However, if four loci are segregating, at least
256 plants are required in the F 2 in order to observe the
desired recombinant. It is important to note that the
desired recombinant can be isolated from the F 2 without
any further evaluation. In the case of a dominant
locus, (e.g., the cross PP × pp), 25% of the F 2 will be
homozygous recessive, whereas 75% would be of the
heterozygous-dominant phenotype (of which only 25%
would be homozygous dominant). The breeder needs
to advance the material one more generation to identify
individuals that are homozygous dominant.
Quantitative variation
Most traits encountered in plant breeding are quantitatively
inherited. Many genes control such traits, each
contributing a small effect to the overall phenotypic
expression of a trait. Variation in quantitative trait
expression is without natural discontinuities, as previously
indicated. Traits that exhibit continuous variations
are also called metric traits. Any attempt to classify such
traits into distinct groups is only arbitrary. For example,
height is a quantitative trait. If plants are grouped into
tall versus short plants, one could find relatively tall
plants in the short group and similarly short plants in the
tall group (Figure 5.8).
Quantitative traits are conditioned by many to
numerous genes (polygenic inheritance) with effects
that are too small to be individually distinguished.
They are sometimes called minor genes. Quantitative
trait expression is very significantly modified by the
variation in environmental factors to which plants in
PP × pp
Pp × Pp
PP Pp pp
PP Pp pp
Figure 5.7 (a) Breeding a qualitative trait conditioned by a recessive gene. The desired recombinant can be observed
and selected in the F 2 . (b) Breeding a qualitative trait conditioned by a dominant gene. The desired trait cannot be
distinguished in the F 2 , requiring another generation (progeny row) to distinguish between the dominant phenotypes.
PP
pp
Parents
F 1 (self)
F 2
F 3 (progeny rows)