Principles of Plant Genetics and Breeding

132 CHAPTER 8
in a decline in both heritability and phenotypic standard
deviation. Once genes have been fixed, there will be no
further response to selection.
Example For example:
X σ p V P V A V E
Parents 15 2 6 4 3
Offspring 20.2 15 4.3 2.5 3
R = ih 2 σ p
Parents
h 2 = V A /V P
= 4/6
= 0.67
0 5 10 15 20 25 30 35 40
Advance = 2.5
(a) (b)
0 5 10 15 20 25 30 35 40
Advance = 10
Figure 8.3 The effect of phenotypic variance on genetic advance. (a) If the phenotypic variance is too small, the genetic
variability from which to select will be limited, resulting in a smaller genetic gain. (b) The reverse is true when the
phenotypic variance is large.
for i at P = 10% = 1.755 (read from tables and assuming
a very large population).
R = 1.755 × 0.67 × 2
= 2.35
Offspring
h 2 = V A /V P
= 2.5/4.3
= 0.58
R = 1.755 × 0.58 × 1.5
= 1.53
Generally, as selection advances to higher generations,
genetic variance and heritability decline. Similarly, the
advance from one generation to the next declines, while
the mean value of the trait being improved increases.