Introduction to Genetic Analysis 662

Introduction to Genetic Analysis 662 - 44200_20_p643-678...

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20.7 Quantifying heritability 661 We would like to separate the so-called additive ef- fect caused by substituting a alleles for A alleles from the variation caused by dominance. The reason is that the effect of selective breeding depends on the additive variation and not on the variation caused by dominance. Thus, for purposes of plant and animal breeding or for making predictions about evolution by natural selection, we must determine the additive variation. An extreme example will illustrate the principle. Suppose that plant height is inFuenced by variation in a gene and that the phenotypic means and frequencies of three genotypes are: A / AA / aa / a Phenotype 10 12 10 ±requency 0.25 0.50 0.25 It is apparent (and a calculation like the preceding one will con²rm) that there is no average difference between the a and A alleles, because each has an effect of 11 units. So there is no additive variation, although there is obviously genetic variation because there is variation in phenotype between the genotypes. The tallest plants are
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