20.7 Quantifying heritability661We would like to separate the so-called additive ef-fect caused by substituting aalleles for Aalleles fromthe variation caused by dominance. The reason is thatthe effect of selective breeding depends on the additivevariation and not on the variation caused by dominance.Thus, for purposes of plant and animal breeding or formaking predictions about evolution by natural selection,we must determine the additive variation. An extremeexample will illustrate the principle. Suppose that plantheight is inFuenced by variation in a gene and that thephenotypic means and frequencies of three genotypesare:A/AA/aa/aPhenotype101210±requency0.250.500.25It is apparent (and a calculation like the preceding onewill con²rm) that there is no average difference betweenthe aand Aalleles, because each has an effect of 11units. So there is no additivevariation, although there isobviously genetic variation because there is variation inphenotype between the genotypes. The tallest plants are
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