AA Aa aaAA Aa aaBB 123BB 123Bb246Bb464bb369bb963In the first case if we selected for long noses, we would tend to drive the a and b alleles to high frequency. If we selected for long noses in the second case we would tend to drive the A and b alleles to high frequency. The important distinction between these two tables is that in the simple two-locusadditivecase on the left, heterozygotes at one locus are intermediate between the two homozygotes regardlessof the genotype at the other locus. In contrast, the table on the right shows that the relationship between genotype and phenotype at one locus depends on the genotypeat the other, interacting locus. In a sense, one locus is modifyingthe expression of the other locus. If selection were to act in favor of nose length in the right-hand epistaticsystem, the way alleles "marched to fixation" would be very different.Now consider how linkage and epistasiscan affect the response to selection. In the
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