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AA - AA BB 1 Bb 2 bb 3 Aa 2 4 6 aa 3 6 9 AA BB 1 Bb 4 bb 9...

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AA Aa aa AA Aa aa BB 1 2 3 BB 1 2 3 Bb 2 4 6 Bb 4 6 4 bb 3 6 9 bb 9 6 3 In 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-locus additive case on the left, heterozygotes at one locus are intermediate between the two homozygotes regardless of 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 genotype at the other, interacting locus. In a sense, one locus is modifying the expression of the other locus. If selection were to act in favor of nose length in the right-hand epistatic system, the way alleles "marched to fixation" would be very different. Now consider how linkage and epistasis can affect the response to selection. In the
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