50 100 150 200 250 300 350 400 Generations

Figure 6.6 Allele frequencies over time for three types of gene action with a low initial allele frequency. In all three cases the equilibrium allele frequency is fixation or near fixation for the A allele. With complete dominance, natural selection initially increases the allele frequency very rapidly. The approach to fixation for the A allele slows as aa homozygotes become rare since heterozygotes harbor a alleles that are concealed from natural selection by dominance. Natural selection initially changes the frequency of a recessive allele very slowly since homozygote recessive genotypes are very rare. As the recessive homozygotes become more common, allele frequency increases more rapidly. With additive gene action the phenotype of the heterozygote is intermediate between the two homozygotes so all genotypes differ in their viability. Additive gene action has the most rapid overall approach to equilibrium allele frequency. The degree of dominance is represented by the dominance coefficient, h. In this illustration the selection coefficient is s = 0.1.

frequency trajectory for additive gene action is intermediate. It combines the rapid initial change in allele frequency of complete dominance with the later-stage rapid approach to equilibrium and fixation of the complete recessive. Equilibrium allele frequency (fixation or near fixation) is reached most quickly with additive gene action.

With completely dominant or recessive alleles, natural selection cannot discriminate between two of the three genotypes since their fitness values are identical. How this lack of difference in fitness values affects natural selection depends on genotype frequencies. In the early generations, the recessive case shows slow change because the heterozygote is selected against and the fittest genotype is rare. In the later generations of the dominance case, heterozygotes shelter the recessive allele from natural selection slowing further change in allele frequency as the recessive homozygote becomes very infrequent. In contrast, the fitness values of all three genotypes are distinct and uniformly different with additive gene action. Additive gene action gives the maximum difference in marginal and average fitness values across the entire range of possible genotype frequencies under random mating.

Gene action is an important factor in understanding the fate of new mutations acted on by natural selection. Imagine a new mutation in a population that has a high relative fitness when homozygous. As covered in Chapter 5, the initial frequency of any new mutation will be low ). A completely or

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