heterozygotes be smaller than that of normal homozygotes AA by factors of and respectively, where is a number between and measuring the degree of dominance ( indicates that A is completely dominant while indicates no dominance). For simplicity, suppose that mating is random. The degree of dominance affects the relative importance of selection on heterozygotes versus homozygotes. If A is not completely dominant (i.e. is not close to zero), then deleterious mutations are primarily removed by selection on heterozygotes because heterozygotes contain the vast majority of deleterious B alleles (assuming that the deleterious mutation rate is not very large). This case is approximately equivalent to the preceding haploid case, where mutation converts normal homozygotes to heterozygotes at rate and selection acts on heterozygotes with selection coefficient ; thus . In the case of complete dominance (), deleterious alleles are only removed by selection on BB homozygotes. Let , and be the frequencies of the corresponding genotypes. The frequency of normal alleles A increases at rate due to the selective elimination of recessive homozygotes, while mutation causes to decrease at rate (ignoring back mutations). Mutation–selection balance then gives , and so the frequency of deleterious alleles is . This equilibrium frequency is potentially substantially larger than for the case of partial dominance, because a large number of mutant alleles are carried in heterozygotes and are shielded from selection. Many properties of a non random mating population can be explained by a random mating population whose effective population size is adjusted. However, in non-steady state population dynamics there can be a lower prevalence for recessive disorders in a random mating population during and after a growth phase. See also Negative selection Dysgenics Viral quasispecies References Mutation Selection