Equilibria under natural selection at a sex-linked locus

Summary

An equilibrium of a pair of allelomorphs at a sex-linked locus can be stable under natural selection if the fitnesses of the mutant hemizygote and homozygote differ from that of the normal in opposite directions, even if the female heterozygote is less fit than the mean of the homozygotes. But if the fitnesses of the mutant homozygote and liemizygote differ in the same direction from those of the normals, stability demands that the heterozygous female should be fitter than either homozygote. The facts as to human glucose-6-phosphate dehydrogenase are discussed in the light of these findings. In this case it seems likely that there is real heterosis, but this may not be true for most such equilibria found in nature.