Stable Equilibrium at a Sex-linked Locus

Abstract

BENNETT¹ has shown that a stable selectively balanced polymorphism between two alleles can exist at a locus in the X-chromosome under conditions which are different from those determining stable equilibrium in the autosomal situation. In particular, it is neither necessary nor sufficient, in the sex-linked case, for the heterozygote in the homogametic sex to be at a selective advantage with respect to both homozygotes. It has previously been found² that this principle of heterozygosity operates in the case of multiple allelic systems in autosomes in the following form: a necessary condition for stable equilibrium is that the viability of each homozygote be less than the mean viability of the zygotic population at equilibrium. It seems at first sight that no such heterozygosity principle operates in the sex-linked situation. It will, however, be shown here that it does do so in a somewhat modified form.