The Existence and Stability of Selectively Balanced Polymorphism at a Sex-Linked Locus

Abstract

Necessary and sufficient conditions are given for the existence and stability of a selectively balanced polymorphism at a sex-linked locus. It is shown that selective superiority of the heterozygote is neither necessary nor sufficient and also that the occurrence of heterozygotes with a frequency greater than that of homozygotes does not necessarily indicate that the heterozygote has a selective advantage over both homozygotes. These results are considered in relation to Wallace's published data for populations of Drosophila pseudoobscura in which selection is acting on the sex-linked condition "sex ratio".