Two-locus drift with sex chromosomes: the partitioning and conversion of variance in subdivided populations.

A set of two-locus descent measures are derived for loci on sex-linked and autosomal loci, with a population structure which includes finite population size, migration, and extinction and recolonization. Two-locus descent measures which involve sex chromosomes increase much faster with inbreeding than those for autosomes. For loose recombination, the two-locus descent measures are shown to be closely approximated by the product of the appropriate one-locus measures. The distribution and partitioning of variance within and among populations is derived, including additive-by-additive epistatic genetic variance. The variance among populations available to group selection is much higher in the homogametic sex with epistasis involving sex chromosomes or haplodiploids. The amount of additive genetic variance within populations due to sex chromosomes or haplodiploids can be significantly increased as a result of population bottlenecks and the conversion of epistatic genetic variance. The proportion of epistatic genetic variance which is converted to additive variance is greater in the homogametic sex than in the heterogametic sex, by as much as a factor of 2.