Social selection in human populations: differential modification of the fitness by the sex of an affected parent.

Social selection model of two alleles at a locus has been constructed where the fitness of offspring is modified differentially by the sex of an affected parent. It has been shown that the equilibrium frequency of deleterious alleles depends on the loss of fitness of an individual due to the trait (gamma), due to an affected mother (beta 1) and affected father (beta 2), and the probability that the heterozygote develops the trait (h). The equilibrium gene frequency is given approximately by alpha/hs for 0 less than h less than or equal to 1 and square root alpha/s for h = 0, where s = gamma + (beta 1 + beta 2) (1 - gamma)/2 for multiplicative models. The same result holds for the additive model if the value of gamma is small.