Social Selection in Human Populations: Fitness Interaction Among Sibs

A social selection model with one locus and two alleles has been constructed by considering the fitness interaction among sibs. It is shown that the equilibrium gene frequency depends on the magnitude of viability reduction (γ) and fertility reduction (α), the additional fitness reduction resulting from an affected sib (β), the probability that the heterozygote develops the trait (h), and the potential number of sibs (n). With mutational input from the wild-type allele to the deleterious gene at a rate of u per generation, the equilibrium frequency of deleterious genes is given approximately by u/hs for 0 < h < 1 and $$\sqrt{u/s}$$ for h = 0, where s = α + γ + β(n - 1)(1 - γ)/2. Thus, the frequency of genetic diseases in a population depends not only on the intrinsic deleterious effect of disease but also on sociocultural reactions to the trait.