The Genetic Variance Maintained by Pleiotropic Mutation

The equilibrium genetic variance of a quantitative trait under mutation–selection balance is analysed using a population genetic model. I assume that mutations have both phenotypic effects on a quantitative trait and deleterious effects on fitness itself. Hence, genetic variance is generated by pleiotropic effects of such deleterious mutations. If the deleterious effect is large, the predicted genetic variance is notably smaller than the equilibrium genetic variance predicted by the mutation–selection balance model with the rare-alleles approximation. The model further suggests that the ratio of the equilibrium genetic variance to the variance of allelic effects of new mutations is proportional to the ratio of total mutation load to the net selection coefficient on new mutations. It is also suggested that the net selection coefficient is equal to the per-generation proportional increment of genetic variance by new mutations.