Variation maintained in quantitative traits with mutation–selection balance: pleiotropic side-effects on fitness traits

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Abstract
A model of genetic variation in a quantitative trait is investigated in which mutations affecting the trait also have deleterious pleiotropic effects on fitness, such that there is a bivariate distribution of effects of the mutants. It is motivated principally by frequent observations of deleterious side-effects of visible mutations. When the correlation,p, of absolute values of mutant effects on the trait and fitness is unity, the model behaves similarly to one of pure stabilizing selection on the trait alone, and the genetic variance quickly increases to a limit with increasing population size (N). Otherwise (ρ < 1), the genetic variance continues to increase as N increases, but may do so very slowly because genes affecting the metric trait can be nearly neutral with respect to fitness and therefore contribute much variance in large populations. The model predicts that the metric trait will appear to be under stabilizing selection because extreme genotypes tend to be less fit, and its apparent strength is evaluated.