Fixation of genes having large or small effects on a trait with an intermediate optimum

Major genes affecting quantitative traits are widespread and well known. However, major genes affecting unimodally distributed quantitative traits appear to be rare in populations of outbreeding organisms. This may largely reflect methods of analysis, or it may reflect the interaction of chance and selection on the frequencies of such major genes. If many quantitative traits have intermediate optima, or at least are subject to selection against extremes, it may be argued that selection will tend to eliminate segregation for genes of major effect more rapidly than for those of minor effect. This argument is tested by Monte Carlo methods. Results suggest that times to fixation do not differ greatly for major and minor genes determining a trait undergoing a moderate degree of centripetal selection in small populations but major genes are in general fixed first. The effect of linkage and the definition of 'large' and 'small' effects are discussed.