A note on Fisher's ‘average effect’ and ‘average excess’

Abstract

The average effect and average excess of a gene substitution are formulated in terms of gene frequencies and inbreeding coefficient. This clarifies their meanings and shows how each is affected by non-random mating. The meanings of various definitions are examined, and one commonly used definition of average effect is found to be invalid with non-random mating. The concept of breeding value is shown to have no useful meaning when mating is not random.