The Effect of Epistasis and Gametic Unbalance

ENETIC load has been defined as the extent to which an equilibrium population is impaired because not all individuals are of optimum genotype. The concept, first discussed by HALDANE (1937), is the basis of theory developed by CROW (1958) and others for assessing the relative impact of various facets of the genetic system (especially mutation, segregation and inbreeding) on the average fitness of populations in genetic equilibrium. LEVENE (1 963) recently discussed various theoretical and experimental difficulties in utilizing the theory of genetic loads in the analysis of the genetic structure of populations. In its simplest form, this theory involves several assumptions among [which those of no epistasis on the fitness scale and linkage equilibrium are particularly restrictive. The purpose of this study is to investigate the effects of epistasis and gametic unbalance (linkage disequilibrium) on the manifestation of genetic loads when inbreeding is imposed on random-mating populations. Since the measures of joint inbreeding at two or more loci are influenced by gametic unbalance, the expressions developed by HALDANE (1949) for joint inbreeding are first generalized to take into account gametic and zygotic associations. The loads maintained at interacting loci are then evaluated in terms of genetic models involving several kinds of epistasis.