Associative overdominance caused by linked detrimental mutations

Abstract

SUMMARYAssociative overdominance due to linked detrimental mutations was investigated using the method of moment equations based on diffusion models. The expectation of the apparent selective value at the marker (neutral) locus has been evaluated. Assume two linked loci, at one of which the steady flux equilibrium is reached under constant mutational input of deleterious mutations (with ratev) having disadvantageshsin heterozygote andsin homozygotes. At another locus, the neutral alleles are segregating with frequencies near 0·5. Let Nebe the effective size of the population andcbe the recombination fraction between the two loci. Then the coefficient of associative overdominance at the neutral locus can be obtained by taking the expectation with respect to chromosome frequencies at steady flux equilibrium. It becomes approximatelywhere (LI−L0) is the inbreeding depression caused by deleterious mutations under complete inbreeding, andNehs≫ l andhs≫vare assumed. More generally, if the inbreeding depression of a chromosome segment with a length of recombination fraction C is (LI−L0) then s′ at the neutral marker at the edge of the segment iswherehsis the average heterozygote disadvantage of detrimentals.The significance of the associative overdominance is discussed in relation to actual observations. It is proposed that the most of the observed heterozygote superiority including inversion chromosomes ofDrosophila, isozyme alleles inAvenaand ABO blood group genes in man could be explained by the associated detrimentals.