The effects of deleterious mutations on linked, neutral variation in small populations.

The effects of recessive, deleterious mutations on genetic variation at linked neutral loci can be heterozygosity-decreasing because of reduced effective population sizes or heterozygosity-increasing because of associative overdominance. Here we examine the balance between these effects by simulating individual diploid genotypes in small panmictic populations. The haploid genome consists of one linkage group with 1000 loci that can have deleterious mutations and a neutral marker. Combinations of the following parameters are studied: gametic mutation rate to harmful alleles (U), population size (N), recombination rate (r), selection coefficient (s), and dominance (h). Tight linkage (r </= 10(-4)) gives significant associative effects, leading either to strong reduction of heterozygosity when the product Nhs is large or to a clear increase when the product Nhs is small, the boundary between these effects being 1 < Nhs < 4 in our simulations. Associative overdominance can lead to heterozygosities that are larger than predicted by the background selection models and even larger than the neutral expectation.