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how inbreeding depression (measured as the average phenotype of a population) will change over time with continued consanguineous mating. Under the dominance hypothesis, recessive alleles that cause lowered fitness are more frequently found in homozygous genotypes under consanguineous mating. This exposes the deleterious phenotype and the genotype will decrease in frequency in a population by natural selection (individuals homozygous for such alleles have lower survivorship and reproduction). This reduction in the frequency of deleterious alleles by natural selection is referred to as purging of genetic load. Purging increases the frequency of alleles that do not have deleterious effects when homozygous, so that the average phenotype in a population then returns to the initial average it had before the onset of consanguineous mating. In contrast, the overdominance hypothesis does not predict a purging effect with consanguineous mating. With consanguineous mating, the frequency of heterozygotes will decrease and not recover until mating patterns change (see Fig. 2.12). Even if heterozygotes are frequent and have a fitness advantage, each generation of mating and Mendelian segregation will reconstitute the two homozygous genotypes so purging cannot occur. These predictions highlight the major difference between the hypotheses. Inbreeding depression with overdominance arises from genotype frequencies in a population while inbreeding depression with dominance is caused by the frequency of deleterious recessive alleles in a population. Models of natural selection that are relevant to inbreeding depression on population genotype and allele frequencies receive detailed coverage in Chapter 6 in this volume.

Inbreeding depression in many animals and plants appears to be caused, at least in part, by deleterious recessive alleles consistent with the dominance hypothesis (Byers & Waller 1999; Charlesworth & Charlesworth 1999; Crnokrak & Barrett 2002). A classic example of inbreeding depression and recovery of the population mean for litter size in mice is shown in Fig. 2.16. Model research organisms

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