Caenorhabditis elegans, are hermaphrodites that can mate with themselves.
There are also cases of disassortative mating, where individuals with unlike genotypes have a higher probability of mating. A classic example in mammals is mating based on genotypes at major histocompatibility complex (MHC) loci, which produce proteins involved in self/non-self recognition in immune response. Mice are able to recognize individuals with similar MHC genotypes via odor, and based on these odors avoid mating with individuals possessing a similar MHC genotype. Experiments where young mice were raised in nests of either their true parents or foster parents (called cross-fostering) showed that mice learn to avoid mating with individuals possessing odor cues similar to their nest-mates' rather than avoiding MHC-similar individuals per se (Penn & Potts 1998). This suggests mice learn the odor of family members in the nest and avoid mating with individuals with similar odors, indirectly leading to disassortat-ive mating at MHC loci as well as the avoidance of consanguineous mating. One hypothesis to explain the evolution of disassortative mating at MHC loci is that the behavior is adaptive since progeny with higher heterozygosity at MHC loci may have more effective immune response. There is also evidence that humans prefer individuals with dissimilar MHC genotypes (Wedekind & Furi 1997).
The effects of non-random mating on genotype frequencies can be measured by comparing Hardy-Weinberg expected frequency of heterozygotes, which assumes random mating, with observed heterozygote frequencies in a population. A quantity called the fixation index, symbolized by F (or sometimes /, although it never will be in this book, since f is reserved for the inbreeding coefficient as introduced later in section 2.6), is commonly used to compare how much heterozygosity is present in an actual population relative to expected levels of heterozygosity under random mating:
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