Disassortative Mating

Disassortative mating is the preferential mating of individuals with dissimilar phenotypes. This means that there is a negative correlation between the phenotypes of mating individuals. For example, the major histocompatibility complex (MHC), mentioned in Chapter 1, is found not only in humans but in mice as well. In mice, genetic variation in MHC induces odor differences. There is disassortative mating at this gene complex in mice that is due to olfactory discrimination of potential mates (Potts and Wakeland 1993). Recent evidence indicates that this may be occurring in humans as well (Wedekind et al. 1995; Wedekind and Furi 1997). Several college males were asked to wear the same t-shirt for two days. A group of college females were then asked to smell the t-shirts and rank them (no pun intended). The t-shirts with the most pleasant (or least obnoxious) smells turned out to be shirts worn by males that shared fewer MHC alleles with the women than expected by chance alone. Moreover, the women indicated that the preferred smells were similar to those of their boyfriends or husbands. A subsequent experiment revealed that both men and women prefer odors from others most dissimilar to themselves for MHC. More direct evidence that MHC affects mating preferences in humans comes from studies in a population of Hutterites, a small, isolated religious sect that has maintained genealogical records since about 400 members originally migrated from Europe to North America in the 1870s. Hutterite couples were less likely to share MHC alleles than by chance, even after corrected for the nonrandom mating among colony lineages and close inbreeding taboos (Ober et al. 1997). Note that the baseline system of mating for detecting disassortative mating in this case is not random mating but rather avoidance of system-of-mating inbreeding.

To see the impact of disassortative mating, consider the simple one-locus, two-allele model with a one-to-one genotype-phenotype mapping and 100% disassortative mating given in Figure 3.8. As with assortative mating, the genotype frequencies in general will change over the generations in this model of disassortative mating. For example, if we start out at Hardy-Weinberg frequencies with p = 0.25, then the initial random-mating heterozygote frequency of 0.375 is altered by a single generation of disassortative mating to 0.565. Note that disassortative mating induces a heterozygote excess with respect to Hardy-Weinberg expectations (f < 0)—exactly the opposite of assortative mating. Moreover, unlike assortative mating, the allele frequencies in general are changing. In the example given above, the allele frequency is changed from 0.25 to 0.326 in a single generation.

Zygotic population

Mechanisms of producing phenotypes

Phenotypes of adult population

Mechanisms of uniting gametes (disassortative mating)

Mated population

Mechanisms of zygotic production (Mendel's first law)

Zygotic population of next generation

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Responses

  • magdalena
    What is the effect of disassortative mating in hardyweinberg?
    8 years ago

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