The fixation index and heterozygosity

• The fixation index (F) measures deviation from Hardy-Weinberg expected heterozygote frequencies.

• Examples of mating systems and F in wild populations.

• Observed and expected heterozygosity.

The mating patterns of actual organisms frequently do not exhibit the random mating assumed by Hardy-Weinberg. In fact, many species exhibit mating systems that create predictable deviations from Hardy-Weinberg expected genotype frequencies. The term assortative mating is used to describe non-random mating. Positive assortative mating describes the case when individuals with like genotypes or phenotypes tend to mate. Negative assortative mating occurs when individuals with unlike genotypes or phenotypes tend to mate (also called dis-assortative mating). Both of these general types of non-random mating will impact expected genotype frequencies in a population. This section describes the impacts of non-random mating on genotype frequencies and introduces a commonly used measure of non-random mating that can be utilized to estimate mating patterns in natural populations.

Mating among related individuals, termed consanguineous mating or biparental inbreeding, increases the probability that the resulting progeny are homozygous compared to random mating. This occurs since relatives, by definition, are more likely than two random individuals to share one or two alleles that were inherited from ancestors they share in common (this makes mating among relatives a form of assortative mating). Therefore, when related individuals mate their progeny have a higher chance of receiving the same allele from both parents, giving them a greater chance of having a homozygous genotype. Sexual autogamy or self-fertilization is an extreme example of consanguineous mating where an individual can mate with itself by virtue of possessing reproductive organs of both sexes. Many plants and some animals, such as the nematode

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