To show how this formula works, Figure 23.6 calculates al-lele frequencies in two populations, each containing 200 diploid individuals. Population 1 has mostly homozygotes (90 AA, 40 Aa, and 70 aa); population 2 has mostly heterozygotes (45 AA, 130 Aa, and 25 aa).
The calculations in Figure 23.6 demonstrate two important points. First, notice that for each population, p + q = 1. If there is only one allele in a population, its frequency is 1. If an allele is missing from a population, its frequency is 0, and the locus in that population is represented by one or more other alleles. Since p + q = 1, then q = 1 - p. So when there are only two alleles at a given locus in a population, we can calculate the frequency of one allele and then easily obtain the second allele's frequency by subtraction.
The second thing to notice is that both population 1 (consisting mostly of homozygotes) and population 2 (consisting mostly of heterozygotes) have the same allele frequencies for A and a. Therefore, they have the same gene pool for this locus. However, because the alleles in the gene pool are distributed differently, the genotype frequencies of the two populations differ. Genotype frequencies are calculated as the
In any population:
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