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Modern Genetic Analysis Chapter 17. Population and Evolutionary Genetics Variation and its Modulation
Genetics In Process 17-1: The calculation of allele frequency

If fA/A, fA/a, and fa/a are the proportions of the three genotypes at a locus with two alleles, then the frequency p(A) of the A allele and the frequency q(a) of the a allele are obtained by counting alleles. Because each homozygote A / A consists only of A alleles and only half the alleles of each heterozygote A / a are type A, the total frequency (p) of A alleles in the population is

Similarly, the frequency q of a alleles is given by

So,

If there are multiple alleles, the frequency for each allele is simply the frequency of its homozygote plus half the sum of the frequencies for all the heterozygotes in which it appears.

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