N

where nAA, nAa, and naa represent the numbers of AA, Aa, and aa individuals, and N represents the total number of individuals in the sample. We divide by 2N because each diploid individual has two alleles at a locus. The sum of the allelic frequencies always equals 1 (p + q = 1); so after p has been obtained, q can be determined by subtraction: q = 1 — p.

Alternatively, allelic frequencies can also be calculated from the genotypic frequencies. To do so, we add the frequency of the homozygote for each allele to half the frequency of the heterozygote (because half of the heterozygote's alleles are of each type):

p = f(A) = f(AA) + / f(Aa) q = f(a) = f(aa) + 1/2 f(Aa)

We obtain the same values of p and q whether we calculate the allelic frequencies from the numbers of genotypes (Equation 23.3) or from the genotypic frequencies (Equation 23.4).

Loci with multiple alleles We can use the same principles to determine the frequencies of alleles for loci with more than two alleles. To calculate the allelic frequencies from the numbers of genotypes, we count up the number of copies of an allele by adding twice the number of homozygotes to the number of heterozygotes that possess the allele and divide this sum by twice the number of individuals in the sample. For a locus with three alleles (A1, A2, and A3)

and six genotypes (A A1, A A2, A A3, A2A2, A2A3, and A3A3), the frequencies (p, q, and r) of the alleles are:

0 0

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