Alternatively, we can calculate the frequencies of multiple alleles from the genotypic frequencies by extending Equation 23.4. Once again, we add the frequency of the homozygote to half the frequency of each heterozygous genotype that possesses the allele:

X-linked loci To calculate allelic frequencies for genes at X-linked loci, we apply these same principles. However, we must remember that a female possesses two X chromosomes and therefore has two X-linked alleles, whereas a male has only a single X chromosome and has one X-linked allele.

Suppose there are two alleles at an X-linked locus, XA and Xa. Females may be either homozygous (XA XA or XaXa) or heterozygous (XAXa). All males are hemizygous (XAY or XaY). To determine the frequency of the XA allele (p), we first count the number of copies of XA: we multiply the number of XAXA females by two and add the number of XAXa females and the number of XAY males. We then divide the sum by the total number of alleles at the locus, which is twice the total number of females plus the number of males:

2nfemales + nmales

Similarly, the frequency of the Xa allele is:

2nx nXAXa

females males

The frequencies of X-linked alleles can also be calculated from genotypic frequencies by adding the frequency of the females that are homozygous for the allele, half the frequency of the females that are heterozygous for the allele, and the frequency of males hemizygous for the allele:

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