The simplest description of single-gene variation isthe list of observed proportions of genotypes in a popu-lation. Such proportions are called the genotype fre-quencies.Table 19-1 shows this frequency distributionfor the three genotypes of the MNgene in several hu-man populations. Note that there is variation betweenindividuals in each population, because there are differ-ent genotypes present, and there is variation in the fre-quencies of these genotypes from population to popula-tion.For example,most people in the Eskimopopulation are MM, while this genotype is quite rareamong Australian Aborigines.More typically, instead of the frequencies of thediploid genotypes, the frequencies of the alternative al-leles are used. The allele frequency is simply the propor-tion of that allelic form of the gene among all the copiesof the gene in the population, where each individualdiploid organism in the population is counted as con-tributing two alleles for each gene. Homozygotes for anallele have two copies of that allele, whereas heterozy-gotes have only one copy. So the frequency of an allele is
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