Introduction to Genetic Analysis 615

Introduction to Genetic Analysis 615 - 44200_19_p611-642...

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The simplest description of single-gene variation is the list of observed proportions of genotypes in a popu- lation. Such proportions are called the genotype fre- quencies. Table 19-1 shows this frequency distribution for the three genotypes of the MN gene in several hu- man populations. Note that there is variation between individuals 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 Eskimo population are MM, while this genotype is quite rare among Australian Aborigines. More typically, instead of the frequencies of the diploid 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 copies of the gene in the population, where each individual diploid organism in the population is counted as con- tributing two alleles for each gene. Homozygotes for an allele have two copies of that allele, whereas heterozy- gotes have only one copy. So the frequency of an allele is
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