Protein gel electrophoresis was a leap forward in our ability to detect genetic variation, but this technique had significant limitations. Researchers could study only enzymes because they needed to be able to stain specifically for enzyme activity and could detect only mutations that resulted in amino acid substitutions that changed a protein’s mobility in the gel. Only with DNA sequencing did researchers finally have an unambiguous means of detecting all genetic variation in a stretch of DNA, whether in a coding region or not. The variations studied by modern population geneticists are differences in DNA sequence, such as a T rather than a G at a specified nucleotide position in a particular gene.

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Calculating allele frequencies, then, simply involves collecting a population sample and counting the number of occurrences of a given mutation. We can look even closer at the example of the Drosophila Adh gene from Fig. 21.4 to focus not on the amino acid difference between the Fast and Slow phenotypes, but on the A or G nucleotide difference corresponding to the two phenotypes. If we sequence the Adh gene from 50 individual flies, we will then have 100 gene sequences from these diploid individuals. We find 70 sequences have an A and 30 have a G at the position in question. Therefore, the allele frequency of A is 70/100 = 0.7 and the allele frequency of G is 0.3. In general, in a sample of n diploid individuals, the allele frequency is the number of occurrences of that allele divided by twice the number of individuals.

Quick Check 2 Data on genetic variation in populations have become ever more precise over time, from phenotypes that are determined by a single gene to gel electrophoresis that looks at variation among genes that encode for enzymes, to analysis of the DNA sequence. Has this increase in precision resulted in the uncovering of more genetic variation or less?