FIG. 21.4

How is genetic variation measured?

BACKGROUND The introduction of protein gel electrophoresis in 1966 gave researchers the opportunity to identify differences in amino acid sequence in proteins both among individuals and, in the case of heterozygotes, within individuals. Proteins with different amino acid sequences run at different rates through a gel in an electric field. Often, a single amino acid difference is enough to affect the mobility of a protein in a gel.

METHOD Starting with crude tissue—in this case, the ground-up whole body of a fruit fly—we load the material on a gel and turn on the current. The rate at which a protein migrates depends on its size and charge, both of which may be affected by its amino acid sequence. To visualize the protein at the end of the gel run, we use a biochemical indicator that produces a stain when the protein of interest is active. Here, we use an indicator that detects the activity of alcohol dehydrogenase (Adh). The result is a series of bands on the gel showing where the Adh proteins in each lane migrated.

RESULTS The Adh gene has two common alleles, distinguished by a single amino acid difference that changes the charge of the protein. One allele, Fast (F), accordingly runs faster than the other, Slow (S). Four individuals are S homozygotes; two are F homozygotes; and two are FS heterozygotes. Note that the heterozygotes do not stain as strongly on the gel because each band has half the intensity of the single band in the homozygote. We can measure the allele frequencies simply by counting the alleles. Each homozygote has two of the same allele, and each heterozygote has one of each.

Total number of alleles in the population of 8 individuals = 8 × 2 = 16

Number of S in the population = 2 × (number of S homozygotes) + (number of heterozygotes) = 8 + 2 = 10

Frequency of

Number of F in the population = 2 × (number of F homozygotes) + (number of heterozygotes) = 4 + 2 = 6

Frequency of

Note that the two allele frequencies add to 1.

CONCLUSION We now have a profile of genetic variation at this gene for these individuals. Population genetics involves comparing data such as these with data collected from other populations to determine the forces shaping patterns of genetic variation.

FOLLOW-UP WORK This technique is seldom used these days because it is easy now to recover much more detailed genetic information about genetic variation from DNA sequencing.

SOURCE Lewontin, R. C., and J. L. Hubby. 1966. “A Molecular Approach to the Study of Genic Heterozygosity in Natural Populations. II. Amount of Variation and Degree of Heterozygosity in Natural Populations of Drosophila pseudoobscura.” Genetics 54:595–609.