Protein Polymorphisms Gel electrophoresis separates protein molecules on the basis of differences in size and electrical charge. If a nucleotide variation in a structural gene results in the substitution of a charged amino acid, such as glutamic acid, for an uncharged amino acid, such as glycine, the net electrical charge on the protein will be altered. This difference in charge can be detected as a change in the rate at which proteins migrate through an electrical field. In the mid-1960s, John Hubby and Richard Lewontin used gel electrophoresis to measure protein variation in natural populations of Drosophila, and Harry Harris used the same techniques to measure human variation. In subsequent years, researchers have used the technique to study genetic variation in a wide range of organisms (Table 26.1), although subsequently developed DNA techniques have in many cases replaced the use of enzyme electrophoresis. Table 26.1. Allozyme Heterozygosity at the Protein Level (This item is displayed on page 643 in the print version) Species Population Studied Loci Examined Polymorphic Loci[*] per Population(%) Heterozygotes per Locus(%) Homo sapiens (humans) 1 71 28 6.7 Mus musculus (mouse) 4 41 29 9.1 Drosophila pseudoobscura (fruit fly) 10 24 43 12.8 Limulus polyphemus (horseshoe crab) 4 25 25 6.1 Source: From Lewontin, 1974, p. 117.