Genetic variation in natural populations: problem of electrophoretically cryptic alleles.

Abstract

Electrophoretic studies show that the average frequency of heterozygous loci per individual is .apprx. 12% in Drosophila and other invertebrates and .apprx. 6% in vertebrates. Only .apprx. 2/3 of all amino acid substitutions change net electric charge; a large fraction of all genetic variation may be undetected by electrophoresis. Peptide mapping of 11 independent alleles coding for alcohol dehydrogenase in D. melanogaster uncovered 1 cryptic variant; the frequency of electrophoretically cryptic variation is apparently low, .apprx. 9% in this sample. A simple model shows that this degree of cryptic variation, if it is typical of other loci, would substantially change current estimates of genetic variation: the average heterozygosity would increase from .apprx. 12% to .apprx. 25% for invertebrates and from .apprx. 6% to 21% for vertebrates. A variety of techniques, including sequential electrophoresis and heat or urea denaturation, were used by various investigators to detect electrophoretically cryptic variation. These techniques appear to be less effective than peptide mapping for detecting cryptic variation, but, like peptide mapping, they suggest that standard electrophoresis may detect most of the protein variation present in natural populations. The charge-state model of protein variation proposes that the alleles detected by electrophoresis are extremely diverse classes consisting of many electrophoretically cryptic alleles. The alcohol dehydrogenase peptide-mapping results are inconsistent with the charge-state model.