Genetic polymorphism: from electrophoresis to DNA sequences.

Recent studies indicate that the amount of protein variation undetected by electrophoresis may be reasonably small. Nevertheless, at the protein level, a typical sexually-reproducing organism may be heterozygous at 20 or more percent of the gene loci. Although the evidence is limited, it appears that at the level of the DNA nucleotide sequence every individual is heterozygous at every locus - if introns as well as exons are taken into account. The evidence available does not support the hypothesis that, at least at the protein level, the variation is adaptively neutral.