Evidence for overdominance inDrosophila melanogaster

In artificial populations ofDrosophila melanogaster the mutantsvermilion (v) andcinnabar (cn), both interfering with eye pigment formation, appeared to be balanced against wild type, indicating that the heterozygotesv/+ andcn/+ have a selective advantage over the respective homozygotes. In a population homozygous forv, the mutantcn in competition with its wild type allele tended to be eliminated. Ifcn was homozygous,v showed a similar tendency. Thus in competion experiments it was found that the heterozygotes for one of the two recessive genes,v andcn, ceased to be heterotic when the other recessive is homozygous. These findings are considered to be evidence for the existence of overdominance.