Abstract
THE tropical species of Drosophila, D. polymorpha, is polymorphic with regard to the pattern of pigmented abdominal bands. The polymorphism is a case of balanced polymorphism, as shown in a previous paper1. The types are inherited as if caused by two alleles of a single autosomic gene. The dark and the light types are the homozygotes and the intermediate type is the heterozygote. The heterozygote has an adaptive value higher than that of the homozygotes. The heterosis of the intermediate type ensures the maintenance of the two alleles in the populations. The probability that the pigmentation pattern is by itself an adaptive character is very small. Another possibility to be considered is that the two main genes responsible for the pigmentation are associated with other genes through an inversion. The differences in adaptive value could, then, be due to the genes carried in the inversion as a whole.
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