Mode of action of a homoeotic gene in Drosophila melanogaster

Polycomb (Pc, Pc2) — one of three closely linked nonallelic homoeotic mutants in chromosome 3 of Drosophila melanogaster that change the development of the hind legs to that of the first legs — is located in the left arm of the chromosome between ri and eg. By using a duplication of the salivary chromosome segment 75A–B through 80, flies were obtained that had two or three doses of Pc alleles. Evidence is given to show that Pc2 and Pc+ are antagonistic in their actions indicating an active competition between homologous gene products. A hypothesis that Pc+ possibly bears a regulatory function comparable to genetic repressors is suggested and briefly discussed.