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The process by which cells select one of several specific programs of genetic information in development was examined in homoeotic mutant Antennapedia of Rappaport in Drosophila melanogaster. The results show that the alteration of determination from antenna to leg induced by this homoeotic mutation occurs in a population of cells related by proximity rather than by pedigree. Clones of cells marked by means of X-ray-induced somatic crossing-over prior to early third instar participated in the formation of both antenna and homoeotic leg, but after this time, the clone was restricted to either antenna or homoeotic leg, indicating that determination occurs in the third larval instar. The results are discussed in relation to other processes of determination.
Postlethwait et al. (Mon,) studied this question.