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Sex determination in Drosophila melanogaster is under the control of the X chrom0some:autosome ratio and at least four major regulatory genes: transformer (tra), transformer-2 (tra-2), doublesex (dsx) and intersex (ix) . Attention is focused here on the roles of these four loci in sex determination. By examining the sexual phenotype of clones of homozygous mutant cells produced by mitotic recombination in flies heterozygous for a given recessive sex-determination mutant, we have shown that the tra, tra-2 and dsx loci determine sex in a cell-autonomous manner. The effect of removing the wild-type allele of each locus (by mitotic recombination) at a number of times during development has been used to determine when the wild-type alleles of the tra, tra-2 and dsx loci have been transcribed sufficiently to support normal sexual development. The wild-type alleles of all three loci are needed into the early pupal period for normal sex determination in the cells that produce the sexually dimorphic (in pigmentation) cuticle of the fifth and sixth dorsal abdominal segments. tra+ and tra-2+ cease being needed shortly before the termination of cell division in the abdomen, whereas dsx+ is required at least until the end of division. By contrast, in the foreleg, the wild-type alleles of tra+ and tra-2  + have functioned sufficiently for normal sexual differentiation to occur by about 24 to 48 hours before pupariation, but dsx+ is required in the foreleg at least until pupariation—A comparison of the phenotypes produced in mutant/deficiency and homozygous mutant-bearing flies shows that dsx, tra-2 and tra mutants result jn a loss of wild-type function and probably represent null alleles at these genes.—All passible homozygous doublemutant combinations of ix, tra-2 and dsx have been constructed and reveal a clear pattern of epistasis: dsx &amp;gt; tra, tra-2 &amp;gt; ix. We conclude that these genes function in a single pathway that determines sex. The data suggest that these mutants are major regulatory loci that control the batteries of genes necessary for the development of many, and perhaps all, secondary sexual characteristics.—The striking similarities between the properties of these loci and those of the homeotic loci that determine segmental and subsegmental specialization during development suggest that the basic mechanisms of regulation are the same in the two situations. The phenotypes and interactions of these sex-determination mutants provide the basis for the model of how the wild-type alleles of these loci act together to effect normal sex determination. Implications of these observations for the function of other homeotic loci are discussed.

SEX AND THE SINGLE CELL. I. ON THE ACTION OF MAJOR LOCI AFFECTING SEX DETERMINATION IN DROSOPHILA MELANOGASTER