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HEREDITARY control of sex had first support when unequal chromosomes were shown to be directly correlated with the sex of an individual or its progeny. Over the years no specific sex loci could be located within these chromosomes, however. Polyploid types within Drosophila showed that the sex chromosomes were not the only factors. Sex determination appeared to rest rather on a balance between the sex chromosomes and the autosomes. In Drosophila the X chromosomes operated by pushing the organism toward the female type, the autosomes pressed toward the male type. The Y chromosome had little effect. Loci for major sex genes utilising the intersex types as test agents were sought but again in no clear-cut instance were they found. In the interim major sex genes appeared through observed mutational types. Sturtevant (1920, 1921) in D. simulans isolated a gene in the second chromosome which could convert diploid females to intersexuals, and rendered males sterile. Phenotypically these intersexuals were femalelike in that they lacked sex combs, had 7 dorsal abdominal tergites, ovipositor of abnormal form, 2 spermathece and lacked the penis. They were male-like in having first genital tergite although abnormal in form, lateral anal plates, claspers, black pigmented tip to the abdomen. The gonads were rudimentary. The gene, as was expected since it was recessive, had no effect on D. simulans ymelanogaster hybrids.

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