Doublesex (dsx) is a gene that is involved in the sex determination system of many insects including the fruit fly Drosophila melanogaster.
The production of dsxf is caused by the presence of the female-specific version of the transformer (tra) gene.
[2] In a sense, the isoform of dsx informs a cell about the organism's sex; for instance, female genitals only develop if dsxf is present.
In conjunction with the gene fruitless, dsx also causes differences in the brain structure and behavior of males and females.
[3] Although the details of sex determination differ in the various species, there is a gene related to dsx in vertebrates (DMRT1) and in nematodes (MAB-3).