In Drosophila it is the ratio of the number of copies of the X chromosome to the number of copies of each autosome (X:A) that determines sex. The presence of a Y chromosome is not the determining factor. If the ratio is 0.5, which will occur in a typical diploid drosophila with a single X chromosome, then male development is triggered. If the ratio is 1, which will occur in typical Drosophila with 2 X chromosomes then female development is triggered.

A ratio greater than 1, which can arise in Drosphila with 3 or more X chromosomes, results in female development. A ratio between 0.5 and 1, which can result in cases of aneuploids, leads to a mixture of traits in the fly.

The mechanism for how X:A determines sex involves alternative splicing of the Sex Lethal gene, which is covered elsewhere.

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