| Xy Sex-determination System |
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The XY sex determination system was first described independently by Nettie Stevens and Edmund Beecher Wilson in 1905 . MECHANISMS Some species (including most mammals) have a gene or genes on the Y Chromosome that determine maleness. In the case of humans, a single gene ('' SRY '') on the Y chromosome acts as a signal to set the developmental pathway towards maleness. Other mammals use several genes on the Y chromosome for that same purpose. Not all male-specific genes are located on the Y chromosome. Other species (including most '' Drosophila '' species) use the presence of two X Chromosome s to determine femaleness. One X chromosome gives putative maleness. The presence of Y chromosome genes are required for normal male development. Humans, as well as some other organism, can have a chromosomal arrangement that is contrary to their phenotypic sex, that is, XX males or XY females. See, for example, XX Male Syndrome and Androgen Insensitivity Syndrome . SEE ALSO
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