In mammals, the distinction between male and female at the chromosomal level is due to the X and Y chromosomes. Typically, females have two X chromosomes (XX) while males have an X and a Y chromosome (XY). The Sry gene on the Y chromosome triggers the formation of the testes. However, there exist a handful of rodent species in which the Y chromosome has disappeared, taking with it the Sry gene. The mechanism by which testes development occurs in these species is not fully understood, and is the subject of much research.
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Source: Phys.org