Evolution
SRY may have arisen from a gene duplication of the X chromosome bound gene SOX3, a member of the Sox family. This duplication occurred after the split between monotremes and therians. Monotremes lack SRY and have a ZW-like sex determination system, likely involving DMRT1, whereas therians (marsupials and placental mammals) use the XY sex determination system. SRY is a rapidly evolving gene. A small number of mammals lack this gene entirely and use an alternative form of sex determination.
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