Eli5 How does the platypus have both an XY and a ZW sex determining chromosome system?

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Eli5 How does the platypus have both an XY and a ZW sex determining chromosome system?

In: Biology

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The Platypus does have an XY chromosome system. However compared to other animals it is the closest to the ZW system. There is a gene called DMRT1 which is what is used by the ZW system to make the body develop a male. It is present on the Z chromosome so that when there is two genes it will be a boy. In mammals the DMRT1 gene have moved to other chromosome and gets suppressed by the Y chromosome. However in the platypus there is a copy of the DMRT1 gene on the X chromosome. So the X chromosome of the platypus have features of the Z chromosome suggesting that these are related. However the platypus does use the XY chromosome system and the presence of the DMRT1 gene on the X chromosome seams like an evolutionary quirk that does not have any major effect.