Why is the sex chromosomes of humans and most animals written as XY while for birds it is ZW
In: Biology
Others have explained why biologists use different names for chromosomes in different chromosomal sex determination mechanisms. But there’s an important assumption in your question:
> Why is the sex chromosomes of … most animals written as XY
It is not. XY is common among mammals (because most use the XY sex determination system), but not animals in general, not even vertebrates.
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