Why is the sex chromosomes of humans and most animals written as XY while for birds it is ZW
In: Biology
Zw is a different biological system. Xy animals, xx is female with two similar chromosomes and xy is male with two distinct ones; In zw animals, males have two of the same chromosome, written zz, but females have distinct z and w chromosomes. Essentially they are opposite ways of randomizing offspring genetics
To differentiate them. The sex-determination works in opposite way: humans (and other mamalians) offspring sex is determined by the sperm that has either X or Y chromosome. Male chromosomes are XY and female XX. Male birds have ZZ chromosomes and females have ZW and the chromosomes in the egg determine the sex of the offspring.
Although XY and ZW control sex determination, they are otherwise complete dissimilar. There are zero genes conserved between humans and birds in these chromosomes, and they even function differently. For example, in humans a mismatched pair (XY) is male while it birds (ZW) it’s female. A ZZ bird is male. Here’s a good non-technical article about it.
https://gizmodo.com/why-do-birds-have-zw-sex-chromosomes-instead-of-xy-1682328912
It’s a scientific designation to make it easier for scientists.
XY and ZW are all sex chromosomes. These don’t look the same between different species and instead they’re grouped together by function.
In XY the Y chromosome is the sex-determining chromosome and those with it become male. Sex is determined by a difference in the sperm.
In ZW the W chromosome is the sex-determining chromosome and those with it become female. Sex is determined by a difference in the egg.
~~The X and Y chromosomes in humans are named for their shape. The X looksike an X, and the Y is missing a tail.~~
Edit: nope, im just misremembering. Chromosomes all look X shaped, the Y has a small tail. X is named for “wait where’s this one’s partner? Call this one X for unknown.” And then Y just seemed right.
Other sex chromosomes discovered have been named along similar one-letter conventions, just for ease and consistency, but I don’t think the W chromosome is a very good W shape.
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Oh, cool, I didn’t even know about this thing, what other kind of sexual chromosomes are there?
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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Because they’re dinosaurs?
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