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Why are farmers trying to prevent mules and hinnies from breeding? Did a quick google and didn't find out.

I feel like this ignores the common ancestry of all life, though.

Like, sapiens-neanderthalensis "interbreeding" is a great example.

We diverged for a while, likely just due to geographic isolation, then re-crossed, and now a significant fraction of people are "hybrids", even if neanderthal proper aren't around anymore.

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Are you referring to Speciation?

But do any have an odd number of chromosomes?

The most crazy examples of animals are usually parthenogenetic insects. And in the distant future, even human males may lose their Y chromosome (although another chromosome would need to take over). Here is a case study featuring the spiny rat: https://greekreporter.com/2022/11/30/rat-without-y-chromosome-human-genetic-future/

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Very detailed answer, thank you!

But if the number of chromosomes changes as species evolve, there would most likely be a point where an organism with one number of chromosomes is mating with one that has a different number of chromosomes.

Is that what causes that? I knew that many plant species had a ridiculous number of redundant chromosomes, but I didn't know it was because of a leak of centrioles. How do the cells separate the DNA during cell division, then?

Hybridization brings genetic diversity. Lots of small differences to choose from.

Without going to read all those sources, where is the new male-determining genre? And is it wholly new, it is it SRY in a weird place? I've heard of a condition in humans where SRY can wind up on the X chromosome in some sperm cells, with the sister Y chromosome not carrying it, leading to (exceedingly rarely) XX male humans or XY female humans.

> That is happening right now. The Y chromosome used to be bigger and is shrinking and will be gone in a few million years.

Can you really extrapolate that it will disappear altogether from the fact that it's shrinking? There's no evolutionary reason that's obvious to me why the Y chromosome needs to be particularly large, but there is an evolutionary reason to have one at all. It's certainly true that there are alternative possibilities, but the probability of losing SRY at a population level is still significantly lower than the probability of losing some other random gene that isn't expressed. Unless there's an established mechanism that means the Y chromosome has to keep shrinking?

Yea Fungi mating types aren’t so much sexes as we understand them and more decoder rings.

It’d be like if we had 4 sex chromosomes instead of two, and multiple letters instead of binary.

XAYZ sex vs LBXK sex, and you can only mate with someone whose every chromosome is different.

That, in very broad strokes, is the method behind some fungi reproduction. Others are pretty analogous to human sexes. It’s a very wide range of strategies with fungi.

There is also the problem of atmospheric seeing, which uncorrected limits the resolution to about 1 km at the distance of the Moon with excellent seeing conditions.

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> The y chromosome is decreasing because there's either pressure to do so, or no pressure to stop it.

It seems that sex chromosomes (as well as "parasitic" beta chromosomes) are always under a pressure to degrade over time. But, IIRC there is also a counter-pressure for new chromosomes to arise before this.

This seems like they are missing something important. Evolution is driven by those genes that get passed on. If absence of the gene produces females and if females do not ever carry the sex gene it's basically impossible for the gene to be lost. They are over extending the prediction based on the chromosome getting smaller but to suggest the key gene will just disappear is just silly. For a genotype to become dominant in a species it needs to convey some sort of advantage. Usually it's a survival advantage as this correlates with reproductive success. But here we are strictly talking about a reproductive advantage. It's just impossible for that to become the dominant trait as it is a direct disadvantage. The gene (or lack there of) can't actually get passed on and therefore it can't become the dominant genotype. This is strictly regarding the whole "extinction" argument btw. Evolution just doesn't work that way

Not relevant to the original topic but it blew my mind when I found out that there are mushrooms with thousands of sexes. Super weird little fungi :)