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It's SRY that gets lost, and it would probably not get lost all at once, but instead get a mutation that makes it work less well and shifts the chances of being born male lower. That will create selective pressure for the mutation near SOX9 (or another that substitutes for SRY) to spread through the population.

I’m all ears. I’ve been inundated with researching other, more pressing and unpleasant things, and would love a reprieve by someone enthusiastic about the topic.

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Why do they actively want to prevent mules from breeding? (besides wanting them available foe work).

Thank you for this rabbit hole! I am stunned that creatures with such a huge difference in chromosome number can produce viable offspring! Reading about chromosomal fusion, genome shock, and stunning, is freaking incredible, and I'm going to be obsessing about this for a while. I pity they people in my life who are about to get the most random, intense, infodump soon.

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That is happening right now. The Y chromosome used to be bigger and is shrinking and will be gone in a few million years. Most mammals have a similar situation to humans, who have an X chromosome with 900 genes and a Y chromosome with 55 genes. One of those 55 genes is the one that causes male sex characteristics to start to develop - don't tell your right wing friends, but everyone is female at first and the males TRANSform at about 9 weeks.

The platypus has 2 equal sized sex chromosomes, and we diverged from them evolutionarily 166 million years ago. We have lost 845 Y chromosome genes in that time, which means we will lose the rest in about 11 million years. All is not lost, though.

We know of a couple of other mammals that lost their Y chromosome and still produce males. In most mammals, a gene called SRY on the Y chromosome (sex region y) triggers another gene on another chromosome called SOX9 to start the process of developing into a male. In a couple of spiny rat species from Japan, the y chromosome has disappeared. The males have a duplication mutation near the SOX9 gene that turns it on and the females don't have that mutation. Humans could evolve (or splice in) a similar mutation to keep producing males once we have lost SRY.

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There are some unisexual salamanders that can be triploid or even pentaploid, and it doesn't really affect their fertility because they are all female and reproduce by mating with males of closely-related species.

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Some creatures like stick bugs or grasshoppers have an X0 sex determination system. So when eggs are fertilized, some sperm may have an X chromosome and some don’t. If the result of fertilization is two X chromosomes (even number of total chromosomes), then the offspring is female. If the result is only one X chromosome (an odd number of chromosomes), then the offspring is male.

I read about that, apparently it'll just get replaced. I don't remember much of that article, I read it a long while ago. Now I might go fall down a wiki hole just thinking about it

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So plants don't have centrioles (I think that's right? I'm a plant ecologist not a geneticist) and as a result, the chromosomal number doesn't matter as much. Because of this, many plants are polyploid - they can be diploid, triploid, or even tetraploid, with additional chromosomal replicates believed to confer some advantages in specific conditions, depending on the species. You could, then, have a triploid plant with an odd number of chromosomes.

EDIT: Apparently fertility in triploid plants is greatly reduced, which I was unaware of, though reproduction is possible: https://journals.ashs.org/hortsci/view/journals/hortsci/51/8/article-p968.xml#:~:text=Triploids%20are%20typically%20highly%20infertile,et%20al.%2C%202011).

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Nope. Nope nope nope.

Several species of insect have odd chromosomal numbers. Both male and female Japanese Oak Silkmoths have 31 chromosomes (not pairs).

The male Indian muntjac has 7; the female has 6.

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I’m a forensic dna analyst and I unfortunately can’t answer your direct question but I can offer some insight on some things you’ve mentioned or other commenters have mentioned.

Haploid and diploid: our fully developed human cells are diploid—we reproduce sexually. We have 23 pairs of chromosomes for a total of 46. This is our diploid number. One “partner” of each pair originated from our bio “mom” (egg donor) and the other from our bio “dad” (sperm donor). Our sex cells or gametes (sperm or eggs) are haploid. They contain just ONE partner from each pair. Our diploid number is 23. Each egg or sperm is completely random whether it has the partner chromosome we inherited from our mom or dad, for EACH pair (if I am the egg source, my egg could have all the partners that originated from my mom or all the partners from my dad or any combination of partners from mom vs dad—but just one partner of each pair, for a haploid total of 23 chromosomes). My haploid egg is then fertilized by a haploid sperm also containing one partner of each chromosome pair inherited from its bio parents, randomly). This is why we share half our dna with each of our parents but may share no dna or all of our dna with our full siblings—random sorting of diploid chromosomes into haploid sex cells randomly combining during fertilization to make a new combo of diploid cells :)

There are some human genetic conditions where we sometimes end up with an extra partner or missing a partner in a chromosome pair, giving an odd number of total chromosomes. This usually occurs when the pairs are being copied and divided to produce eggs or sperm by our bio parents. Trisomy 21 (downe syndrome) is an example most people have heard about—an extra copy of chromosome 21 is inherited from one of bio parents. This can also happen with the X and Y chromosome (one copy of X is Turner syndrome, two copies of X with Y is klinefelter syndrome, whereas typically we inherent either two x or one x and one y).

Sorry for the book. Hope it helps. Always glad to help :)

Not soon... but eventually.

https://www.sciencealert.com/the-y-chromosome-is-slowly-vanishing-a-new-sex-gene-could-be-the-future-of-men

> The sex of human and other mammal babies is decided by a male-determining gene on the Y chromosome. But the human Y chromosome is degenerating and may disappear in a few million years, leading to our extinction unless we evolve a new sex gene.

> The good news is two branches of rodents have already lost their Y chromosome and have lived to tell the tale.

> A new paper in Proceedings of the National Academy of Science shows how the spiny rat has evolved a new male-determining gene.

(the paper is Turnover of mammal sex chromosomes in the Sry-deficient Amami spiny rat is due to male-specific upregulation of Sox9 - https://www.pnas.org/doi/10.1073/pnas.2211574119 )

Chasing links and searches:

NYT A Gene Mystery: How Are Rats With No Y Chromosome Born Male?

> ...

> Both female and male Amami spiny rats have only one X chromosome, an arrangement only known to occur in a handful of rodents among mammals. Arata Honda, associate professor at the University of Miyazaki and the lead author of the paper, said in an email that he was partly motivated to study Amami spiny rats in the hope that learning about them might reduce their risk of extinction.

> No one knows how or why, but at some point the rats lost their Y chromosome and, along with it, an important gene called SRY that’s considered the “master switch” of male anatomical development in most mammals.

And this also lead's to the OP's question: https://en.wikipedia.org/wiki/Ryukyu_spiny_rat

> The Ryukyu spiny rat (Tokudaia osimensis) is a species of rodent in the family Muridae. Endemic to Amami Ōshima island in the Amami Islands of the Ryukyu archipelago of Japan, its natural habitat is subtropical moist broadleaf forest. The karyotype has an odd diploid number, 2n = 25. Like its relative T. tokunoshimensis, it has lost its Y chromosome and SRY gene.

http://www.departments.bucknell.edu/biology/resources/msw3/browse.asp?id=13001850

> The unique chromosomal complement of this species (2n = 25, with no X in the female or visible Y in the male) first documented by Honda et al. (1977) and corroborated by Kimiyuki et al. (1989)

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> the Y continuing to dwindle down to nothing and then we’re just like stick bugs.

I don't think that would be likely given that human sex development is decided by the presence or absence of the SRY gene. Without that gene activating, you develop female. So without a Y chromosome, the SRY would have to find somewhere else to live.