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CyberneticPanda t1_jbi6tye wrote

Reply to comment by actuallyserious650 in Is there a fertile creature with an odd number of chromosomes? by TheBloxyBloxGuy

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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PuddyVanHird t1_jbj7tin wrote

> 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?

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CyberneticPanda t1_jbjp2bu wrote

The Y Chromosome is mostly non-coding DNA. We know that it has been losing genes for millions of years. The reason it shrinks while others don't is that it has no duplicate partner to repair itself from, like every other chromosome has including the X chromosome, though X only has a partner in women. We also know that it has been lost in other mammals. Some of them found alternative ways to keep producing males. We don't know for sure, but it is a reasonable hypothesis that others did not and went extinct.

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PuddyVanHird t1_jbjp9ku wrote

Interesting, thanks.

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Lurker_IV t1_jbkt0a3 wrote

Producing and carrying offspring is far more costly and risky than just producing sperm. One excellent example of this is flatworm penis-fencing where they battle to impregnate their opponent while avoiding it themselves.

Some point in our evolutionary history as mammals some mutation made it impossible for one side to get pregnant at all and only able to impregnate others thus freeing up resources for males to focus on getting as many females pregnant as they could. This strategy also carries the danger of relying entirely on others to reproduce. If females develop the ability to select only female offspring and not males then this can eliminate y-chromosomes entirely, something that has been theorized to have happened more than once already in our evolutionary past until a y-chromosome able to overcome this selectivity happened.

There are entire books on the topic of male-female reproductive strategies and cost-benefit analysis at the genome level which I won't go into as I don't have a teaching degree.

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[deleted] t1_jbi7q92 wrote

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CyberneticPanda t1_jbi8ed9 wrote

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.

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