PuddyVanHird t1_jbj7tin wrote on March 9, 2023 at 1:07 PM

Reply to comment by CyberneticPanda 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.

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?

CyberneticPanda t1_jbjp2bu wrote on March 9, 2023 at 3:17 PM

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.

PuddyVanHird t1_jbjp9ku wrote on March 9, 2023 at 3:18 PM

Interesting, thanks.

Lurker_IV t1_jbkt0a3 wrote on March 9, 2023 at 7:27 PM

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.