I know many animals use an X/Y sex determination system, but not all of these animals use the same X/Y genes. That is to say the X/Y sex determination system has convergently evolved in many species, and they often use different chromosomes as their X and Y than humans do.

I'm interested specifically in the variant of the X/Y genes that humans and most mammals use and their origin. (side note, is there a better term for referring specifically to therian x/y genes vs the X/Y genes of say snakes or insects?)

I'm frankly curious what we know about how the sex chromosomes we use now evolved and when it happened. For example do we know of any non-therian mammals that have autosomal chromosomes that look similar to our sex chromosomes that presumably suggest where our sex chromosomes split off from?

I'm aware that monotremes have their own odd variant of X/Y sex determination system, but it looks like neither the platypus, nor the Echidna X/Y chromosomes are similar to our own. So clearly our sex determination system came after we split from monotremes, but are there any therian mammals that don't share a last common ancestor with us that used our X/Y genes for sex determination?

Not sure if this is asking for more then we can know, but if the monotremes' similarity to Z/W chromosomes in birds suggests pre-mammal ancestors used z/w sex determination system then how come we now have only one non-autosomal chromosome now? How would a theoretical pre-mammal z/w sex chromosomes have become autosomal?

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    $\begingroup$ As I am sure you know by now, the Stack Exchange format works best with one question per question. I count 3 related questions in yours. You could split them up into 3 questions or re-word to make what you actually want to know clearer. $\endgroup$
    – bob1
    Mar 3 at 3:31
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    $\begingroup$ try googling "vertebrate sex chromosome evolution": nature.com/articles/nrg.2015.2 $\endgroup$ Mar 3 at 3:35


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