xilog t1_jbps2iq wrote on March 10, 2023 at 7:54 PM

Reply to comment by Tropenpinguin in Why are Y chromosomes shorter than the X chromosome? by Which-Community-5851

> Also sex-determination doesn't only involve the Y chromosome, but around 60 genes working in concert all over the genome.

But aren't all of those triggered by SRY, which is (usually) located on the Y-chromosome?

Tropenpinguin t1_jbqmuh9 wrote on March 10, 2023 at 11:23 PM

SRY is like a conductor. It triggers the multiple genes to express testes or suppress ovaries. But those genes also trigger and suppress other genes. And while SRY is the trigger in some mammals, it's not necessary to kick start sex determination.

For example the platypus has five pairs of sex chromosomes. Females are XXXXXXXXXX and males are XXXXXYYYYY. Despite that many Y, on none of them SRY is found. One of those other interacting genes triggers sex determination. What's even more interesting those various involved genes can be found in similar combination throughout all vertebrae, but the trigger differs.

So while SRY is the trigger in some, it's not the only one.

xilog t1_jbqqfgg wrote on March 10, 2023 at 11:49 PM

TIL! Thanks :)

Tropenpinguin t1_jbqpoga wrote on March 10, 2023 at 11:44 PM

I actually had to look some infos up again and found another fun (?) fact.

In South America there are nine species of vole (genus Akodon) in which a quarter of female are XY, not XX. Their Y chromosome is complete with SRY, yet they still develop ovaries and produce viable eggs. That suggests they must have a entirely new master switch gene that can suppress SRY.