It‘s the Y chromosome in Indian muntjacs that‘s added on.

Or rather the Y chromosome doubled at some point, with one of them becoming a non sex determining variant.

Since a ‚functioning‘ Y chromosome that determines sex only actually needs a single gene to switch from the normal female type to the male type; it can get pretty wonky without much trouble.

I‘d assume in other species it‘s the same.

Also the X chromosomes in Indian muntjacs is stuck into a different chromosome.

So the female muntjac has two ‚regular‘ chromosome pair, one pair with a tiny X portion stuck to the two tops. While the male muntjac has the same two regular pairs; and then the next pair has the X portion only stuck to a single one in the pair, and the Y being a tiny additional chromosome.

So really, the males have a higher number of chromosomes because their Y chromosome is free floating and doesn‘t ‚attach‘ to the spot where the second X would go in females

As single X chromosome individuals are perfectly viable in virtually all species, it seems the Y chromosome can really do whatever it wants and things will still work.

Btw the Chinese muntjac has 46 chromosomes and can interbreed with the Indian muntjac with 6/7 chromosomes.

And the change from the 46 variant to the 6/7 one is pretty recent.

Like they have the same number of genes. They just Stuck all those 46 chromosomes together in a very chaotic way for some reason.

While staying perfectly healthy throughout.

https://www.jstage.jst.go.jp/article/cytologia/70/1/70_1_71/_pdf

For the chromosomes.

Since the Y chromosome is the ‚inferior‘ variant of the X chromosome, I.e. it carries virtually zero essential genetic information, because only XX individuals are ‚whole‘; it is much more free to just ‚be‘ and still work.

It really only needs the SRY gene and it‘ll work.