[Verginius Rufus]

Maybe this is a dumb question, but how widespread is it for the sex of an individual to be determined by having either XY or XX chromosomes? Is it true of all hominids, or of all primates, or of all mammals? Or is it more widespread than that?

Obviously there is some way that the developing organism is programmed to develop as male or female, but perhaps in some species it is a different chromosome than it is among humans (Homo sapiens or earlier human species).

[allmendream]

All mammals I believe. Fish, for example, start out male then turn female. Birds have the sex determining factor from the female rather than the male.