In the case of fetal gender determination, Mr. D, it's not the chromosomes that become active. They're already there. It's hormones that fire up to either lets us remain female or make us male. I suspect you probably meant hormones, actually.

It's that critical, very early hormone-influenced period that scientists are increasingly finding is what influences our subsequent sexual preferences. That is, those hormonal influences on the fetal brain are now thought to be responsible for people later being gay, straight, or transgendered.