We don't have to imagine. We can look at birds, where the sex chromosomes are the opposite. I haven't looked at them, so I don't know how much is a consequence of the chromosomal structure. But, for some reason, I'm skeptical that most people who pontificate their role have looked either. The points about hormones and development are more reasonable.
Are the opposite? I assumed the XX/XY goes back to the very beginnings of gender i.e. fishes... how comes very different chromosomes can make the same hormones i.e. AFAIK birds do have testosterone?
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