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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?
The sheer number of ways sex can be determined amongst vertebrates is amazing, let alone other animals or microbes (there are fungi with 10,000 'sexes'/mating types...). I will restrict my examples to vertebrates.
As a rule, in most vertebrates (including humans and other organisms in which it is genetically determined) everything needed to make all the biology of both sexes is present in every individual, but a switch needs to be thrown to pick which processes to initiate.
Many reptiles use temperature during a critical developmental period with no sex c... (read more)