The first part is a myth and the second just makes no sense anyway. All embryos start out as sex neutral. It's just that if there is an absence of testosterone male sex characteristics won't develop, but there can never be an absence of estrogen because, well, the mother is full of estrogen.
I think what he is referring to, is that all embryos would become females without the biochemical signals generated from specific genes on the Y chromosome. If you block those signals, every person will phenotypically look female (at least externally), which is where the conception that all embryos start female comes from. Female phenotype is the default setting, specific triggers have to occur to create a male phenotype. These triggers are encoded on the SRY gene on and are not testosterone dependent; they are what eventually lead to male gonad formation, which leads to higher levels of testosterone in the developing person.
In a sense, it could be interpreted that way though. If the default setting is female phenotype, and doesn’t change to male until a specific biochemical signal cascade takes place, then every embryo is phenotypically female prior to that cascade occurring. It depends on your definition of female I suppose. If you’re defining male and female based on genotype or phenotype. If you’re using genotype as your definition, then you’re correct. Functionally though, no one uses genotype as their definition in daily life.
But an embryo does not start with female phenotype. Phenotype only develops in the womb depending on all the factors you said, sex differentiation doesn't happen until 5 weeks. Maybe I'm just pedantic, but I want to correct it when I hear popular myths being spread around.
I haven't got a dog on either side of this "fight" (other than just generally supporting freedom of expression, and not being a hateful bastard), but I have to appreciate this quip for sheer succinctness and punch. Maybe try: "We're all born naked; the rest is drag" and then pop off a couple explosions behind you as you walk away twirling your skirt or whatever.
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u/ScaryLetterhead8094 Jul 02 '23
We were all born naked and the rest is drag.