Is there any inherent reason why the body becomes female unless “told” by the Y-chromosome to be male as opposed to the other way around?
In: Biology
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“Human embryos start out female” is the stock answer to “why do men have nipples?” but the reality is more complicated.
The truth is that human embryos start out as both male and female, but also neither. To oversimplify, embryos essentially start with precursors of both testes and a uterus. If the Y chromosome is present, hormones will cause the testes to develop and the uterus to degenerate. If the Y chromosome is not present (or defective, as in [Swyer syndrome](https://en.wikipedia.org/wiki/XY_gonadal_dysgenesis)) the uterus will develop and the testes will degenerate. This is what lead to the conception that humans “default” as female because that’s the “default” development path which is altered only by the presence of the Y chromosome.
As to why humans are like this, it’s because all mammals are like this (birds, for instance, are the opposite, they “default” to male) so it’s probably just because the ancestor of all mammals happened to evolve that way, there may not be any evolutionary advantage or anything like that.
Contrary to the traditional belief that human embryos “default” to female in fetal development, recent studies suggest that the development of both male and female characteristics is an active process.
Every developing embryo, regardless of its sex, initially contains both male and female reproductive tracts, known as the wolffian duct and the müllerian duct, respectively.
In the presence of testosterone and the anti-müllerian hormone (AMH) gene products from the Y chromosome, the female müllerian ducts are destroyed.
However, a protein called COUP-TFII has been identified as a key player in actively eliminating the wolffian duct in a developing female embryo, giving it female characteristics.
This suggests that the development of female sex organs does not proceed by “default” but requires the coordinated action of specific signaling proteins.
There is no such thing as defaulting to female during fetal development. When sperm are produced during meiosis, they contain either an X chromosome or a Y chromosome. If a sperm carrying an X chromosome fertilizes an egg, the baby is female. If a sperm carrying a Y chromosome fertilized an egg, the baby is male.
A study conducted in 2017 found that the idea that embryos “default” to female was completely false.
The male pathway happens “first”, and female pathway “second” – they are literally separated in time so that the fetus “tries” to be male first, and then tries to become female after it failed to become male. Remember both processes are active, the feminization process just happens later in development than the masculinization process does.
In most cases with fetal hormonal deficiencies and other sex gene errors, the fetus can use the mother’s hormones from her blood to activate differentiation if it becomes necessary, as is the case with Y chromosome bearing fetuses with androgen insensitivity.
Thus my hypothesis (relatively unsupported) is that female “default” is because female has that backup secondary pathway of “just use mom’s estrogen to kickstart gonad differentiation”
My first attempt at this was auto-modded for some reason, so, I’ll try again.
It’s a lower-energy-requirement situation.
Mother Nature is lazy. If you need to make both P and V shapes, it’s simply less steps to start out with the V-ish state and turn ~50% of those shapes into P shapes, than to start out with a P shape, and turn 50% of those into Vs.
The y chromosome in the xx, xy chromosome pairs responsible for sexual characteristics is a mutation. The early stages of fetal devlopment are pretty much the same with major organs like eyes, heart, lungs, and brain. The devlopment of testies from ovaries is a modification that happens later in devlopment, without a lack of distinguishing features, and those differences being prevalent in the male, the ‘default’ is female. The female fetuses also have modifications to their sexual organs as they devlop but the are less obvious.
ELI5 Answer:
Males were only invented as a way for females to trade DNA with each other. That’s what a son is.
The birds, snakes and butterflies have it the other way around — males have two identical, “complete” (ZZ) chromosomes, while females have one “depleted” (WZ) chromosome. So other evolutionary lines show us, that a different system is possible.
I think it is more that a neutral gender develops first, which resembles females, but isn’t then it branches off to one or the other.
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