Human Development - Blog Posts

So as someone who's taken a college biology class with an entire unit dedicated to sex differentiation it's been bugging me immensely to see so many people (mostly on threads) say that "we're all female at conception" There's some truth to female anatomy being the "default baby" because it takes less developmental energy to create/maintain but "we're all female to start" is a way oversimplification of this to the point it's just not true. I'm not claiming to be an expert by any means but I would like to spread the knowledge I do have bc I think it's honestly cool as hell

To start I've seen this talked about a lot in relation to Trump trying to sign an executive order that categorizes people's sex by "the gametes they have at conception" There's so many issues with this. A zygote is not producing gametes at conception, it's a single cell. A zygote carries the genetic coding for which sex you are but genes are not the final determination and only make up part of your sex. For example, you may have an SRY gene which is the catalyst for male development but it may be damaged and not expressed. A person may present entirely as one sex but not be able to produce gametes, Sex isn't just one characteristic.

Once a fetus begins developing its organs it doesn't just start out with female anatomy which can then "become male anatomy if you have the genes for it" We actually start out with undifferentiated gonads (which are still not producing gametes) and two duct systems, one that can become the female reproductive organs and one that can become the male. They're called the Müllerian and Wolffian ducts. (Science illustration so you know I'm not just pulling this out of my ass)

This is where our genes come into play deciding if our gonads become ova or testes and which ducts develop into our reproductive organs. Shout out to all my intersex folks, a lot of intersex conditions become expressed at this stage of development. For example, a person with androgen insensitivity syndrome would have XY chromosomes but would have missing or non-functioning androgen receptors. This means their gonads become testes and the hormone that degrades the Mullerian ducts is released, but their body cannot use any androgens to continue male development. This means without androgens the wolffian ducts also degrade and the person would have external female genitalia and internal testes. My knowledge on the more micro-biology and chemical reactions part of sex determination is more limited so I can't really talk about it in a comprehensive way but I just wanted to share this really cool human fact.