Can someone who knows embryology poke some holes (lol) in their new devastating argument that all embryos are female at conception and hence everyone is legally a woman? My understanding was that the embryo was not formed with ?? chromosomes, but XX or XY and just takes a little bit of time for the Y to kick in and start making you have balls, but that's why men have nipples.
Basically could you take a blastula or whatever the earliest stage is and peep the chromosomes and know that if all goes well, this will be a boy?
(Also I wonder if any pooners are seething about this stupid argument)
To answer your question, yes you could determine it before any development occurs by looking at a freshly-fertilised embryo.
Biological sex ultimately comes down to gene expression, there are a few genes that affect downstream components of sex differentiation, but the Sex-determining region Y protein is invariably responsible for sex differentiation in humans. One quick aside here, this is true only for placental mammals.
During gestation, there are bipotent cells around the urogenital ridge (precursor to your junk) that
could differentiate to develop into
either male or female gonad tissue. SRY is a transcription factor, this means its purpose is to influence the expression of other genes, in this instance a gene called SOX9 which is yet another transcription factor. I won't go into detail about what SOX9 does because it's a complicated signalling cascade, but just understand that it essentially flicks on all the switches in the balls factory. The effect of SRY and SOX9 is two-fold, it inhibits female development, and initiates male development. If SRY is absent or otherwise non-functional, female sexual differentiation occurs. Ultimately, the bipotent cells will develop one way or the other based on the influence of SRY.
Here's some factors that can obfuscate that process:
- Non-functional SRY gene (female differentiation, XY chromosomes)
- XX-male syndrome, where an X-chromosome somehow has the SRY gene (male differentiation, XX chromosomes)
- Ovotesticular syndrome (variety of causes, extremely rare, usually involving chimerism) - this is the one example where I'd say someone cannot be accurately defined as one sex or another.
So in most cases, sexual differentation is determined by the presence of a Y-chromosome, which is determined at conception
not during gestation. To account for all scenarios, it's more accurate to say it is determined by the presence of a functional SRY gene, which is again
determined at conception.
Even when you account for the smokescreen of DSDs (intersex is not an accurate descriptor for what these disorders truly are), biological sex is almost always binary and determined at conception.
There's a few factors that can complicate sexual development downstream, these are typically more involved with hormones:
- (Complete/Incomplete) Androgen Insensitivity Disorder (androgen receptors don't work, the person produces male sex hormones but they don't have a phenotypic effect). These people usually have internal testes and are therefore biologically male.
- 5-a-reductase 2 deficiency (supressed or non-existent conversion of testosterone to 5a-dihydrotestosterone, this impacts sexual differentation during gestation, but masculinisation occurs during puberty) - there's anecdotes of people being raised and girls and suddenly developing into unambiguous males at puberty.
I am a biologist, but developmental biology/embryology is not my field. Some stuff I've said may be wrong or inaccurate.
The main takeaway here is biological sex is determine by the presence of a functioning SRY gene (can be simplified to Y-chromosome, but DSDs make this cloudy), which is determined at conception. Also, it's not even accurate to say that a foetus is female until differentiation occurs, humans are androgynous (given that the urogenital cells are still multipotent).
