Thread by @FondOfBeetles

On “we all start as females” (by request).

At the level of anatomy, “female” describes a particular reproductive system - eggs in ovaries, oviducts, uterus, cervix, vagina and vulva.

This reproductive system begins to differentiate at around six weeks post-fertilisation, when the embryonic gonads - two balls of cells clumped in your pelvic area - turn into ovaries and not testes.

The ongoing development of internal and external genitalia follows this gonadal differentiation into ovaries.

This is what is meant by “organisation” - the coordinated, sequential development of multiple tissues that have evolved around a given reproductive function.

The trigger for these balls of cells to turn into ovaries and not testes is genetic. This is what is meant by sex being “determined” by genetics.

Given those genes were inherited at fertilisation, it is reasonable to say sex is determined at fertilisation.

This means you can look at that genetic information in a fertilised egg and understand what will happen in six weeks to those balls of cells, despite the balls of cells not yet existing.

In an IVF clinic, for example, you can sort embryos by this genetic information.

The genetic switch that turns the balls of cells into ovaries and not testes is the absence of a Y chromosome gene called SRY.

The female switch is often conceptualised in the OFF position. That is, unless you have the genes required to flip the switch to ON, you will differentiate ovaries and not testes.

Plenty of developing systems use ON/OFF switches to drive different tissue fates. Sometimes the ON position makes something happen, sometimes it stops something different happening.

ON is not inherently more interesting or important than OFF.

The OFF position is often labelled a “default” pathway.

That is, we ask: what will happen to these precursor cells or this precursor tissue if it doesn’t get positively switched to do something else instead? What do they turn into “by default”?

Because differentiating as male - making testes and not ovaries - requires a positive switch (the SRY gene), female differentiation has been framed as the “default” pathway.

This leads to the claim that because the differentiation of female reproductive anatomy is “default”, the embryo before differentiation of any reproductive anatomy is phenotypically female.

This makes no sense to me; it’s really random.

And while many who anchor sex as a karyotype/genotype argue that embryos are, in fact, female (or male) from fertilisation before any sex differentiation, not even this framework can translate into the idea that all embryos are female at fertilisation.

It is a bonkers claim, leaning heavily into misogynistic ideas that the absence of a penis and the presence of a precursor embryonic outlet tube - the cloaca, in both sexes - means female.

Because females are no-penis cavities, amirite?

In fact, what we know is that female development is not the result of “do nothing”. You need plenty of female-specific, positive signalling to make ovaries. They don’t just drift into existence for lack of anything better to do; they are busy little balls of cells.

The embryonic gonads - the first point of sex differentiation - are bipotential. They are “competent” to differentiate as either ovaries or testes.

Before differentiation, they are no more female than they are male.

Internal genitalia develops from two sets of ducts, one pair that can be fashioned into female bits and one pair that can be fashioned into male bits. All embryos contain both sets of ducts.

This set up is no more female than it is male.

External genitalia develops from a tissue field that is, like the gonads, bipotential.

And again, before differentiation, this tissue field is no more female than it is male.

The wording in Trump’s executive order is fine (well, I dislike “conception” but that’s a minor quibble).

Sex is determined at fertilisation. We can predict the sex class of an embryo well before any sex phenotype develops, and with almost-perfect reliability.

Human embryos do not all start as female. This is a scientifically-illiterate claim.

@Kindun526 But regardless, are you suggesting that de novo mutation post-fertilisation can switch someone’s sex?

How interesting.

How do you assess that their sex has been switched?

@Shatterface I mean, I could make the argument they have converged to the baseline of only one active X chromosome, which is a mechanism to control gene dose in females.