okay - basic biology question...

Each egg has an X chromosome. Each sperm has either an X or a Y chromosome. So sex (XX or XY) is determined when the egg is fertilised. Male and female embryos develop in the same way for the first 7 weeks, after which male and female sex organs start to differentiate. So, for instance, a 6 week old XY embryo and a 6 week old XX embryo will appear identical, but they are internally programmed to adopt different fates.

Embryo development of other animals in comparison with humans is fascinating - they look very similar. Don't think anyone is using this as proof of trans-speciesism though.

Iirc (it's not an experiment I have actually tried ), if you stick a load of sperm in a centrifuge, the x's will sink and the y's float, as the x's are marginally heavier than the y's. Apparently this is how they select for sex in some animal breeding, or at least how they used to.

I'm not sure how you gender-select for lady brains, breeding-wise...

Well at about 3 days post conception (ivf) it is possible to tell the sex, because you can select for sex-linked diseases this way.

That’s right Sonic. Although the chromosomes are defined at conception, sex included, male and female foetuses develop in exactly the same way initially. That’s why males have nipples.

The answer to the question s phrased it is 'maybe' - as not everyone is either XX or XY.
Everyone has at least one X chromosome.
People with anything other than XX or XY suffer a genetic condition. They will have various physical symptoms. It's similar to people with Downs syndrome.

There are various disorders of sexual development but they don't have anything to tell us about trans - because trans people have standard XX or XY chromosomes

Its basically because there aren't any sex differences in a cluster of cells in the early stages of development. The fetus is growing a brain and the start of the central nervous system, arms and legs.

Technically - if I'm being a pedantic arse - whether the individual sperm will be carrying an X or a Y is determined at spermatogenesis - each individual sperm is already a boy sperm or a girl sperm before conception.

I think I read that the Y chromosome doesn't start to express itself (and differentiate the fetus) until later on, about 6 weeks I think.

But the "path" that the fetus is put on is determined at conception by the X or Y sperm. The Y just doesn't cause a diversion (I am using traffic terminology for some reason! I'm not a biologist...) until 5-6 weeks when development goes down a different route than an XX one would.

Hopefully one of our resident scientists can explain if I've understood properly

Sex determined at conception as stated above. Genitals visibly differentiated by 11 weeks gestation.
Makes a mockery of all that “assigned at birth” crap, doesn’t it!

The answer to the question s phrased it is 'maybe' - as not everyone is either XX or XY.

But it's also 'yes'to your subsequent question that the individual's chromosomes are fixed at the moment of conception.

However the epigenetics may be influenced by environment, including uterine environment, so it is possible (and still a fairly new field of study) that expression of X or Y linked characteristics could be influenced.

There is some (unverified) suggestion that hormones during pregnancy might affect masculinity or femininity. So if you believe in gendered souls, then you might believe gender can be affected by hormone washes.

Yup!

All depends on whether the sperm which fertilizes the egg carries an X or Y chromosome.

Well-informed my arse.

You are correct.

The sperm arrives with either an X or a Y chromosome and whichever it is determines the sex of the embryo.

An egg has an X, and a sperm has an X or a Y... So sex is determined at the point of conception.