Sex determined by chromosomes - exceptions?

[BonnyBo]

I’ve seen on twitter a rebuttal of sex being characterised by chromosomes and wondered if anyone scientifically literate could parse it for me?

It’s not the usual DSDs, I don’t think. I understand that with DSDs, they are disorders of development and are sex specific but this seems to be a family of XY individuals, some of whom have given birth and aren’t showing chimerism.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2190741/

Does this, as a twitterer claimed, mean that sex isn’t contingent on chromosomes?

Genetic mosaicism is when you have different chromosome combinations in different cells in different tissues in different parts of the body, as oposed to all your cells in your whole body being having the same chromosome tally (apart from the sex cells, sperm and eggs). It causes all sorts of odd and unexpected results.

For example, my friend was initially told that her baby had Down Syndrome when pregnant after initial testing because they picked up male cells with trisomy 21 - an extra copy of chromosome 21, which is the cause of downs syndrome. However, her child does not have Downs Syndrome because it was a case of moascicism and that happened after the cell lines split shortly after conception so the cell that went on to form the placenta had the extra copy but the cell that went on to form the embryo and baby did not.

All genes on chromosomes and the myriad possibilities for things to happen differently from how they usually happen. And even mutations are 'normal' because that is the basis for evolution as some will have been beneficial/positive over the years that humans have evolved.

Here’s the bit where it says the ovarian tissue has a tiny bit of XX.

Yeah but elsewhere in the paper they don't even mention the <1% 46 XX and suggest the 5.9% 45 X may be artifactual so they're not saying that it's the <1% 46 XX cells that have made this woman uniquely fertile (though her XY daughter is not), and such a tiny proportion of cells even if real wouldnt be enough, they're saying, or speculating, that there's some other mutations in genes going on in the 93% cells with XY chromosomes on the X chromosome which have 'sex reversed' her ie sent her down the female development path to develop functional ovaries and a uterus instead of testicles, as would normally happen if you had predominantly XY cells in the tissues that turn into gonads - testicles or ovaries.

Fascinating. Thank you.

Fascinating case though now I've read through it all. Nothing revolutionary here though, just confirms that there are many, many different genes involved in the pathway of sexual differentiation in humans/mammals and that you can get very unexpected outcomes from 'the norm' with unusual mutations, which may or may not be on the sex chromosomes. And that's what's happened to this family, unusual mutation(s) has been propagated through it and given odd results. Still only 2 possible sexes though and still the body you end up with is determined by the particular gene variants you have encoded by the DNA that makes up the chromosomes in your cells, and the hugely complex interplay between them all.

This is the trouble really, it's massively, massively complex and the interplay between 100s of thousands of genes encoded on the 46 chromosomes (ish, some can get added or lost) that everyone has in every cell of their body, can lead to millions of slightly different outcomes. The underlying mechanism of sex determination remains the same though and you end up either male or female, whether you look like that from the outside or not and whether you're fertile or not. I spent 4 years studying molecular biology and genetics at university so can get the gist of this paper, as can other people on these threads with scientific training. But people on twitter who didn't even pass Biology GCSE (that barely touches on inheritance of genes) are now trying to understand how it works to prop up ideological causes without doing the work - which in this country is science GCSEs, A levels and Science/Medicine university degrees.

Not that I'm saying people shouldn't try to understand it btw, I know eff all about astrophysics but I still like reading about it, just that it's hard because the people who do understand it, at the molecular level, haven't traditionally been very good at explaining it in terms that people that haven't studied it at depth can digest it. I do think that is changing now though, which might be one positive thing to come out of the trans delusion, and there are people on twitter etc doing sterling work explaining the genetics of sex determination such as Zachary Elliot. Plus these threads of course where I have learnt as much about feminism over the years as I learnt about DNA at university.

Hypothetically speaking, if there were some arrangement of chromosomes that made a "3rd sex" possible, it still wouldn't change the fact that Málaga airport personnel all have the standard male chromosome arrangements.

Not even hypothetically possible. There are millions of possible arrangements of the atoms within the molecule that we call DNA and millions of possible combinations of genes on chromosomes and lots of different arrangements of chromosomes in cells with movement of bits of DNA between them, but there are only 2 possible outcomes of sexual differentiation in mammals - set up for having large gametes/eggs - female, or small gametes/sperm - male, whether or not a particular individual actually does have/produce them.

There is no third sex possible from all that diversity and nobody is intersex, even if they have tissues with different chromosomal arrangement (mosaicism). There is no other possibility than egg or sperm resulting from meiosis (the specialised type of cell division that create sperm or eggs). There's no Spegg or Esperm and hence there's only ever 2 sexes possible. Sex is the outcome of all that genetics. It doesn't mean that someone with a Y chromosome can't end up female, or vice versa, the other way round from 'the norm'. But still, binary, and only 2 possible oucomes. Which is pretty remarkable really given all the background genetic diversity and shows how sexual reproduction has been honed by evolution over millions of years.

Humanity used to aspire to be ‘only a little less than the angels’. Now some of them aspire to be mushrooms. Oh well…..🍤🧌

@AIstolemylunch thank you for all your explanations

Fascinating. There is brief mention of mosaicism , not all cells in the body will share the same genetic material I understand.
Was there mention of any consanguinity?

Sorry, will read whole thread to check for experts.

The more I read, the more I realise what miracles we are.

That's funny. Just read the thread and saw all the scientists using the word fascinating. Now I have a mental picture of peering at a tra like a specimen whilst murmuring the word fascinating to myself.

We have had this paper posted here on MN previously as a 'gotcha'. However, just as AIstolemylunch says, this paper doesn't do what most people who use it as a 'gotcha' think it says. I don't believe this woman or her mother produced both sperm and ova. Only one. Their bodies are formed to produce one gamete whether successfully or not.

The supposed 'gotcha' is all about Chromosomes and those only. But many of the posters on MN are fully aware that Chromosomes are just one aspect of the categorisation of human sex.

As the saying goes..'science - it's like magic, but real.'

I doubt the article about determining sex will be quoted by TRAs. It says

determining sex does not mean “observing” or “identifying sex” in the colloquial sense. Instead, determining sex is a technical term for the process by which genes trigger and regulate differentiation down the male or female path in the womb. This determines the structures that can support the production and release of either gamete type, and thus, the individual’s sex.

and

an individual’s sex is defined with respect to gamete type and identified by the structures that support the production and release of either gamete.
Thus, while there are various mechanisms that control male and female development, there are still only two endpoints: male and female.