Genetics seem to be the magical source of explanation for things in our fiction these days, from X-Men to Gattaca to Alphas. We can look back now and see the ridiculousness of "radiation" being the origin story of so many comic book heroes, as it seems obvious that a bite from a radioactive spider or being nearby a nuclear explosion would be more likely to kill than to grant super powers.
In the future we'll look back and see "because genes" in the same way, akin to the Dungeons and Dragons storyteller who finds themselves trapped and says "because a wizard did it."
Our growing knowledge of the human genome can tell us a lot, but there are limits. A huge portion of biology is influenced by other factors and we've even seen how environmental and sociological factors can change your genes.
Getting your genes sequenced is not like going to the fortune-teller and learning your destiny. Yet people will casually use "genetic" and "biological" interchangeably and point to genes as if they are a magical explanation for the world around them. And it's amazing how often this gets used to justify transphobic bigotry.
One of my favorite examples is the 1999 case Littleton v Prange. The court declared a marriage between a trans woman and a cis man to be invalid by defining sex as only determined by chromosomes, pointing out that they "do not change with either hormonal treatment or sex reassignment surgery." According to this, anyone with XY chromosomes is considered male and since both individuals had XY chromosmes, the marriage was not valid.
There's only one problem.: they never tested either individual's chromosomes. For that matter, I don't believe they have ever tested anyone's chromosomes as a criteria of getting a marriage license.
The court tried to point to what they saw as an objective standard for sex by invoking genetics and chromosomes, but like many bigotries they were simply unaware of how deep their biases ran. It appears they looked at Christie Lee Littleton and decided she was male before ever considering the evidence, and only looked for a justification to confirm their biases after the fact. By declaring that to be fact without ever testing their chromosomes - or anyone else getting married - it makes their argument as ridiculous as radioactive spiders.
What they really care about is birth assigned sex and they are only assuming that will line up with chromosomes.
By knowing someone's birth-assigned sex, you can guess at what their chromosomes are, and have a good chance of being right, just about 99.9%. But extend that to the millions of people who live in Texas and you've got thousands of people who you've guessed wrong about.
In fact, when chromosomal mapping technology was first becoming widely available, it was common for high school biology classes to have students test their own chromosomes as a part of a lab. They had to stop that practice because too many students were getting results other than what they'd expected and it was causing distress. Rather than address the reality of what that means, schools decided to bury their head in the sand and keep on pretending that everyone can accurately guess their XX/XY chromosomes.
Now CrossFit is making headlines for refusing to allow Chloie Jönsson, a trans woman competitor, to participate in their women's games. Despite being in flagrant violation of California law, the company claims this does not count as discrimination because... you guessed it: because chromosomes.
Chloie was born, genetically - as a matter of fact - with an X and a Y chromosome... Chloie may have felt herself emotionally, and very conscientiously, to be a woman in her heart, and that she ultimately underwent the legal and other surgical procedures to carry that out, cannot change that reality... Our decision has nothing to do with "ignorance" or being bigots - it has to do with a very real understanding of the human genome, of fundamental biology, that you are either intentionally ignoring or missed in high school. (CrossFit response to Jonsson's attorney)
Once again, this argument is made absent of any actual genetic testing. Nothing I have read has indicated that CrossFit has medical documentation of any of their competitors' chromosomes. What they are putting forth as a "matter of fact" is nothing more than a guess - a stand in for their biases.
Nevermind the fact that the Olympics and many other major athletic organizations have abandoned chromosomes as a standard for determining gender due to the overwhelming medical evidence showing that they have little to no influence on competitive advantage as opposed to other factors such as hormones. All the other reasons given to bar trans athletes from competition are easily debunked.
But CrossFit doesn't need to review any of the medical literature because they know many people will hear "human genome" and just take it as a given that genes determine everything. As long as they speak from a position of authority and back it up with a high school-level understanding of biology, maybe no one will notice that they don't actually investigate or care about the chromosomes of any of their other competitors.
It's not about the chromosomes, it's about birth-assigned sex - and discriminating based on birth-assigned sex is explicitly against the law.