I recently came to think about the potential person inside me. I’m not pregnant. No, I was just thinking about how an embryo differentiates into a boy or a girl in the womb.
Unless I’m misinformed (which is of course possible, almost everything we know is ultimately hearsay), this is broadly the case: All embryos start out female. If the genome has a Y chromosome, then barring some unusual conditions the child will be a boy. The Y chromosome doesn’t do much itself; it’s small and kind of degenerate since it passes virtually unchanged from father to son without the opportunity to be “refreshed” by mixing with other chromosomes.
What it does have is a gene that sets off a sequence of cascading activations that leads to the formation of testicles. They in turn produce androgens, leading to a bunch of changes all over the body.
Sometimes this doesn’t work for some reason or another, and people with an XY-genome can still grow up and develop into women.
What’s interesting about this is that the capacity to build both male and female bodies is apparently contained in all human genomes, male and female alike. Bar the singular “start button”, the helices of women also have the capacity to grow men, and vice versa.
When you’re used to the idea that one genome builds a particular body, this realization is a little odd. I just grokked that my genome, without adding any information whatsoever (just remove the Y and replace it with a copy of my X), could have built a woman instead.
And then I wonder what she would have been like. We can see parts of ourselves as the opposite sex in our parents, children or siblings. Fraternal twins can easily be of opposite sexes. But that’s as close as we can get. The idea that we could have (very close to) an identical twin of the opposite sex is something else. I have no model I can use to visualize her. Like a sister, but “twice as similar”? I’ve seen brothers and sisters. I’ve seen identical twins. I’ve never ever seen anything like this. Nothing that helps me picture my shadow twin sister. How alike would we be? What would she look like? What would she think like?
Here is an unethical experiment begging to be tried.
UPDATE: It appears that identical twins of different sexes are indeed possible, just extremely rare and always the result of some weird chromosomal fluke. I’ve looked for pictures to satisfy my curiosity, but haven’t found any. The closest example I did find is Nicole and Jonas Maines. Now, they weren’t born boy and girl, instead Nicole is transgendered which makes it slightly different. But since she transitioned early I suppose she looks very much like someone born a girl with (nearly) the same genome would. The picture of them is interesting and close to what I expected but I’m a little surprised that they don’t look that much more similar than regular siblings.