Settle in folks, and I’ll tell you a tale of how you got your genitals.
In order to exist, there must have been an egg. Human eggs can only contribute X chromosomes, because that’s the only chromosome they have to offer. Sperm could offer an X or a Y chromosome, but that doesn’t really factor in to this conversation until later.
In the beginning, a fetus develops the precursors to “female” internal reproductive tracts. All fetuses, even ones that later turn into men utterly terrified of being perceived as feminine, started off with something called Mullerian ducts. But, PLOT TWIST!, you also had the precursor to “male” internal reproductive tracts. Wolffian ducts often later become the plumbing that rules the world. We could call these things mesonephric and paramesonephric ducts, and these terms better describe what’s going on but are harder to say.
Anyway. You had all of this chaos inside your sweet pea sized self right about the time your momma realized her period was late. Plenty of moms don’t realize they’re pregnant and NextGen already has equipment to build into any kind of human they want. With no outside intervention, ovaries will prevail and dissolve the Wolffian ducts. I’m not even kidding. If boys didn’t have outside help, they’d literally be taken apart at the seams by a six week old soon-to-be-girl.
Ok, now this is the part where I want desperately to go in to detail about an aspect of sex determination that just will not make sense without a couple years of general chemistry, organic chemistry, and biology, and still we’d need lots of diagrams and pictures. So I’ll do my best to explain the high lights and not get bogged down in the wild trenches of science we have recently become reasonably confident that we might kinda understand.
If a Y chromosome is also present, a testis will develop instead of an ovary. Probably. *but it will not change the visible presence of external genitalia* Not by itself.
In order to wash away the presence of the Mullerian ducts, the testis have to make a cell capable of producing a hormone capable of conquering a would be uterus. That takes some time and is not guaranteed to succeed.
Ok so let’s say the tiny testis does succeed in vanquishing the Mullerian ducts. Now the fetus has a testis or two, has the potential plumbing to grow a penis and no longer has a would be uterus. That is not enough to actually grow a penis. Alas, yet another force is needed. The testis has to grow another type of cell that is capable of producing another hormone. This hormone you’re familiar with so I’ll use its given christian name: testosterone. Testosterone by itself cannot complete the process of developing ejaculatory ducts and the vas deferens and seminal vesicles, but the process cannot happen without it either. (Truly, pre-infantile men need a whole lotta help) Testosterone can be converted to another thing by a different thing and then that other thing (kinda like a boiled down, super concentrated testosterone) can finally build you a penis and scrotum and prostate.
But at any point along the way, development can be thwarted. If the testosterone is not thoroughly converted to the super concentrated version, the kid is going to appear to have a vagina at birth, even with XY chromosomes. There are so many wild and fascinating ways development can change and shift. If you can’t sleep one night and are going to wander down an internet rabbit hole, look up this stuff. It’s incredible.
So, let’s stop pretending chromosomes determine sex, let alone gender. They really don’t have that kind of power. It takes a whole team to build a penis, and that can only happen if you can defeat the uterus and ovary to begin with. Humans are extraordinary creatures full of mystery and magic. If someone wants you to call her a girl and use words like ‘she’ – just do it. It’s such an easy way to recognize someone’s complex, holy and divine self. We humans want to pretend we understand the world around us, but we’re too little to see the big picture and too big to see the small pieces. We try to understand through religious lenses, but those languages have limitations too. When Jesus was asked to boil down the whole goals of life he said to love. Love God and love each other. If you master that, then move on to the details. But if you spend a whole life time trying to be good to people, you’ll keep yourself plenty busy and probably never master it. That’s ok. Just keep putting one foot in front of the other. Maybe some day I’ll tell you the story of how you got those feet too.