How is the kid’s sex decided during pregnancy and what is intersex?
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Development of sex is a very complicated process with many stages, that depend on each other. While this process should pick between male development route and female one, there are a lot of places to sidetrack this process. When this process produces someone, who has both male and female features – such person is called “intersex”.
Now, I try to ELI5 the process:
**Stage 0. Chromosomes.** Human genome is split into 46 chromosomes, which are organized into 23 pairs. One pair is special – it is a “sex pair”. Women have two similar sex chromosomes (called X chromosomes). Men have two distinct ones: one X and one Y. Each parent gives their child one chromosome from each pair. Mother can only give X chromosome, but father can give either X or Y. If the child gets XX – it’s a girl, if XY – it’s a boy.
…Usually. In reality, there are many ways to fail this process:
* Sometimes the child gets more than 2 chromosomes from the pair
* Chromosomes themselves don’t matter: genes inside them matter. The female body is the default, but it can be switched to male with a single gene – SRY. This gene usually sits on the Y chromosome – but it doesn’t have to: it can work from any chromosome! Y chromosome with damaged SRY looses it’s function and produces a girl. Some other gene can mutate into fake SRY and produce a boy.
* Non-sex pairs can also contain genes, necessary for sex development. Many sex genes are just signals – they require pair detector genes. Those detector genes can mutate, and either stop working, or make false detections.
**Stage 1. Internal reproductive organs:** ovaries or testicles. Those are developed by the command of (or absence of) SRY gene. They also give signals to the later stages – later stages do not use sex chromosomes directly! If this stage gets sidetracked – all others will too.
**Stage 2. External reproductive organs.** Those are developed by command from stage 1. There are many genes at work at that stage: faulty genes can make incomplete organs. This is also the stage, that the doctors check to assign gender at birth.
**Stage 4. Brain.** This stage influences sexual orientation and self-identified gender. It is commanded by stage 1. There are several known ways for this stage to fail: one of them involves the mother’s immune system attacking a signal hormone.
**Stage 5. Puberty.** This stage happens 9-16 years after birth. It is again commanded by stage 1. This stage is special, because it can be relatively easy controlled with medication.
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