How is the kid’s sex decided during pregnancy and what is intersex?

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How is the kid’s sex decided during pregnancy and what is intersex?

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There are two main ways of determining the sex of a fetus prior to birth. One is, basically, looking: an ultrasound at 20 weeks of pregnancy gives you (well, professionals) a pretty good look at fetal anatomy, and that includes the genital region.

The other, which has only become mainstream in the past 10 years, is through genetic testing earlier. A pregnant person can have their blood drawn around 12 weeks of pregnancy and that blood can be tested for the fetus’ genetic material, including whether said fetus is XX , XY, or another combination.

And that also leads to intersex: not all people are born with either XX or XY sex chromosomes, nor are all people born with what you might think of as the “standard” set of reproductive organs and systems. “Intersex” is a broad term that encompasses a range of individual conditions and body types that fall under that umbrella, outside of the binary. (This is different from someone having a non-binary gender identity, as sex and gender are not interchangeable.)

Biological sex is determined by a person’s genome, which is defined in the moment or conception, when gametes from each of their parents become a zygote (a full set of DNA). What’s in that DNA will determine how they develop.

For most people, it comes down to whether they got an X or Y chromosome from their father, but there are some exceptions outside XX and XY.

We’re not like crocodiles or whatever where our incubation temperature can change which sex we turn out to be. 😅

What do you mean by “decided”? Do you mean how to doctors find out, or what actually determines it biologically?

If your question is the latter, the thing that determines the sex is the sperm cell. All eggs have an x chromosome and all sperm have either an x or a y. If a x chromosome sperm fertilizes an egg, that fertilized egg will have 2 x chromosomes and will become female. If a y chromosome sperm fertilizes an egg, that fertilized egg will have 1 x and 1 y chromosome and will become male.

Intersex is an umbrella term for when a person does not fall neatly into either male or female. Sometimes this is because of sex chromosome abnormalities, other times it’s other genetic conditions, and other times it’s the result of developmental abnormalities that are not genetic.

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.