How do sex chromosomes determine our reproductive body parts?
Biologic sex is much less straightforward than most of us have been taught. Humans come in a variety of combinations of body parts (anatomy), hormones, and X and Y chromosomes.
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Q: How do sex chromosomes determine our reproductive body parts?
A: Biologic sex is much less straightforward than most of us have been taught. Humans come in a variety of combinations of body parts (anatomy), hormones, and X and Y chromosomes.
Most people either have XY chromosomes and develop typical male anatomy or have XX chromosomes and develop typical female anatomy. But this isn’t true for everyone. Some people are XXY or XO and have slightly different anatomy and hormone levels than their typical XY or XX counterparts. Even when the sex chromosomes themselves are XX or XY, body parts don’t always develop into strictly female or male anatomy. For example, some XX individuals are born with testes, penis, and scrotum, and some XY individuals are born with a vulva and vagina but no uterus. Other times, the external anatomy looks like something in between a penis and a vulva and is called “ambiguous genitalia.” These are just a few examples - there are lots of different combinations that exist!
This variety is possible because of how our body parts form in the first place. For the first 6 weeks after sperm meets egg, all embryos have the same anatomy, regardless of whether they will later develop male or female parts. We all start out exactly the same! Externally, we have what looks like a tiny penis with folds of skin on either side. As the embryo develops, the tiny “penis” may become either a clitoris or a penis. The folds of skin may become the vulva or the scrotum. Internally, we all start with two sets of tubes. One set of tubes can turn into a uterus and fallopian tubes, and the other set can turn into the vas deferens (tube that carries sperm) and other male-typical structures.
How those parts develop depends on a complicated dance between many different genes and hormones. If a certain gene is altered, a receptor is missing, or a hormone level is too high or too low, you can end up with different combinations or slightly altered body parts. This actually happens in about 1-2% of humans. That’s about as common as having red hair! Some variations are obvious at birth, such as ambiguous genitalia. But others can be subtle, so not everyone who has a difference may even be aware of it.
So, even at the level of chromosomes and body parts, things are not always black and white (or pink and blue)! Humans exist on a spectrum of biologic sex, not a binary. This is the reality of human bodies, even though it’s not commonly taught or recognized.
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For more information:
Video animation of typical fetal sexual differentiation
Video animation of differences in development
Video lecture on fetal sexual differentiation
Differences in Sexual Development FAQ on Intersex Conditions [archived link]
BuzzFeed Video - What it’s like to be Intersex
This post was updated by Those Nerdy Girls from the original written on February 17, 2023.
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Great article! Thanks for shedding light on why biological sex isn't black and white.