So I’m aware there’s other possible abnormalities of other chromosomes such as ring shaped chromosome 20, but I’m wondering why down syndrome with chromosome 21 is so relatively common? What about it makes it more likely to get an extra/abnormality?
Edit: Seems to be equally common and it’s simply that because chromosome 21 doesn’t have very many “important” genes so babies with down syndrome can still survive, while the others will always miscarry or die shortly after birth. Confirm? thanks
In: Biology
Humans are meant to have 2 autosomal (non-sex) chromosomes. 3 chromosomes results in overexpression of genes during transcription, which can interfere with cellular function.
It just so happens that the body is more “resilient” to overexpression of Chromosome 21, that someone with Trisomy 21 could live well into adulthood. Trisomy 13 and Trisomy 18 can live for maybe a few weeks after birth, but other autosomal trisomies almost always end up with miscarriages or stillbirths. That’s why you never see them.
The sex chromosomes are a different beast. Humans only require one functional X chromosome for life. To guarantee the existence of (genetic) females, the body’s able to inactivate the extra X chromosome to prevent overexpression. You can actually see this from a mouth swab under a microscope; the cells from females have dark spots while the cells from males don’t. Because of this ability, combinations like 45XO, XXY, XYY, and XXX can exist in a comparatively mild way. Some people don’t even realize they’re living with these until they wonder why they can’t have kids.
Humans and our ancestors evolutionarily never needed to worry about having anything more or less than 2 autosomal chromosomes, which is why nature’s much more strict than it is with sex chromosomes.
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