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
Sounds like you have the right answer. Trisomy can happen in any chromosome, but most result in a non-viable fetus. This can also happen in the sex chromsomes (resulting XXX, XXY or XYY chromosomes) but these have less severe consequences than Down syndrome so they are not as apparent as Down.
Down Syndrome is caused by a full or partial extra copy of chromosome 21, meaning a person has 3 copies of that chromosome instead of 2. This is called a trisomy. Trisomy 21 (Down syndrome) is more common simply because it’s one of the few trisomies that are survivable, which is because it has the fewest number of genes besides the Y chromsome, so the extra copy does the least damage.
Any chromosome can be copied too many times by accident, but the vast majority of those will result in nonviable embryos that die before ever developing. Trisomies of the 13th and 18th chromosome are also sometimes survivable, but they result in much more serious issues and life expectancy in babies born with those trisomies is typically less than a year, so it’s uncommon to encounter a person with those trisomies.
Trisomies of the sex chromosomes (X and Y) as opposed to the numbered chromosomes are usually less severe. The typical presentation is XY in males and XX in females, but a person can be XXY, XYY, or XXX. People with these trisomies typically have full, normal lives, although they may have some health issues and/or mild cognitive impairments.
So in short, it’s not necessarily that chromosome 21 is more prone to abnormalities, it’s just that, because chromosome 21 has the fewest genes (besides the y chromosome), the extra copy does the least damage, which makes the extra copy survivable.
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.
> 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?
Pretty much this. Down’s syndrome is caused by having 3 copies of chromosomes 21. You could end up with 3 copies of any chromosome, but most cause an early miscarriage.
It’s a little more complex than the genes being “important” or not, but just having extra copies of those genes and making them more active isn’t fatal.
Just to add one additional piece of information: The chromosomes are numbered by length, so 1 is the longest and 22 is the shortest (X/Y are separate). That means 21 is the second-shortest non-sex chromosome. It’s not coincidence that we find the one well-survivable extra chromosome towards the end of the list.
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