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
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.
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