How does the body know which chromosomal abnormality is good enough to become a person and others are better to be ended?
In: Biology
Down syndrome is also called trisomy 21, this means that you got three instead of the usual two copies of your 21st chromosome in body cells, resulting in all the genes on that chromosome being over-expressed. Trisomy 13 and trisomy 18 individuals also get past birth with a reasonable frequency although it is still much rarer than Down’s syndrome and they have much shorter lives. If you have too many sex chromosomes that is not much of a serious condition at all, the Y chromosome is not that relevant and the excess X chromosomes (also the second one in normal females) are for the most part silenced anyway. This is why you can even survive with just one X chromosome (an example of monosomy, only chromosome pair in humans where it is non-lethal). Monosomies are more severe than trisomies partly because having one copy of your genetic material will result in a lot of genetic damage being expressed.
Notice that your chromosomes are numbered from long to short, and trisomy 21 is thus affecting your second-to-last shortest chromosome (sex chromosomes don’t get a number). Now it just happens that there are no quintessential genes located on this short chromosome and that you can survive with all of them expressed about 1,5 times what they usually are. The trisomy has the combined effects of all of these, it really results in a wholly different recipe for a human and with chromosome 21 it just happens to be viable. This is pretty much a coincidence because of all the things that happen to lie on that chromosome, genes are spread pretty randomly. Chromosomal abnormalities are pretty extreme things genetically because they often affect an entire chromosome at a time, but also many mutations in individual genes will be ‘lethal’, although also often only if it’s present in both copies.
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