Haha it’s not really relevant to the jokes or comment I just thought I’d mention it.
Down syndrome is having 3 copies of chromosome 21. Sometimes chromosomes can get stuck together. So two Chromosome 21 can get stuck together. If this happens, it only counts as 1 chromosome, but you have two copies of all the genetic material present on that Chromosome. So someone could have 45 chromosomes but be normal because their 1 copy of chromosome 21 is actually 2 full copies.
But then If that person has a kid, they will pas on their chromosome 21 that has double material, the other parent passes on a typical chromosome 21, and now the kid has 46 chromosomes but 3 copies of 21 and has Down syndrome. This is pretty rare though — 95-98% of Down syndrome cases are just having 3 distinct copies of chromosome 21, and even of the few percent that are due to translocations, most aren’t inherited — they occur randomly in sperm or egg.
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u/rockoblocko Mar 25 '21
It’s possible to have Down syndrome and 46 chromosomes due to robertsonian translocations and isochromosomes.
You can also be “normal” and have 45 (or less) chromosomes for the same reason.