We all have a 23 chromosomes that occur in pairs to make a total of 46. Down Syndrome happens when an individual gets 3 #21 chromosomes, instead of two. That is called "Trisomy". This can happen with other chromosomes, like 18, 9, 13, to name a few, but I found no obvious reference of trisomy with chromosome #23. Maybe you are thinking of "polyploidy", the condition of having 3 sets of chromosomes instead of two.
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