Trisomy 21 comes in a few forms, with variable intellectual compromise. In the classic form, there is a problem in meiosis where either the sperm or egg has a pair of 21’s, rather than the one it is supposed to. There is a transposition type, where essentially a part (large or small) of 21 becomes attached to another chromosome, resulting in a partial trisomy, and there can be an issue with mitosis after the egg is fertilized, so that only a fraction of the cells carry the trisomy. This is a “mosaic” trisomy, and in this form the disability varies depending on how far along the development was when the error occurred.
tl;dr: It’s complicated.
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