Why is down syndrome called trisomy 21

 "Trisomy" basically means that instead of there being two copies of a chromosome there are three (tri=3). (All normal chromosomes have only two copies- they're all pairs, as you will notice if you look at any picture of a chromosome).  

Trisomy + 21 means that the 21st chromosome has an extra copy, which is essentially the definition of Down Syndrome.  

Hope this helps

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