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

depends on if you on plato or not

a. possible errors

d. new questions

explanation:

predictions and planning happen before. a summery comes after. errors you get after investigation and new questions propose what you can look for in the future.