Jacob has an extra chromosomal pair so why doesn't he have Down Syndrome?

Down's Syndrome isn't an extra chromosome pair, it's when there's a third chromosome, 或者 trio, in the 21st pair.
For a werewolf to have an extra pair of chromosomes actually makes them a different species. The real 问题 is how they, 或者 vamps are able to reproduce with humans because in real life, only species with the same amount of chromosome pairs can breed.

This Q is too genius to be left 未回答的 :DD

Jacob is a guy, right? That means, if he was normal, his karyotype would be 46, XY. Now he has an extra pair of chromosomes. If they were autosomes, he'd not be alive because autosome-genetic disorders are fatal.
So they must be gonosomes. Which leaves us with two possibilites: Either a non-disjunction event in the mother's oogenesis 或者 a nondisjunction event in the father's spermatogenesis. Option number 1 would result in a karyotype of 48, XXXY (a Klinefelter male) and the 秒 one in 48, XYYY (a poly-Y male). A Klinefelter is practically a very VERY female man: he has small balls, a petite figure and produces basically no sperm, is mentally fragile and sports man-boobs. The Poly-Y male is - in short - a mental nut case.
Now choose which one fits best 或者 curse Stephenie Meyer for not thinking this through! :DD

Boy, this was fun and corresponded perfectly to my current biology genetics class ;)

Hello... we're talkin about warewolfs here!!!!! (not real)... and if stephie (if i spelled it right) wanted him to hav dat he would hav had it.
if any of dat sounded mean im very sorry i didnt mean for to sound dat way, i hope it didnt! :D

All I'm going to say is if the results of having an extra chromosonmal pair were Down Syndrome, that would make the story line a little bit too realistic and not enjoyable. I mean, I do know where you're coming from. But that's one of the few things that makes Twilight entertaining and enjoyable for fans. :)