Originally Posted by Hanseat
No, people with trisomie 21 are perfectly capable of reproducing sexually. Only the chance that their offspring (sorry for the biological term but that's just it) will inherit this genetic defect increases greatly (I don't fell like doing a Punett square right now but go ahead...).
The point is though that there is a good chance that their child will be perfectly normal!
Females with Down syndrome are sub-fertile. The prevalance of affected offspring is about 30 - 50%.
There are only a few documented cases of paternity in males with this condition, ie male fertility is significantly diminished.