What are the chances of people with down's syndrome passing this particular condition to their children?

35-50%: While people with Down syndrome (trisomy 21) have reduced fertility, they can have children. If a child is born to a couple where one parent has trisomy 21, studies have shown that the child has a ~35-50% chance of also having down syndrome.

Statistically 50%: A person with Down syndrome has a statistical risk of 1 in 2 or 50% of passing on the extra 21 chromosome. However, there is decreased fertility in males and most do not father children. Females are more likely to reproduce but there is decreased survival of embtos with a 47 chromosome count, so actual risk is probably 30-40%.