Using your knowledge of meiosis and the formation of gametes, consider the following information. down syndrome is caused by nondisjunction of chromosome pair 21. if a person with down syndrome has a child with a person who does not have down syndrome, what is the probability that the child will have down syndrome?

50 percent
0 percent
100 percent
75 percent

As can be seen from the Punnett Cross, there is a fifty (50) percent chance that if one of the parent has a Non-disjunctive Chromosome 21 that the offspring being born will also carry the Non-disjunctive Chromosome as well.

I added an extra diagram for additional information purposes.  In your spare time, for a better understanding of the special case of Down-Syndrome and other genetic situations caused by a Non-Disjunctive Chromosome pair you could just do a little reading on the Trisomic Zygote which is as a result of the Non-Disjunctive Chromosome from one parent and another Chromosome from the other parent.