Question

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

Answer 1

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. 

Answer 2

Its not 0 percent and its not 100 percent. So, that leaves us with A or D, It is most likely a 50/50 chance.