Trisomy 21, or Down syndrome, occurs when there is a normal diploid chromosomal complement but one (extra) chromosome 21. Although fertility is reduced in both sexes,
Females have higher fertility than males. Van Dyke et al. (1995; Down Syndrome Research and
Practice 3[2]:65-69) summarize data involving children born of Down syndrome individuals. Given the fact that conceptuses with 48 chromosomes (four chromosome 21's) are not likely to survive early development, what percentage of surviving offspring would be expected to have Down syndrome if both parents have Down syndrome?
A) None of the surviving offspring would be expected to have Down syndrome.
B) All the children would be expected to have Down syndrome.
C) One -half of the surviving offspring would be expected to have Down syndrome.
D) One -third of the surviving offspring would be expected to have Down syndrome.
E) Two -thirds of the surviving offspring would be expected to have Down syndrome.

Question

Quizplus · Accepted Answer

If both parents have Down syndrome, they each contribute an extra chromosome 21. Conceptuses with 48 chromosomes (four chromosome 21's) are not viable. Of the viable conceptuses, two-thirds would have Down syndrome (trisomy 21) and one-third would have a normal chromosome count.

Trisomy 21, or Down Syndrome, Occurs When There Is a Normal