Trisomy 21, or Down syndrome, occurs when there is a normal diploid chromosomal complement but one (extra) chromosome 21. While there is reduced fertility in both sexes, females have higher fertility than males. Van Dyke et al. (1995; Down Syndrome

Question

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 #21 chromosomes) 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) One-third 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) Two-thirds of the surviving offspring would be expected to have Down syndrome.
E) None of the surviving offspring would be expected to have Down syndrome.

Answer 1

Answer: D