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 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) None 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) One-half of the surviving offspring would be expected to have Down syndrome.

Question

Quizplus · Accepted Answer

When both parents have Down syndrome, their genotypes are Trisomy 21, meaning they each have an extra chromosome 21. When they reproduce, the possible combinations for their offspring include: one normal set of chromosomes (without the extra chromosome 21), one set with an extra chromosome 21 (resulting in Down syndrome), and one set with two extra chromosomes 21 (which is typically not viable and does not survive early development). Therefore, of the surviving offspring, one-half would have a normal set of chromosomes, and one-half would have Down syndrome. However, since the offspring with two extra chromosomes 21 (which would make up one-third of all conceived offspring assuming all combinations are equally likely) are not likely to survive, the surviving offspring's distribution changes. This leaves us with two-thirds of the surviving offspring having Down syndrome (since they represent two out of the three viable groups: normal, Down syndrome, and non-viable).

Trisomy 21,or Down Syndrome,occurs When There Is a Normal Diploid