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 21s)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, they each have an extra chromosome 21. During gamete formation, they can pass on either one or two chromosome 21s. The possible combinations for the offspring are: (1) normal (two chromosome 21s), (2) Down syndrome (three chromosome 21s), or (3) non-viable (four chromosome 21s). Since conceptuses with four chromosome 21s are not likely to survive, the viable combinations are normal and Down syndrome. Among these, two-thirds would have Down syndrome and one-third would be normal.

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