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 32 : 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- half of the surviving offspring would be expected to have Down syndrome. B)Two- thirds of the surviving offspring would be expected to have Down syndrome. C)None 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)All the children would be expected to have Down syndrome.