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

Multiple Choice

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 rates 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.Assume that children are born to a female with Down syndrome and a normal 46-chromosome male.What proportion of the offspring would be expected to have Down syndrome?

A) One-third of the offspring would be expected to have Down syndrome.
B) Two-thirds of the offspring would be expected to have Down syndrome.
C) All the children would be expected to have Down syndrome.
D) None of the offspring would be expected to have Down syndrome.
E) One-half of the offspring would be expected to have Down syndrome.