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

question 37

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.