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

question 12

Multiple Choice

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.

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