Trisomy 21, or Down Syndrome, Occurs When There Is a Normal

question 12

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

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