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