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 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 chromosome 21’s) 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?
◦ Two-thirds of the surviving offspring would be expected to have Down syndrome.
◦ All the children would be expected to have Down syndrome.
◦ None of the surviving offspring would be expected to have Down syndrome.
◦ One-third of the surviving offspring would be expected to have Down syndrome.
◦ One-half of the surviving offspring would be expected to have Down syndrome.
Sickled and normal red blood cells.
A child with Down syndrome.
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