Trisomy 21, or Down syndrome, occurs when there is a normal diploid chromosomal karyotype but one (extra) chromosome 21. While there is reduced fertility in Down syndrome individuals, both sexes make viable gametes. (Van et al. 1995; Down Syndrome Research and Practice 3(2): 65-69). During gamete formation, all three copies of chromosome 21 synapse. During the reductional division, one daughter cell receives 2 copies of chromosome 21, while the other receives 1 copy of chromosome 21. 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?
O One-third of the offspring would be expected to have Down syndrome.
O Two-thirds of the offspring would be expected to have Down syndrome.
O All the children would be expected to have Down syndrome.
O None of the offspring would be expected to have Down syndrome.
O One-half of the offspring would be expected to have Down syndrome.