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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 (considering individuals with Down syndrome), females have higher fertility than males. [Van Dyke et al. (1995. Down Syndrome Research and Practice3(2):65-69) summarized data involving children born of Down syndrome individuals.]When a zygote inherits four of the #21 chromosomes (48 chromosomes total) it will not survive early development in utero. Considering this fact, what proportion of surviving offspring would be expected to have Down syndrome when both parents have Down syndrome?(HINT: Consider events during prophase I, metaphase I, and anaphase I. It may help to use a Punnett square to track the #21 chromosomes during the formation of gametes in a person with Down syndrome, and possible fertilization events that follow.)One-third of the surviving offspring would be expected to have Down syndrome.Two-thirds of the surviving offspring would be expected to have Down syndrome.One-half of the surviving offspring would be expected to have Down syndrome.None of the surviving offspring would be expected to have Down syndrome.All the children would be expected to have Down syndrome.

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