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

Question

Quizplus · Accepted Answer

When a female with Down syndrome (who has an extra chromosome 21, making her karyotype 47,XX,+21) has children with a normal 46-chromosome male, the offspring have a 50% chance of inheriting the extra chromosome 21 from the mother. This results in half of the offspring having Down syndrome.

Trisomy 21,or Down Syndrome,occurs When There Is a Normal Diploid