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

Question

Quizplus · Accepted Answer

When a female with Down syndrome (47 chromosomes) mates with a normal male (46 chromosomes), the offspring can inherit either the extra chromosome 21 or not. Thus, there is a 50% chance (one-half) that the offspring will have Down syndrome.

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