A couple has a child with Down syndrome when the mother is 39 years old at the time of delivery. Which is the most probable cause?
A)The woman inherited this tendency from her parents.
B)One member of the couple carried a translocation.
C)One member of the couple underwent nondisjunction in somatic cell production.
D)One member of the couple underwent nondisjunction in gamete production.

Question

Quizplus · Accepted Answer

The most probable cause of Down syndrome in this case is nondisjunction in gamete production, specifically in meiosis I, where an extra copy of chromosome 21 is present in one of the gametes. This is more likely to occur in older mothers, as the risk of nondisjunction increases with age. A translocation (choice B) would involve a rearrangement of chromosomes, and the parents would have been aware of this prior to the birth of their child. Inheriting a tendency for Down syndrome (choice A) is not a known cause of the condition. Nondisjunction in somatic cell production (choice C) does not result in a viable embryo, so it would not be a possible explanation for the birth of a child with Down syndrome.

A Couple Has a Child with Down Syndrome When the Mother