Question

a couple has a child with down syndrome. the mother is 39 years old at the time of delivery. which of the following is the most probable cause of the childs condition?
nondisjunction occurred in one member of the couple during somatic cell production.
the woman inherited this tendency from her parents.
one member of the couple carried a translocation.
nondisjunction occurred in one member of the couple during gamete production.

Explanation:

Down syndrome is typically caused by nondisjunction during gamete production. Nondisjunction in somatic cell production would not lead to a child with Down syndrome as gametes (sperm and egg) are involved in creating the zygote (the initial cell of the child). A translocation - while a less common cause of a form of Down syndrome (translocation Down syndrome) - is less likely than the most common cause which is nondisjunction during gamete formation. Inheriting a tendency from parents is not the direct cause of the extra chromosome 21 (in most cases of Down syndrome).

Answer:

Nondisjunction occurred in one member of the couple during gamete production.