A male child is born with an XYY constitution of sex chromosomes. The parents are normal XX and XY individuals, so their child's genotype must be the result of nondisjunction in the formation of one of their gametes. When (and in whom) could the nondisjunction have taken place?

Question

Quizplus · Accepted Answer

Nondisjunction in meiosis II of the father could have resulted in the formation of a YY sperm, which would combine with a normal X-bearing egg from the mother to produce an XYY zygote. This is the only answer choice that would result in a YY sperm, as nondisjunction in mitosis (choice A) or meiosis I (choice B) would result in a Y-bearing sperm and a normal X-bearing sperm. Nondisjunction in the zygote at the first mitotic division (choice D) would result in mosaicism rather than a wholly XYY individual. Nondisjunction in meiosis I of the mother (choice E) could result in an X-bearing egg and a nonviable 0X egg, but could not produce an XYY zygote.

A Male Child Is Born with an XYY Constitution of Sex