Question

A color-blind, chromatin-positive male child (one Barr body) has a maternal grandfather who was

color blind. The boy's mother and father are phenotypically normal. What needed to occur to have
this genetic outcome?
A) Nondisjunction in meiosis I of the father occurred.
B) The chromosomal doubling effect of meiosis II occurred in the father.
C) The mother must be heterozygous and underwent nondisjunction at meiosis II.
D) The mother is homozygous recessive and underwent nondisjunction in meiosis I or II.

Answer 1

The genetic outcome in this case is due to nondisjunction in meiosis I of the mother, leading to the inheritance of the color-blindness allele from the mother.

Step-by-step explanation:

The genetic outcome in this case is due to nondisjunction in meiosis I of the mother. Since the boy's maternal grandfather was color-blind and the mother is phenotypically normal, it suggests that the mother carries the color-blindness allele on one of her X chromosomes. During meiosis I, the mother's X chromosomes failed to separate properly, leading to one of the X chromosomes with the color-blindness allele being passed on to the son. The father's normal vision indicates that he does not carry the color-blindness allele.