Women have sex chromosomes XX, and men have sex chromosomes XY. Which of a woman's grandparents could not be the source of any of the genes on either of her X chromosomes?

A) mother's mother

B) father's mother

C) mother's father

D) father's father

D) Father's father. The woman receives an X chromosome from both her parents. Her mother did the same, so it could be either of her mother's parents. The father, however, only receives an X chromosome from his mother, so it cannot be his father's genes.