Which of a woman’s grandparents could not contribute to her X chromosomes?

The correct answer is the father’s father, as he cannot contribute an X chromosome to his granddaughter. In human genetics, males possess one X and one Y chromosome (XY), while females carry two X chromosomes (XX).

A woman inherits one of her X chromosomes from her mother and the other one from her father. In this case, the father’s father (the woman's paternal grandfather) does not have an X chromosome to contribute; instead, he can only pass on a Y chromosome to his male children (the father of the woman) and the X chromosome comes solely from the grandmother.

On the other hand, the woman’s mother and her mother’s parents (grandmother and grandfather) can each potentially contribute X chromosomes to the woman. Thus, the mother’s mother and father both contribute X chromosomes, while the father’s mother contributes an X chromosome to the woman as well. This distinct inheritance pattern highlights the role of X and Y chromosomes in determining genetic contributions and helps clarify why the father’s father cannot contribute an X chromosome.