Which grandparents cannot contribute genes to a man's Y chromosome?

The Y chromosome is passed directly from father to son, meaning that only paternal ancestors contribute to a man's Y chromosome. This means that the father's father is the only grandparent who contributes genes to the Y chromosome of a man.

Both maternal grandparents, the mother's mother and the mother's father, do not contribute to a man's Y chromosome. Thus, the maternal grandparents are excluded from contributing any genetic material to this specific chromosome because the Y chromosome is inherited only from the male line, specifically from father to son.

Therefore, identifying both the mother's mother and the father's mother as grandparents who cannot contribute genes to a man's Y chromosome accurately reflects the genetic inheritance pattern of the Y chromosome.