Though the answer to that question remains unrevealed, Stamos, who lost his mother Loretta in 2014 and his father Bill in 2001, reflected on what life must have been like for his grandfather without his parents.
"I couldn't imagine growing up without both parents. Right when I saw that, I was like oh, my heart, you know?" he said. "My grandfather was a very successful man, a very successful businessman, a very proud father and grandfather, too. I guess he had to make up for lost family."
Who Do You Think You Are? airs Sundays at 10 p.m. ET/PT on TLC.