John Stamos takes a turn in digging deeper into his family history.

On Sunday's episode of "Who Do You Think You Are?," the 53-year-old actor learns the heartbreaking truth about his grandfather: He was an orphan by the time he was 13 years old.

In ET's exclusive sneak peek, Stamos -- who is of Greek heritage -- is understandably shocked and saddened by the news. "Really? Orphan?" he asks. "That's sad, isn't it? How'd his parents die?"

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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.

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