Except Ferguson never would have guessed what he ended up discovering about his family.
"We followed my great-grandfather's line, and I had no information about him at all," Ferguson tells PEOPLE. "He passed away before my dad was even born."
But only one day into researching his great-grandfather's life, the Modern Family star was stunned at the findings.
"The exploration started with a crazy reveal, and we found some really dark stuff in the very distant past," says Ferguson, who learned that his great-grandfather was accused of murdering his own aunt.
"It was overwhelming, because when I've watched [Who Do You Think You Are?] as a fan, usually there's this big reveal, this important part of the puzzle, near the end of the episode," he says. "And here we are dealing with it in the beginning."
While the murder case was never actually resolved, Ferguson still struggled with the accusations against his great-grandfather.
"I knew he was the man who raised my grandmother, who I loved so much and I know what kind of woman she turned out to be," he says. "I can't imagine anyone who was bad who could raise such a wonderful person."
First, the ShockDespite this shock at his initial findings, Ferguson found himself only wanting to dig deeper.
"I thought it couldn't get any crazier than that moment, and it just kept getting more interesting as we went along," he says. "It really does affect you in a very profound way. It's like a ball of yarn. It just keeps unraveling and you want to get to the end."
With plans to continue probing his family's past, Ferguson is grateful for the opportunity to be on the show.
"Justin [Mikita, Ferguson's husband] and I want to start a family one day, and that's one of the reasons I really wanted to do this," he says. "I want to pass on information – good and bad – about our history. It's a really, really important thing."
Ferguson's episode of Who Do You Think You Are? will air July 30 at 9 p.m. ET on TLC. See an exclusive clip below.