How a Family Secret About His White Ancestor Paved the Way for Henry Louis Gates Jr.’s Finding Your Roots (Exclusive)

For almost as long as Henry Louis Gates, Jr. can remember, questions about his heritage have gnawed at him.

As he explains in this week’s issue of PEOPLE, Gates’ intense curiosity about his lineage was partly the inspiration for the popular PBS docuseries Finding Your Roots, which over the course of 10 seasons has welcomed guests and uncovered their family trees, often with surprising results.

Edward Norton learned that he and Julia Roberts are genetic cousins, Kerry Washington discovered she was conceived through a sperm donor, and Viola Davis found out that she’s related to Anita Hill.

For Gates, uncovering his own lineage began in 1960 when he first created his own family tree.

“I was 9,” says Gates, who also recently debuted Gospel, a four-hour PBS docuseries exploring Black spirituality and an accompanying special, Gospel Live! “I did it the day after we buried my father’s father, Edward St. Lawrence Gates. And he looked like a White man. He was so white, we called him 'Casper' behind his back.”

Shortly afterwards, Gates found a clue about his lineage inside his grandfather’s tattered scrapbook, which contained the 1888 obituary for his great-great-grandmother Jane Gates. It hailed her as an “estimable colored woman” and midwife freed from slavery in 1864.

That night, notebook in hand, he determinedly interviewed his parents.

“I was trying to figure out how could somebody with my color, the shape of my nose, my lips, my hair texture — how could a guy with straight hair and white skin be my grandfather?” he recalls. “And how in the world was I related to a slave and a midwife?”

When the groundbreaking Roots premiered in 1977, it reignited Gates’ longing to trace his own roots, a quest that continued in 1994 as he traveled across 3,000 miles of East Africa while taping a documentary with his then 14- and 12-year-old daughters (Liza, now 43, and Maggie, 41, whom he shares with his first wife, Sharon Lynn Adams).

The Harvard professor became further excited in 2000 after a fateful meeting with Rick Kittles, a renowned Black geneticist. “I went into orbit when I found out that DNA could be used to trace your ancestry back 500 years,” he says.

A few years later, his “secret dream” of hosting a documentary series would become a reality after the concept of a TV show unraveling Black Americans’ roots hit him. “I was standing in the bathroom with tears running down my face, because I knew it was a good idea,” he recalls.

African American Lives eventually launched in 2006 with guests including Oprah Winfrey, Maya Angelou, Morgan Freeman and Chris Tucker, and it evolved into Finding Your Roots, which launched in 2012.

The show has “dramatically morphed” over its 10 seasons, drawing a wide range of celebrities. Some revelations have been welcome, such as Andy Samberg finding out the identity of his grandmother in season 5, which had been previously unknown to his mother, who was adopted as an infant. Likewise, Gates revealed to Bob Odenkirk this season that he has ties to King Charles and Nathan Lane.

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But other reveals have gone over less smoothly, including Ben Affleck’s discovery that he was descended from slave owners, a detail that was omitted from his episode at the actor’s request (he later apologized).

“When you spit in that test tube, you never know what’s going to pop out,” explains Gates.

Finding Your Roots has grown so much in popularity that viewers now stop him at the airport for selfies and have asked the host for the opportunity to have their own genealogies explored. Because of that, three non-famous guests will appear this season.

“It turns out that the stories of non-celebrities are just as riveting, just as emotional and just as challenging,” Gates says.

In April, Gates plans on attending a family reunion in Cumberland, Md., where Finding Your Roots’ genetic genealogist CeCe Moore will finally reveal the identity of his white great-great-grandfather.

“My great-grandfather, born in 1859, died in 1945, and he had no idea what the name of his father was,” he says. “But CeCe is going to reveal his identity at long last. And then I’m going to go up to my great-great-grandmother’s grave and say, ‘Grandma, you could run, but you can’t hide!'"

For more on Henry Louis Gates Jr., pick up the latest issue of PEOPLE, on newsstands Friday, or subscribe here. 

Finding Your Roots with Henry Louis Gates, Jr. airs Tuesdays at 8 p.m. ET on PBS.