It wasn’t a question of whether his ancestors had been slaves. Rep. John Lewis (D-Ga.), a son of the Old South and one of the country’s leading civil rights pioneers, knew that to be true.

But what he learned about one of his enslaved ancestors, guided by Harvard professor Henry Louis Gates Jr., caused him to break down and cry.

Gates says that Lewis’s story is one of the most powerful of his new PBS series, “Finding Your Roots With Henry Louis Gates Jr.” The 10-part program premieres Sunday.

“Finding Your Roots” is the fourth installment of Gates’s popular franchise, in which he digs into the genealogy of famous Americans. This go-round Gates looks into the backgrounds of 25 individuals, including actors Michelle Rodriguez, Robert Downey Jr. and Samuel L. Jackson; media personalities Barbara Walters and Sanjay Gupta; religious leaders Angela Buchdahl, Yasir Qadhi and Rick Warren; and political figures Condoleezza Rice and Cory Booker, the mayor of Newark. The series runs through May 20.

The previous three specials drew a combined 25 million viewers, a testament to the continued interest in genealogy more than 30 years after the television series “Roots” sparked interest in searching family histories, particularly among African Americans. The availability of information online, along with more widespread use of DNA testing, has made it easier for people to explore their family trees.

“What’s your favorite subject? Your favorite subject is you,” Gates said in explaining the popularity of genealogy. “You are, in part, the sum of your ancestors” he said, and researching one’s background helps people figure out “how you became uniquely you.”

Gates, 61, who is editor-in-chief of The Root, an online magazine owned by The Washington Post that features news and commentary about African American culture, said his interest in family history started when he was 9 years old. He remembered attending his grandfather’s funeral and being struck by the physical features of the man in the casket. “He looked white. I wanted to know why. The next day I bought a composition book and interviewed my parents about their ancestry. I’ve been a genealogy junkie since then.” Gates eventually learned that his grandfather inherited his looks from an Irish ancestor.

Booker will actually get to meet his white relatives in one episode. Gates used a family history, old public records and DNA testing to track down Booker’s great-grandfather, a white doctor in Louisiana, who fathered an out-of-wedlock child with Booker’s great-grandmother.

“There is no African American we have ever tested who is 100 percent black. None,” Gates said. “It deconstructs the notion of race. . . . We are a mixed people.”

As the director of Harvard’s W.E.B. DuBois Institute for African and African American Research, Gates’s work often looks at this country’s complex racial history. But it is not his scholarly work, which includes more than a dozen books, countless articles and documentaries, that has brought him the most recognition.

“When people stop me on the street, they don’t say, ‘Hey, I liked your latest book on literary theory.’ They say, ‘I love the genealogy series!’ ”

Still, the teacher in Gates has included lesson plans for elementary and middle school classrooms, which he hopes will use the series to teach history and science.

The first two PBS series focused solely on famous African Americans, but letters from viewers persuaded him to explore people of other races. The 2010 “Faces of America” included Meryl Streep and Eva Longoria; comedian Stephen Colbert and cellist Yo-Yo Ma.

Not everyone is game for digging up their roots. Gates said Colin Powell agreed to participate in one of the series, only to call back minutes later and back out. He added that singer Alicia Keyes also withdrew when a relative objected.

“When I started the series, I thought the moving moment for my guests, in the case of the African American ones, would be when I revealed what tribe the came from in Africa, but it wasn’t. It was revealing the names of slave ancestors. People would cry,” Gates said.

“Nobody cried when I told them what tribe they came from in Africa. They were intellectually fascinated but not emotionally engaged, as when you’re looking at your great-great-grandparents and they were slaves.”

Lewis, the son of sharecroppers who grew up on a farm in Alabama, had not done any exploration of his own into his family background. There had been been “conversations over the years with my mother and my grandparents, but they didn’t go back too far.”

Gates and his team were able to determine that his great-great-grandparents, Tobias and Elizabeth Carter, were given land and money when they were freed after the Civil War.

They also found Tobias Carter’s name on a voter registration from from 1867. It was during Reconstruction, when African Americans were briefly allowed the benefits of full citizenship, including voting and holding seats in Congress.

“I cried when that was revealed,” Lewis said in an interview last week. “It was just unbelievable. Maybe it was something in my DNA or bloodline or whatever you want to call it, but I couldn’t deviate from it. . . . I had to pursue it. That was so powerful for me, and I just cried.”

It was through the efforts of activists such as Lewis, a leader in the Student Nonviolent Coordinating Committee, who in 1965 was nearly beaten to death by Alabama state troopers during a protest march for voting rights, that African Americans were again allowed to register to vote.

When he was growing up, Lewis said, his parents, grandparents and great-grandparents didn’t have the right to vote.

“I have a greater sense of, I guess, selfhood. I know I come from a strong and determined people, and I think that’s what made me much more determined and persistent,”Lewis said.

He said he is eager to see the reaction of his brothers and sisters when they see the segment.

“I probably will cry some more.”

“Finding Your Roots With Henry Louis Gates Jr.”

8 p.m. Sunday on PBS