Henry Louis Gates Jr. was born in West Virginia, went to Yale and is a professor at Harvard. But when you get the host of PBS’ “Finding Your Roots” talking about genealogy, he sounds like he might be from Utah. He approaches it with a religious zeal.

“I believe our ancestors are in a kind of purgatory waiting to be discovered,” Gates said.

He first became interested in genealogy when he was 9 years old and his father showed him a photo and the obituary of his great-great grandmother. Four decades later, in 2000, he got a letter from a geneticist that began, “Have you ever seen ‘Roots’?”

“I thought — what kind of idiot asks if I’ve seen ‘Roots’?” But the letter went on to say, “We can now do in a test tube what [‘Roots’ author] Alex Haley took years to do.”

“And shortly after that, I got up in the middle of the night and — boom! — I had an epiphany. I mean, it was a gift from God.” Gates said he realized “we could trace people’s genealogy till a paper trail ran out. And when it did, we could do the DNA to reveal their more distant ancestry. And 15 years later, here we are.”

That’s exactly what he and his team have been doing on “Finding Your Roots” since 2012. (A new batch of episodes start airing Tuesday at 7 p.m. on Ch. 7.) Each hour uncovers the family tree of two or three celebrities — and, without fail, it proves to be an emotional experience.

“I have never had anyone who wasn’t deeply moved, but most people will cry sooner or later,” Gates said. “I cry sometimes. Sometimes things happen that are so painful and unimaginable.”

In an upcoming episode, actor Tony Shalhoub learns that his father was orphaned during World War I as what is modern-day Lebanon was devastated by the war and plague. His aunt, then just a teenager, managed to get herself and her surviving siblings to the United States.

(Nicole Rivelli | Amazon via AP) Tony Shalhoub, shown here in a scene from "The Marvelous Mrs. Maisel," learns about his family history in "Finding Your Roots."

“It was really much more kind of wracking and devastating than I had thought,” Shalhoub said. “It kind of caught me off guard, and it was like really reaching back and touching something deep.”

Guests often find connections they never imagined, including fashion designer (and former “Project Runway” judge) Zac Posen, who learns that many of his forefathers were in fashion — as tailors or shoemakers.

“I was pretty much in a lot of disbelief,” Posen said.

(Photo courtesy of Amy Sussman/Invision/AP) Zac Posen was surprised by what he learned about his family in "Finding Your Roots."

Gates can only recall one person who didn’t cry while the cameras were rolling — Francis Collins, the former head of the Human Genome Project and the current director of the National Institutes of Health. And he later learned that Collins burst into tears when he got home.

Not that “Finding Your Roots” is a sad show — guests (and viewers) are more often fascinated and delighted by what they learn. The program shows viewers that everyone has a story, because while the guests are all famous, their ancestors are mostly anything but.

“We are all immigrants,” Gates said.

His own forebears, as well of those of his Black guests, “didn’t come here willingly. They were dragged here in chains.” And the vast majority of immigrants didn’t travel in the luxury cabins of cruise ships.

“They weren’t in first class, baby, they were in the bottom of the boat and getting seasick with no windows, horrible sanitation facilities,” Gates said. “They were all metaphorically in the same boat. Our great country came out of that turmoil and chaos, and we all have to remember that.”

Gates and PBS first partnered on a genealogy series in 2006 with “African American Lives,” which focused solely on Black people. Beginning with “Faces of America” in 2010 and continuing with “Finding Your Roots” since 2012, families of all ethnicities have been examined.

“I used to think that everybody else knew their family tree but Black people,” he said. “Turns out nobody knows their family tree.”

And he hopes “Finding Your Roots” reminds viewers that “all of our ancestors came here from someplace else … and we have all contributed to the great blend of what makes this the great country it is.”

He’s fond of pointing out that every person on Earth’s genome is 99.9% the same as every other person’s.

“I am so deeply proud of this series for our country, especially now, to show what we have in common — both as Americans and as human beings — despite our apparent differences,” Gates said. “The stories we find in our guests’ family trees demonstrate over and over that we are fundamentally a blended nation, bonded by shared values. We draw strength from our diversity.”