First, DNA testing confirmed Thomas Jefferson had a sexual relationship with his slave Sally Hemings. Now it's rewriting another chapter in presidential history, this one from the Roaring '20s.

Genetic analysis has proved that President Warren G. Harding fathered a child out of wedlock with long-rumored mistress Nan Britton, according to AncestryDNA, a division of Ancestry.com.

Britton set off a Jazz Age scandal when she went public with her tale of forbidden love in the White House, boldly publishing her story in a 1927 best-selling memoir, "The President's Daughter." But historians long questioned her claims, and Harding defenders vilified her as a liar for nearly 90 years.

Based on DNA from Britton's grandson and descendants of Harding, the results are 99.9 percent certain, Ancestry said. The findings were first reported Thursday by The New York Times.

The child born of their union, Elizabeth Ann Blaesing, was the only known offspring of the 29th president. She died in 2005. Britton died in 1991.

James Blaesing, 65, who grew up hearing the story of his grandfather, the president, from Britton, his grandmother, said he has always wanted to prove she was telling the truth. He was delighted by the DNA results.

"You know what this is? It's a love story," he said of his grandparents. "It was true love, especially on her side, and I know he felt the same way. And he got trapped."

Harding was a U.S. senator from Ohio when the affair began. It ended with Harding's sudden death during his presidency in 1923.

Harding's family long maintained that Britton's book was a lie or a childhood fantasy or was perhaps dreamed up by Democratic opponents of the Republican president. Some people maintained that Harding was sterile because he had mumps as a child.

Blaesing said that if his mother and grandmother were alive today, they would be pleased.

"My grandmother -- right now she has the biggest smile on her face," he said. "She is so happy that this is out there."

AncestryDNA spokesman Stephen Baloglu said the Harding story shows the power of DNA in rewriting history.

"The family connection is definitive," he said. "We were happy to help the Harding family members solve this longstanding family mystery."