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A descendant of Jamaican slaves, Ms. Duncan had already identified the Scottish slave owner who was her mother's great-great-grandfather through archival records. But the DNA test confirming her 10 percent British Isles ancestry gave her the nerve to contact the Scottish cousins who had built an oil company with his fortune.

"It's one thing to feel satisfied to know something about your heritage, it's another to claim it," said Ms. Duncan, a writer in Manhattan. "There's a kind of checkmateness to the DNA."

The family's 11 castles, Ms. Duncan noted, were obtained with the proceeds of her African ancestors' labor. Perhaps they could spare one for her great-great-great-grandfather's black heirs? In case the paper records she had gathered were not persuasive, she invited male family members to take a DNA test that can identify a genetic signature passed from father to son. So far, no one has taken her up on the offer. Her appeal, Ms. Duncan said, is mostly playful. Less so is her insistence that the Scots stop referring to their common ancestors as simply "Virginia and West India merchants."

"By acknowledging me, the Scots are beginning to acknowledge that these guys were slaveholders," she said.

Other slave descendants, known as the Freedmen, see DNA as bolstering their demand to be reinstated as members of the Indian tribes that once owned their ancestors. Under a treaty with the United States, the "Five Civilized Tribes" — Choctaws, Chickasaws, Creeks, Seminoles and Cherokees — freed their African slaves and in most cases made them citizens in the mid-1800's. More recently, the tribes have sought to exclude the slaves' descendants, depriving them of health benefits and other services.

At a meeting in South Coffeyville, Okla., last month, members of the Freedmen argued that DNA results revealing their Indian ancestry underscore the racism of the tribe's position that their ancestors were never true Indians.

"Here's this DNA test that says yes, these people can establish some degree of Indian blood," said Marilyn Vann, a Cherokee Freedwoman who is suing for tribal citizenship in federal court. "It's important to combat those who want to oppress people of African descent in their own tribe."

As the assets of some tribes have swelled in the wake of the 1988 federal law allowing them to build casinos, there has been no shortage of petitioners stepping forward to assert their right to citizenship and a share of the wealth. Now, many of them are wielding genetic ancestry tests to bolster their claim.

"It used to be 'someone said my grandmother was an Indian,' " says Joyce Walker, the enrollment clerk who regularly turns away DNA petitioners for the Mashantucket Pequot tribe, which operates the lucrative Foxwoods Resort Casino in Connecticut. "Now it's 'my DNA says my grandmother was an Indian.' "

Recognizing the validity of DNA ancestry tests, some Indians say, would undermine tribal sovereignty. They say membership requires meeting the criteria in a tribe's constitution, which often requires documenting blood ties to a specific tribal member. DNA tests cannot pinpoint to which tribe an individual's ancestor belonged.

But if tribes are perceived as blocking legitimate DNA applicants to limit payouts of casino money, experts say, it could damage their standing to enforce the treaties conferring the financial benefits so many covet.

"Ancestry DNA tests are playing a part in the evolution of what the American public thinks matters," said Kim Tallbear, an American Indian studies professor at Arizona State University. "And tribes are dependent on the American public's good will, so they may have to bend."

Under no such pressure, Israeli authorities have so far denied John Haedrich what he calls his genetic birthright to citizenship without converting to Judaism. Under Israel's "law of return," only Jews may immigrate to Israel without special dispensation.

Mr. Haedrich, a nursing home director who was raised a Christian, found through a DNA ancestry test that he bears a genetic signature commonly found among Jews. He says his European ancestors may have hidden their faith for fear of persecution.

Rabbis, too, have disavowed the claim: "DNA, schmeeNA," Mr. Haedrich, 44, said the rabbi at a local synagogue in Los Angeles told him when he called to discuss it.

Undeterred, Mr. Haedrich has hired a lawyer to sue the Israeli government. As in America, he argues, DNA is widely accepted as evidence in forensics and paternity cases, so why not immigration?

"Because I was raised a gentile does not change the fact that I am," Mr. Haedrich wrote in a full-page advertisement in The Jerusalem Post, "a Jew by birth."

Shonda Brinson, an African-American college student, is still trying to figure out how best to apply her DNA results on employment forms.

In some cases, she has chosen to write in her actual statistics — 89 percent sub-Saharan African, 6 percent European and 5 percent East Asian. But she figures her best bet may be just checking all relevant boxes.

"That way, of the three categories they won't be able to determine which percentage is bigger," Ms. Brinson said.