All her life, Rachel Fair has been teased by other black Americans about her light skin. "High yellow," they call her, a needling reference to the legacy of a slave owner who, she says, "went down to that cabin and had what he wanted."

So it was especially satisfying for Ms. Fair, 64, when a recent DNA test suggested that her mother's African ancestry traced nearly to the root of the human family tree, which originated there 150,000 years ago.

"More white is showing in the color, but underneath, I'm deepest Africa," said Ms. Fair, a retired parks supervisor in Cincinnati. "I tell my friends they're kind of Johnny-come-latelies on the DNA scale, so back up, back up."

Ms. Fair is one of thousands of African-Americans who have scraped cells from their inner cheeks and paid a growing group of laboratories to learn more about a family history once thought permanently obscured by slavery. They are seeking answers to questions about their family lineages in the antebellum South - whether black, white or Native American - and about distant forebears in Africa.

The DNA tests are fueling the biggest surge in African-American genealogy since Alex Haley's 1976 novel, "Roots," inspired a generation to try to trace their ancestors back to Africa. For those who have spent decades poring over plantation records that did not list slaves by surname and ship manifests that did not list where they came from, the idea that the key lies in their own bodies is a powerful one.

But the joy that often accompanies the answers from the tests is frequently tempered by the unexpected questions they raise. African-Americans say the tests can make the ugliness of slavery more palpable and leave the hunger for heritage unsatisfied. Some are unsure what to make of the new information about far-away kin, or how to account for genes that undermine a racial identity they have long internalized.

The interest in using genetics to construct a family tree comes despite warnings from scientists that the necessary tools to tell African-Americans what many want to know the most - precisely where in Africa their ancestors lived and what tribal group they belonged to - are still unreliable.

The most that blacks who use DNA tests can hope to learn now is that their genetic signature matches that of contemporary Africans from a given tribe or region from a DNA database that is far from complete. To assign an ancestral identity based on that match is highly suspect, scientists say; a group whose DNA has not been sampled may be a more precise match, or the person might match with several groups because of migration or tribal mixing.

Each test can also trace only one line of a person's many thousands of ancestors, making the results far more murky than the promise held out by some testing companies.

Still, the popularity of the DNA tests seems a testament to the unremitting craving for a story of origin. However flawed or scientifically questionable, the results provide the only clue many African-Americans have to the history and traditions that members of other American ethnic groups whose immigration was voluntary tend to take for granted.

"There's just something about knowing something after years of thinking it was impossible to know anything," said Melvin Collier, 32, a black student at Clark Atlanta University who recently learned that his DNA matches that of the Fulani people of Cameroon. "It's still pretty overwhelming."

Some African-Americans, more interested in searching out recent relatives who in many cases can be dependably identified with a DNA match, are asking whites whom they have long suspected are cousins to take a DNA test. And in a genetic bingo game that is delivering increasing returns as people of all ethnicities engage in DNA genealogy, some are typing their results into public databases on the Internet and finding a match that no paper trail would have revealed.

"I've been sitting here for years with nothing left to try and then, boom, this brand new thing," said B. J. Smothers, a retired urban planner in Stone Mountain, Ga., who says the results of a DNA test have brought her closer than she had ever been to discovering the identity of her father's grandfather. "DNA is our last hope."

Ms. Smothers's father, 88, knew that his father was born a slave in Wilcox County, Ala., but the DNA test showed that he has a European paternal ancestry, a result shared by nearly a third of African-Americans who take the test. The news was not exactly a surprise. But as eager as she is to discover the identity of her great-grandfather, Ms. Smothers is also bracing for a wave of new anger.

"I am kind of preparing myself for what I am going to feel when I find the family, when it's real," she said. She regularly looks for matches to her father's DNA in the online databases where amateur genealogists publish their genetic identities along with more prosaic contact information. Some day, she is certain, she will find a match that will lead to her white relatives.

Family reunions via DNA are not always warm affairs. When Trevis Hawkins, 37, a black oncology nurse from Montgomery, Ala., e-mailed a white man with the same surname whose DNA matched his this year, the man seemed excited. But after Mr. Hawkins gave him the address to his family Web site, which includes pictures, he never heard from him again.

One African-American, upon confirming a match with a white man whose ancestors had owned his, told him he owed reparations and could start by paying for the test, said Bennett Greenspan, chief executive of Family Tree DNA, which offers tests for $129 and up.

But Charles Larkins, whose great-grandmother was a slave, says proving or disproving his suspicion that her owner was his great-grandfather would be cathartic.

Mr. Larkins recently e-mailed Hayes Larkins, the slave owner's white great-grandson, to ask whether he would take the DNA test. Because the Y chromosome, which determines maleness, is passed virtually unchanged from father to son, scientists can use it to determine whether two men share a common ancestor.

"I'm not going to be like the Jefferson descendants, denying anything happened," Hayes Larkins said, referring to a 1998 DNA test that indicated that Thomas Jefferson had fathered at least one child with his slave Sally Hemings, which his white family had denied.

The two Mr. Larkins are waiting for the results to arrive.

For Nickesha Sanders, who already knew her great-great-grandfather was a white slave owner in Tennessee, the appeal of the DNA test was the promise of a link to Africa. "I wanted to be able to connect to my history before slavery," said Ms. Sanders, 26, a student at Texas Southern University. "I wanted it to be more than, the boat stopped at the shores, then slavery, emancipation, civil rights, all that struggle."

To find out about her maternal ancestors, Ms. Sanders paid $349 for a test that analyzes mitochondrial DNA, which is passed on largely intact from mothers to their children and serves a similar purpose as the Y chromosome for scientists tracing ancestry.

The results, from a Washington company, African Ancestry, indicated that Ms. Sanders shared a genetic profile with members of the Kru people of Liberia, who, she was pleased to learn, were known for inciting slave rebellions. But the news did not mean as much to her grandmother, who had hoped to find proof of the American Indian blood she had always been told ran in the family, a frequent quest for African-Americans taking the tests.

The results have propelled some test-takers to plan visits to their newly adopted homelands and to find others here who have been told they share the same ancestry. In online discussion forums, African-Americans with the same DNA test results call each other "cousin." After a lifetime of knowing only that their family came from Africa, some liken the new association to adopted children finding their birth mother.

"Africa is not a country; it's a continent," said LaVerne Nichols Hunter, a retired mathematics teacher in Pittsburgh, whose DNA test results placed her ancestors in Cameroon, Sierra Leone and Liberia.

But if DNA test-takers are making too much family history out of too little genetic information, social scientists say, it is not a phenomenon unique to the new technology.

"Identity is a process," said Alondra Nelson, a sociologist at Yale who studies the intersection of race and genetics. "Narratives and stories about family and kinship are always to some extent people making meaning out of their experiences with whatever tools they have."

When a radio host in Chicago revealed at a Kwanzaa festival last year that he was of Mende descent, several attendees who had received the same DNA result gathered to trade notes, a moment some said they found especially meaningful because slave owners made a point of separating Africans from the same tribes to prevent them from communicating.

But Kwame Bandele has learned enough about the civil war in Liberia, which the tribe his paternal DNA test identified is involved in, to feel deeply troubled by the kinship. A manager at General Electric, Mr. Bandele has tried to persuade the company to provide ultrasound machines for pregnant women in refugee camps.

He sends out e-mail with news about the war to friends, but feels he should be doing more.

"There was a massacre with machetes the other night," he said. "My people are in bad shape."

Ray Winbush, a psychology professor at Morgan State University, said being told that his ancestors hailed from the Takar people of Cameroon served to underscore his disconnectedness, both from an ancestral tribe he knows little about and from an American society that can still be a hostile place for African-Americans.

"It's like being lost and found at the same time," Mr. Winbush said.