Even as population geneticists battle over the meaning of race, cline and "biogeographical ancestry," a small industry has emerged out of the quest to understand human migration and identity. At least 11 companies offer individuals the ability to trace their African, Native American, Asian or European roots through DNA markers. One, Oxford Ancestors, will even help men learn if they carry the "heroic Y chromosome that flowed through the veins of the High Kings of Ireland" or inherited it "directly from Gen-ghis Khan."

Hyperbolic promises aside, many population geneticists urge caution in interpreting these ancestry tests. Detractors say these companies rarely clarify that they are tracing just a tiny fraction of ancestry or that many lineages might share the same markers. And taken together, critics emphasize, the claims made for this technology misleadingly convey the sense of race and family as a matter of biological precision.

Rick Kittles, a geneticist at Ohio State University, acknowledges these pitfalls but has pressed forward with his firm, African Ancestry, because of what he considers a critical social need. The Washington, D.C.-based company compares a client's DNA with that of ethnic groups now living -mainly in West and Central Africa, aiming to recover the family ties destroyed by the slave trade. Like the other firms, its technicians examine either a section of mitochondrial DNA or markers on the Y chromosome and then try to match them up with databases of maternal and paternal lineages.

With 13,690 maternal and 11,747 paternal lineages, African Ancestry offers unusually rich detail. Even so, Kittles acknowledges that each of the tests follows only a single ancestral line out of as many as 1,024 over 250 to 300 years (10 generations). The analysis cannot detect all the historical groups that may have contributed to a person's ancestry or even trace a single line beyond the populations Kittles has sampled so far.

But unlike other firms, Kittles is careful to say he is helping his clients connect to modern Africans, not historical figures or tribes, and he is not trying to categorize ethnicity or race. Instead of boiling identity down to genetics, he aims to highlight the interweaving of biology, history and culture. One concrete link to a part of Africa can be deeply healing, he says. Otherwise, most African-Americans can follow their history back only to enslavement. "It creates this void in the psyche of African-Americans, this missing piece of their identity," Kittles explains.

African Ancestry is also adding 200 lineages every few months, broadening out geographically from the original population samples. Kittles hopes that this expanding database may illuminate migration patterns from east and south African regions into the west and central areas, which had been the heart of the transatlantic slave trade. By tapping into the continent's high genetic diversity, this approach might overcome the difficulty of interpreting DNA markers shared across populations on the continent.

Besides technical improvements, with anthropologist Mark Shriver, Kittles has called for a code of conduct that would require companies offering personalized genetic histories to explain both the promises and limitations of their science. "He's careful and responsive to the ethical concerns," says Duana Fullwiley, a medical anthropologist at Harvard University. Still, Fullwiley and others remain cautious. For one thing, they see Kittles's desire to bridge the injury of slavery as overly idealistic. Ancestry tracing "may heal certain wounds," she acknowledges. But "it doesn't give us lost history back." Moreover, rather than resolving questions about identity, the test may open up new questions about genetic pedigree.

With his extensive database, Kittles has pinpointed African-American ancestry in more detail than other scientists thought possible, according to Joanna Mountain, a Stanford University population geneticist. Africa "is not just one homogeneous pot of people," Mountain says. Furthermore, individual African-Americans, who have a very mixed ancestry in their genomes, may be more similar to one another than to any group within Africa. "It's a neat story that's arising," she observes.