An individual's genes are a link to the past that stretches across any break in family name or birthplace through the generations. But not all genes are equally useful in tracing ancestries. The genes present on chromosomes are mixed extensively in every generation, making them a crude guide. In contrast, mitochondrial DNA is passed down from mother to child relatively unchanged, offering an individual the chance of identifying a distinct modern population, such as an ethnic group, having the same ancestors. Such reconstructions may still be imprecise, however, because mitochondrial sequences originating in one ethnic group can easily leak to others as women migrate.
Nevertheless, Bert Ely of the University of South Carolina wanted to see if he could use mitochondrial DNA sequences to trace the African roots of black Americans, as some companies have begun offering. Ely and his colleagues amassed 3,725 mostly preexisting sequences from sub-Saharan Africa. Specifically they looked at the HSV-1 region, which changes fast enough to potentially resolve recent ancestries. They searched their database for matches to samples from 74 South Carolina and Georgia locals who identify themselves as descendents of Africa's Gullah or Geechee peoples, which number in the hundreds of thousands, and to samples of African-Americans obtained from the U.S. Army. Of these test sequences, about half matched multiple African ethnic groups, and 40 percent had no matches, the group reported October 12 in BioMed Central Biology. The database is likely missing many groups, especially rare ones, Ely admits. "But it's very unlikely for any particular individual [that] you could trace them to any specific tribe, or even ethnic group or country," he says. Growing the database may turn up mitochondrial sequences specific to cohesive populations but will probably increase the number of multiple matches as well, he explains.
"They have an extremely limited database and thus have limited power to say anything about ancestry," says University of Chicago medical geneticist Rick Kittles, scientific director of African Ancestry, a firm that traces lineages. Kittles says his company matches people probabilistically based on the frequency of their particular sequence variations in different African populations. The researchers might also be overlooking some near misses to sequences in their database, notes Jason Eshleman, a molecular geneticist at Trace Genetics in San Francisco, Calif. But ultimately, he says, ethnic and linguistic groups don't have sharp biological lines, so "you don't know going in what the level of resolution you're going to get is."