Tracing Americans' African roots is no mean feat

11 May 2009 by Peter Aldhous in New Scientist Magazine

• WHEN Barack Obama sought his African roots, he had it easy: his father came from Kenya's Luo tribe. For most African Americans, the quest is more daunting - and the debate over the value of the genetic tests used by companies to connect black Americans to particular ancestral homelands just got more intense.

• To map the genetic diversity of Africans, Sarah Tishkoff of the University of Pennsylvania in Philadelphia and her team studied about 2400 people from all across Africa, documenting the variation in 1327 genetic markers spread across the genome. They also sampled African Americans living in Chicago, Baltimore, Pittsburgh and North Carolina.

• The team found that African Americans have a complex genetic heritage, with most individuals having links to multiple ancestral populations. Overall, 71 per cent of the genetic heritage of the African Americans studied could be traced back to diverse populations from various parts of west Africa; 8 per cent to other African populations; and 13 per cent to Europeans (Science, DOI: 101126/science.1172257).

• Given the tumultuous history of the slave trade and subsequent mingling and migration within the US, this rich genetic heritage is hardly surprising. Where, though, does it leave the claims of companies such as African Ancestry, which promises to connect its customers to "the place where your ancestry began, more than 500 years ago" and sells T-shirts that allow African Americans to "Represent Cameroon!" or one of four other west African countries?

• African Ancestry tests genetic markers on the Y chromosome, which is passed on from father to son, or mitochondrial DNA, which is inherited down the maternal line, passing to both sons and daughters. The company then compares these results with a large database of African genetic variants of these markers.

• Tishkoff says that the same mitochondrial or Y chromosome variants are usually present in several African populations, making it hard to be sure about the origins of any lineage. And the lines traced via the Y chromosome and mitochondria are just two of hundreds of lineages contributing to an individual's ancestry. "What about all the rest?" asks Tishkoff's colleague Christopher Ehret of the University of California, Los Angeles.

• Rick Kittles, a geneticist at the University of Chicago who co-founded African Ancestry, defends his company's service. Increasingly, he says, customers are testing different branches of their family, in some cases tracing up to 12 distinct lineages. "And guess what? They're all different," Kittles says. "I don't see any conflict here."