http://www.slate.com/articles/health_and_science/science/2006/03/how_african_are_you.html

The newer "genetic admixture tests" examine DNA from genes inherited from all of a person's grandparents, great-grandparents, and so on. A few of these genes reflect the part of the world where those ancestors lived. Like postcards, they track the movement of people from the lands of their ancestors to their current address. Scientists studying these genetic variations now focus on sites that vary between people by one chemical letter. They're called "single nucleotide polymorphisms," or SNPs. Some of these SNPs are important: They may contribute to traits like skin color or resistance to regional diseases like malaria. Others vary among populations just because of chance. For geneticists, finding the SNPs that mark populations is a challenge. For the most part, the same SNP might be found in Africans, Europeans, and people from every other part of the world. It's now possible to test quickly for hundreds of SNPs by using special microchips that bind to the distinctive DNA sequences. These tests examine hundreds of SNPs at once; if among these a person has many that are common in Africa, it is likely that she has some African ancestors. Admixture testing works best in groups like African-Americans, whose ancestors in Africa and Europe lived far from each other. Most of the ancestry of today's African-Americans can be traced to West or Central Africa, with a minority from other parts of the continent. (Gates' family is a bit exceptional in terms of origin.)