How African Are You?

By John Hawks
Updated Wednesday, March 15, 2006, at 1:43 PM ET?

Recently on PBS's "African American Lives," host Henry Louis Gates had his DNA tested to learn about his ancestry. Gates' family suspected its paternal ancestry could be traced to a white slave owner. But DNA testing showed that his Y chromosome did not match the man's white descendants. A second, newer test gave Gates another result he didn't expect: His DNA showed that only half of Gates' ancestry was African. The rest were apparently European.

DNA testing for genealogy has become increasingly popular, as a Newsweek cover story in February attests. Especially attention-getting have been efforts to trace genetic relationships along the male lineage. In January the New York Times wrote up attempts to trace Irish genealogy through the male line to Niall of the Nine Hostages, a fifth-century Irish warlord. Other tests have also shown that as many as 14 million men may share the Y chromosome of Genghis Khan. But tests that seek a single, Y-chromosome male lineage are limited: They leave out the vast majority of ancestors. Newer tests can survey all the DNA that can be inherited from either parent, but at a cost of precision: They don't tell which ancestors lived where, and they can't detect traces of ancestry.?

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.

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