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Numerous studies have estimated the rate of European admixture in African Americans; these studies have documented average admixture rates in the range of 10% to 20%, with some regional variation, but also with substantial variation among individuals. For example, the largest study of African Americans to date, based on autosomal short tandem repeat (STR) markers, found an average of 14% European ancestry with a standard deviation of approximately 10%, and a range of near 0 to 65%, whereas another study based on ancestry informative markers (AIMs) found an average of 17.7% European ancestry with a standard deviation of 15.0%. By using nine AIMs, Parra and colleagues found substantial variation of European ancestry proportions in African-American populations across the United States, ranging from just over 10% in a Philadelphia group to more than 20% in a New Orleans population. Similar levels (11% to 15%) of European ancestry also were reported by Tishkoff and co-workers, based on more than 1,000 nuclear microsatellite and insertion/deletion markers. . . .

The new wave of genealogical testing also has reopened one of America's ugliest wounds by confirming with science what historians have contended for generations: In slavery times and beyond, large numbers of black women were impregnated by white slave owners or other white men in positions of power.

About 30% of black Americans who take DNA tests to determine their African lineage prove to be descended from Europeans on their father's side, says Rick Kittles, scientific director of African Ancestry, a Washington, D.C., company that began offering the tests in 2003. Almost all black Americans whom Kittles has tested descended from African women, he says.