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The rich mosaic of African-American ancestry. Among the 365 African-Americans in the study, individuals had as little as 1 percent West African ancestry and as much as 99 percent. There are significant implications for pharmacogenomic studies and assessment of disease risk.

There are also significant implications for affirmative action and preferential college admissions.

THIS is the thing that will finally destroy such programs, as soon as someone can find a viable way to bring some sort of lawsuit.

Something not mentioned by this article is that there are a lot of "white" Americans who have some African ancestry; can't remember the source but I remember reading somewhere that that is true of something like 25% of "white" Southerners.

What we need to find is some blond-haired, blue-eyed person that tests out to have 5% West African Ancestry, and then demands to participate in some sort of Affirmative Action Program.

In the long-term Affirmative Action will collapse under the legal inability to define who gets preferences and who doesn't.

From here.

Is mixed race ancestry fairly typical for an American? In two ways, it is. First, more than 50 million whites, according to his analyses, have at least one black ancestor.

Another way to approach the question is to group together all the whites and blacks in America and calculate their mean degree of admixture. Shriver's data shows that on average, they would be about 12 or 13 percent African.

Yet, from another perspective, a sizable degree of racial mixing is highly unusual. There simply aren't many African-Americans or European-Americans who are mostly white but also substantially black. Shriver pointed out, "There is a very small degree of overlap in the population distributions." In America, most of the whites are extremely European and most of the blacks are quite African.

Despite the notorious arbitrariness of the "one drop" rule, the actual American population conforms to its strictures surprisingly closely.

Granted, the "one drop" rule would be laughed out of existence if anyone attempted to impose it on a land with a more genetically blended population, such as Puerto Rico (which Shriver has begun to study). Yet, it appears possible that the rule survives in the U.S. because it's not too wildly inaccurate. Only a small fraction of the population is more than half, but less than 90 percent European.

Among self-identified whites in Shriver's sample, the average black admixture is only 0.7 percent. That's the equivalent of having among your 128 great-great-great-great-great-grandparents (who lived around two centuries ago), 127 whites and one black.

It appears that 70 percent of whites have no African ancestors. Among the 30 percent who do, the black admixture is around 2.3 percent, which would be like having about three black ancestors out of those 128.

In contrast, African-Americans are much more racially mixed than European-Americans. Yet, Shriver's study shows that they are less European that was previously believed.

Earlier, cruder studies, done before direct genetic testing was feasible, suggested that African-Americans were 25 or even 30 percent white. Shriver's project is not complete, but with data from 25 sites already in, he is coming up with 17-18 percent white ancestry among African-Americans. That's the equivalent of 106 of those 128 of your ancestors from seven generations ago having been Africans and 22 Europeans.