[s_q_b]

There's a lot more racial admixture in the American population than most realize, especially among populations who have been in the country for long periods of time. At least 4% of self-identified "white" Americans have recent (within the last 200 years) African ancestry, pointing to a historical level of permeability in the racial barrier.

This is not overly surprising because "race" is a social construction, not a biological category, and individuals of ambiguous racial admixture are not uncommon today. Take for example Congressman G.K. Butterfield, who is constantly subject to questions about his membership in the Congressional Black Caucus among other topics, despite having two parents of visible African ancestry. http://en.wikipedia.org/wiki/G._K._Butterfield

The real point is that racial categories are silly. Humans are remarkably genetically similar across the globe. There is greater genetic variation in two chimpanzees from the same troupe than there is between humans from the tip of Patagonia and the Khoisan in Africa.