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.
I've always read that Blacks living in the USA are ~ 25% European.
>> 17-18 percent white ancestry among African-Americans <<
Corresponds pretty closely to what I think I would have guessed, based simply on personal observation.
But what about Indian (”Native American”) ancestry? A lot of blacks have it, as do whites. A lot of African-Americans have what look to me like Indian facial features. And when it comes to whites, my experience indicates it’s hard to find a white southern family whose oral tradition doesn’t include an Indian ancestor (usually female) way back in the 1700’s or early 1800’s.