In 2002, Mark Shriver claimed 30% of white Americans have on average about 2% African ancestry, the average for the population as a whole coming out to about 0.7%. Shortly thereafter, in a different interview, Shriver lowered his estimate, purporting "about 10 percent of [the European-American population] have some African ancestry". Subsequently, another principal of DNAprint revised the estimate still further downward: "Five percent of European Americans exhibit some detectable level of African ancestry". That too was an overestimate. 23andMe, examining the genomes of vastly larger numbers of people using thousands of times as many SNPs, estimates "about 2%" of "European Americans" have any detectable autosomal black ancestry. And three quarters of that 2% have only "about 0.5%" African ancestry (i.e., less than Shriver in 2002 claimed the average American carried).
Exceptions to the "One Drop Rule"? DNA evidence of African ancestry in European Americans. J. L. Mountain1, J. M. Macpherson1, C. B. Do1, B. T. Naughton1, R. A. Kittles2, N. Eriksson1 1) 23andMe, Inc, Mountain View, CA; 2) Institute of Human Genetics, University of Illinois at Chicago, Chicago, IL.
Genetic studies have revealed that most African Americans trace the majority (75-80%, on average) of their ancestry to western Africa. Most of the remaining ancestry traces to Europe, and paternal lines trace to Europe more often than maternal lines. This genetic pattern is consistent with the "One Drop Rule,” a social history wherein children born with at least one ancestor of African descent were considered Black in the United States. The question of how many European Americans have DNA evidence of African ancestry has been studied far less. We examined genetic ancestry for over 77,000 customers of 23andMe who had consented to participate in research. Most live in the United States. A subset of about 60,000 shows genetic evidence of fewer than one in 16 great-great-grandparents tracing ancestry to a continental region other than Europe. They are likely to consider themselves to be entirely of European descent. We conducted two analyses to understand what fraction of this group has genetic evidence of some ancestry tracing recently to Africa. We first identified individuals whose autosomal DNA indicates that they are predominantly of European ancestry, but who carry either a mitochondrial (mt) DNA or Y chromosome haplogroup that is highly likely to have originated in sub-Saharan Africa. Of the 60,000 individuals with 95% or greater European ancestry, close to 1% carry an mtDNA haplogroup indicating African ancestry. Of approximately 33,000 males, about one in 300 trace their paternal line to Africa. We then identified the subset of these European Americans who have estimates of between 0.5% and 5.0% of ancestry tracing to Africa. This subset constitutes about 2% of this set of individuals likely to be aware only of their European ancestry. The majority (75%) of that group has a very small estimated fraction of African ancestry (about 0.5%), likely to reflect African ancestry over seven generations (about 200 years) ago. We estimate that, overall, at least 2-3% of individuals with predominantly European ancestry have genetic patterns suggesting relatively deep ancestry tracing to Africa. This fraction is far lower than the genetic estimates of European ancestry of African Americans, consistent with the social history of the United States, but reveals that a small percentage of “mixed race” individuals were integrating into the European American community (passing for White) over 200 years ago, during the era of slavery in the United States.