The genetic ancestry of African, Latino, and European Americans across the United States. K. Bryc1, E. Durand1, D. Reich2,3,4, J. Mountain1 1) 23andMe, Inc., Mountain View, CA; 2) Harvard Medical School, Department of Genetics, Boston, MA, USA; 3) Howard Hughes Medical Institute, Boston, MA, USA; 4) Broad Institute of MIT and Harvard, Cambridge, MA, USA.

   Within the past 500 years, North America has been the site of a dynamic mixing of people from populations that were previously separated by oceans and other geographic features. The interactions between Native Americans, European settlers, and Africans brought to the New World via the Trans-Atlantic slave trade shaped the early history of what became the United States. We studied the genetic ancestry of 5,269 self-reported African Americans, 8,633 Latinos, and 148,789 European Americans who are 23andMe customers living in the US and show that the legacy of these interactions is visible in the genetically-inferred ancestry of modern Americans.
   We shed light on the unique regional differences in genetic ancestry within and across the United States. In addition to well-established variability in individual ancestry proportions, we detect systematic clines in the amount of African and European ancestry in African Americans from different states. We demonstrate that the pervasive mixed ancestry in modern Americans and the relationship between self-reported identity and genetic ancestry reflect regional social and political history. We find that levels of Native American and African ancestry in European Americans, Latinos, and African Americans are highly correlated with the population density of African Americans and Latinos in each state with one notable exception.
   We provide evidence that a minor, but measurable, proportion of self-reported European Americans carry African ancestry. Likewise, we demonstrate that an appreciable fraction of European Americans carry Native American ancestry. In European Americans, we find strong differences in European subpopulation ancestry across the US, which are consistent with known major historical migrations from different regions of Europe. The genetic diversity within each identity, combined with significant overlap of ancestry profiles between identities, highlights the complexity of using self-reported or physician-inferred ancestry in a biomedical context, since the genetic ancestry of individuals within each identity is widely variable. Our results inform the geography of historical admixture in the US, have sociological implications for self-identity, and inform the use of self-reported race and ethnicity in a medical setting.

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