Over the last several hundred years, the United States has been the site of ongoing mixing of peoples of continental populations that were previously separated by geography. Native Americans, European immigrants to the Americas, and Africans brought to the New World largely via the trans-Atlantic slave trade came together in the New World. Mating between individuals with different continental origins, which we refer to here as “population admixture,” results in individuals who carry DNA inherited from multiple populations. Although US government census surveys and other studies of households in the US have established fine-scale self-described ethnicity at the state and county level (see the US 2010 Census online), the relationship between genetic ancestry and self-reported ancestry for each region has not been deeply characterized. Understanding genetic ancestry of individuals from a self-reported population, and differences in ancestry patterns among regions, can inform medical studies and personalized medical treatment.1 The genetic ancestry of individuals can also shed light on the history of admixture and migrations within different regions of the US, which is of interest to historians and sociologists.