That genetic ancestry of self-described groups varies across geographic locations in the US has been documented in anecdotal examples but has not previously been explored systematically. Most early studies of African Americans had limited resolution of ancestry because of small sample sizes and few genetic markers, and recent studies typically have limited geographic scope. Though much work has been done to characterize the genetic diversity among Latino populations from across the Americas, it is unclear the extent to which Latinos within the US share or mirror these patterns on a national or local scale. Most analyses have relied on mitochondrial DNA, Y chromosomes, or small sets of ancestry-informative markers, and few high-density genome-wide SNP studies have explored fine-scale patterns of African and Native American ancestry in individuals living across the US.