Fine scale population genetic structure of African Americans. E. Y. Durand, M. Macpherson, B. T. Naughton, J. Mountain, C. B. Do 23andMe, Inc., Mountain View, CA 94043.

   Admixture deconvolution refers to the inference of the geographic origin of chromosomal segments. It has a wide range of applications, from disease mapping to learning about history. In particular, the fine-scale study of recently admixed populations such as African Americans illuminates recent human migrations and is critical for better understanding medical genomics in less homogenous populations. However, current methods for admixture deconvolution generally yield only coarse information because they are limited in the number of ancestral populations they use (typically 2 or 3). Therefore the ancestry of chromosomal segments is identified only at the level of continental origin. It has been estimated that African Americans typically have 65-85% of their ancestry tracing to Africa and 15-25% tracing to Europe, but little is known about their subcontinental distribution of admixture. Little is known, also, about the contribution of Native Americans to the African American gene pool. Leveraging a panel of more than 7,500 individuals with known ancestry derived from a combination of several publicly available datasets and over 5,000 23andMe, Inc., customers reporting four grandparents with the same country-of-origin, we developed a novel method that permits the accurate assignment of chromosomal segments to more than 20 geographic regions. We applied this method to data for 8,500 unrelated African American 23andMe customers who had been genotyped at more than one million sites. We traced back the ancestry of chromosomal segments to subregions of Europe and Africa, thus obtaining an unprecedented fine-grained picture of the ancestry of African Americans. We also found that many African Americans have a significant amount (e.g. more than 5%) of Native American ancestry, consistent with admixture between the two groups as recent as four generations ago. Using a subset of 3,000 individuals who self reported their place of birth, we compared the distribution of African, European and Native American ancestry across different regions in the United States.

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