Pervasive Mixing Found in Genetic Ancestry of 160,000 Americans

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Different racial and ethnic groups have shaped the history of North America over the course of five millennia. To see how the genetic ancestry of different populations varies across geographic regions, researchers studied the genomes of 5,269 African Americans, 8,663 Latinos, and 148,789 European Americans—all self-described, all customers of personal genomics company 23andMe. The work was published in the American Journal of Human Genetics this week. 

“The signatures of recent historical migrations can be seen in the DNA of present-day Americans,” Harvard’s Katarzyna Bryc says in a news release. They found pervasive mixed ancestry: All three of the largest groups in the U.S. have ancestry from Africa, Europe, and the Americas. But all three groups had asymmetrical male and female contributions, with more European male and more Native American and African female ancestors. 

Regional ancestry differences, they found, seemed to reflect historical events: early Spanish colonization, waves of immigration from throughout Europe, and forced relocation of Native Americans. (The country’s complex mixing is also why the U.S. has been ignored by many population geneticists until now.)

More than 6 million Americans who self-identify as European (that’s about 3.5 percent) carry at least 1 percent African ancestry—most likely from five to 10 generations ago. And it varies by state, as illustrated in the infographic above. As many as 5 million might have at least 1 percent Native American ancestry.

African Americans in Georgia and South Carolina have the highest average percentage of African ancestry. One in every 20 African Americans carries Native American ancestry, and more than 14 percent of African Americans from Oklahoma carry at least 2 percent Native American ancestry—likely reflecting the Trail of Tears following the Indian Removal Act of 1830. Here’s an illustration of the mean proportion of African ancestry for African Americans across the U.S. 

Among self-described Latinos, those from states in the southwest have the highest levels of Native American ancestry. On average, Science reports, Latinos carry 18 percent Native American ancestry, 65.1 percent European ancestry (mostly from the Iberian Peninsula), and 6.2 percent African ancestry. 

Individuals mostly identified with the majority of their genetic ancestry—rather than according to the “one-drop rule.” In this study, people with up to 28 percent African ancestry are more likely to describe themselves as European American than as African American, whereas those with more than 30 percent African ancestry are more likely to describe themselves as African American. The average African American genome, Science reports, is 73.2 percent African, 24 percent European, and 0.8 percent Native American.

“Our study not only reveals the historical underpinnings of regional differences in genetic ancestry but also sheds light on the complex relationships between genetic ancestry and self-identified race and ethnicity,” Bryc says in a Cell Press release, adding that the findings may undermine “use of cultural labels that separate individuals into discrete, non-overlapping groups.” You can see all their open-access maps and figures here

Images: 23andMe 

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Read more: http://www.iflscience.com/health-and-medicine/researchers-analyzing-genetic-ancestry-160000-americans-find-pervasive-mixing