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AncestryDNA Study Examines Post-Colonial US Population Structure

NEW YORK (GenomeWeb) – Using identity-by-descent connections, AncestryDNA researchers have teased out fine-scale population structure within the modern US population.

By examining genotyping data from some 770,000 people from the US, AncestryDNA's Catherine Ball and her colleagues uncovered closely connected clusters of related individuals that reflected recent, post-colonial immigration patterns. As they reported in Nature Communications, the researchers identified population patterns in immigrant clusters from various geographical regions, as well as in isolated US populations such as the Amish.

"With much greater granularity than previously possible, our analyses demonstrate the impact of subtle, complex demographic forces in shaping the patterns of genetic variation among contemporary North Americans," Ball and her colleagues wrote in their paper.

The researchers analyzed the genome-wide genotypes of 774,516 people born or living in the US, who'd been genotyped at 709,358 autosomal SNPs as part of the AncestryDNA direct-to-consumer genetic test. They searched for signatures of recent demographic history among this cohort by examining identity-by-descent (IBD) patterns, as individuals with recent common ancestors tend to share chromosomal stretches.

As the researchers reported, they uncovered clusters of individuals that fell into four broad categories: intact immigrant, continental admixed, assimilated immigrant, and post-migration isolated groups. To place these clusters in historical context, the researchers relied on a combination of admixture estimates and genealogical data.

With this combination of IBD patterns and genealogical data, the researchers uncovered clusters of people of Scandinavian, Jewish, and Irish ancestry, groups that immigrated to the US within the past 200 years. They dubbed these intact immigrant groups as they largely reflected population structure that existed prior to immigration, despite subsequent admixture. They also uncovered clusters of people of Polynesian ancestry and African Americans.

They linked the clusters they uncovered through IBDs to particular immigrant groups using known ancestral birth locations. The Irish cluster included mostly Irish ancestral birthplaces, while the Scandinavian cluster largely had Norwegian ancestral birthplaces, though they noted this genealogical trace was not possible for the African-American cluster.

Most of these groups also exhibited geographical localizations within the US that corresponded with known migration patterns. For example, the Finnish and Scandinavian clusters localized to the Midwest.

Ball and her colleagues also uncovered continentally admixed clusters that included Colombians as well as groups from Central America and the Caribbean. These clusters have ancestry signatures that trace to multiple continents and ancestral birth locations outside the US.

The largest category, however, was the assimilated immigrant cluster, which included people of mixed European ancestry. They harbored highly similar allele frequencies and few ties to non-US populations, though exhibited evidence of gene flow within the US. They could be distinguished by ancestral birthplaces within the US, the researchers noted.

Genealogical data uncovered a north-to-south trend, particularly east of the Mississippi River, that the researchers said could reflect westward expansion of European settlers and limited north-south migration due to cultural differences.

Some populations, the team found, became isolated after immigrating to the US. The Amish, for instance, clustered in certain parts of the Midwest and Pennsylvania, while the Mormons clustered in Utah and Appalachia residents clustered in the Cumberland Mountain range.

"Our results yield a detailed historical portrait of North America after European settlement and support substantial genetic heterogeneity in the United States beyond that uncovered by previous studies," Ball and her colleagues wrote.

With more data, the researchers said they'd expect to improve the resolution of their clustering as well as uncover additional clusters that include more recent immigrants to the US, such as people of Southeast Asian or Chinese descent.