Mixed Race Studies

Tales of African-American History Found in DNA

The New York Times
2016-05-27

Carl Zimmer

The history of African-Americans has been shaped in part by two great journeys.

The first brought hundreds of thousands of Africans to the southern United States as slaves. The second, the Great Migration, began around 1910 and sent six million African-Americans from the South to New York, Chicago and other cities across the country.

In a study published on Friday, a team of geneticists sought evidence for this history in the DNA of living African-Americans. The findings, published in PLOS Genetics, provide a map of African-American genetic diversity, shedding light on both their history and their health.

Buried in DNA, the researchers found the marks of slavery’s cruelties, including further evidence that white slave owners routinely fathered children with women held as slaves.

And there are signs of the migration that led their descendants away from such oppression: Genetically related African-Americans are distributed closely along the routes they took to leave the South, the scientists discovered…

…The history of African-Americans poses special challenges for geneticists. During the slave trade, their ancestors were captured from genetically diverse populations across a portion of West Africa. Adding to the complexity is the fact that living African-Americans also may trace some of their ancestry to Europeans and Native Americans…

…Most of the Native American DNA identified by Dr. Gravel and his colleagues in African-Americans occurs now in tiny chunks. The scientists concluded that most of the mingling between Africans and Native Americans took place soon after the first slaves arrived in the American colonies in the early 1600s.

The European DNA in African-Americans, on the other hand, occurs in slightly longer chunks, indicating a more recent origin. Dr. Gravel and his colleagues estimate that its introduction dates to the decades before the Civil War…

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The Great Migration and African-American Genomic Diversity

PLOS Genetics
2016-05-27
27 pages
DOI: 10.1371/journal.pgen.1006059

Soheil Baharian
Department of Human Genetics
McGill University, Montreal, Quebec, Canada
Genome Quebec Innovation Centre, Montreal, Quebec, Canada

Maxime Barakatt
McGill University and Genome Quebec Innovation Centre, Montreal, Quebec, Canada

Christopher R. Gignoux
Department of Genetics
Stanford University School of Medicine, Stanford, California

Suyash Shringarpure
Department of Genetics
Stanford University School of Medicine, Stanford, California

Jacob Errington
Department of Human Genetics
McGill University, Montreal, Quebec, Canada
Genome Quebec Innovation Centre, Montreal, Quebec, Canada

William J. Blot
Division of Epidemiology
Vanderbilt University School of Medicine, Nashville, Tennessee
International Epidemiology Institute, Rockville, Maryland

Carlos D. Bustamante
Department of Genetics
Stanford University School of Medicine, Stanford, California

Eimear E. Kenny
Department of Genetics and Genomic Sciences
The Icahn School of Medicine at Mount Sinai, New York, New York

Scott M. Williams
Department of Genetics
Institute for Quantitative Biomedical Sciences, Dartmouth College, Hanover, New Hampshire

Melinda C. Aldrich
Division of Epidemiology, Department of Thoracic Surgery
Vanderbilt University School of Medicine, Nashville, Tennessee

Simon Gravel
Department of Human Genetics
McGill University, Montreal, Quebec, Canada
Genome Quebec Innovation Centre, Montreal, Quebec, Canada

Fig 3. Pairwise genetic relatedness across US census regions among (A) African-Americans, (B) European-Americans, and (C) African-Americans and European-Americans. (D) Census-based prediction for African-Americans (see Materials and Methods). On each map, the line connecting two regions shows the average relatedness between individuals in those regions, and the thickness and opacity of the lines are on a linear scale between the minimum and maximum values shown above the map. Relatedness between regions with fewer than 10,000 possible pairs of individuals is not shown (see Materials and Methods for details). All numbers are in units of cM. (E) Decay of average IBD (shown in logarithmic scale) as a function of distance using IBD segments of length 18cM or longer from HRS (dots), compared to the analytical model (lines).

Abstract

We present a comprehensive assessment of genomic diversity in the African-American population by studying three genotyped cohorts comprising 3,726 African-Americans from across the United States that provide a representative description of the population across all US states and socioeconomic status. An estimated 82.1% of ancestors to African-Americans lived in Africa prior to the advent of transatlantic travel, 16.7% in Europe, and 1.2% in the Americas, with increased African ancestry in the southern United States compared to the North and West. Combining demographic models of ancestry and those of relatedness suggests that admixture occurred predominantly in the South prior to the Civil War and that ancestry-biased migration is responsible for regional differences in ancestry. We find that recent migrations also caused a strong increase in genetic relatedness among geographically distant African-Americans. Long-range relatedness among African-Americans and between African-Americans and European-Americans thus track north- and west-bound migration routes followed during the Great Migration of the twentieth century. By contrast, short-range relatedness patterns suggest comparable mobility of ∼15–16km per generation for African-Americans and European-Americans, as estimated using a novel analytical model of isolation-by-distance.

Author Summary

Genetic studies of African-Americans identify functional variants, elucidate historical and genealogical mysteries, and reveal basic biology. However, African-Americans have been under-represented in genetic studies, and relatively little is known about nation-wide patterns of genomic diversity in the population. Here, we study African-American genomic diversity using genotype data from nationally and regionally representative cohorts. Access to these unique cohorts allows us to clarify the role of population structure, admixture, and recent massive migrations in shaping African-American genomic diversity and sheds new light on the genetic history of this population.

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