The history of African Americans has been shaped by two great journeys. The first brought millions of Africans to the southern United States as slaves. The second, known as the Great Migration, began around 1910, when six million African Americans left the South for New York, Chicago and other cities across the country.
In a study published on Friday in PLOS Genetics, a team of geneticists explored how these journeys have left genetic clues in the DNA of living African-Americans. The variations in their genes carry clues about the cruelties of slavery and the routes many took to escape it.
But Dr. Esteban G. Burchard, a physician-scientist at the University of California, San Francisco, who was not involved in the study, said its importance went well beyond history. A detailed map of genetic variations in African-Americans will help show how genes influence their risk for diseases.
“This has tremendous medical relevance,” he said. Until recently, most research into the link between genes and disease has focused on people of European descent. “We’re missing out on a lot of biology and diversity,” said Simon Gravel, a geneticist at McGill University.
The history of African-Americans poses special challenges for geneticists like Dr. Gravel. During the slave trade, their ancestors were captured from genetically diverse populations across a swath 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. The mixture of genetic variants makes it challenging to pinpoint those that may cause disease.
In the new study, Dr. Gravel and his colleagues analyzed the DNA of 3,726 African Americans participating in three separate medical studies.
The scientists were able to pinpoint stretches of DNA in the subjects that originated on different continents. According to their calculations, the ancestors of the average African-American today were 82.1 percent African, 16.7 percent European and 1.2 percent Native American.
Dr. Gravel and his colleagues also estimated when those genes were introduced.
When two parents from different ethnic backgrounds have a family, their children share long stretches of identical DNA. Over the generations, the stretches get smaller. The lengths therefore serve as a kind of genealogical clock.
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.
The scientists gave some attention to the X chromosome in particular because of its role in sex determination. One X chromosome is inherited from mothers; fathers may contribute a Y or X. The researchers observed that the X chromosome of African Americans has a greater African ancestry than other chromosomes.
This variation is consistent with European men and African women producing children — in other words, slave owners coercing sex from their female slaves.
The databases that Dr. Gravel and his colleagues studied also included information about where the subjects now live. The scientists used this information to trace the movements of African Americans through the United States — through a lens of DNA.
The researchers found very strong genetic connections between African Americans in the Deep South and those in the Northeast and Midwest. Genetic similarities in African Americans cluster along the very train lines that their forebears took as they escaped the Jim Crow South: the Illinois Central train to Chicago, for example, and the Atlantic Seaboard train up the East Coast.
The scientists were intrigued to find that European Americans who live in the South are more closely related to African Americans in the north or west than to present-day African Americans in the South.
“The first people to migrate out of the south were the ones with the most European ancestry,” said Dr. Gravel.
Alondra Nelson, the dean of social science at Columbia University and the author of “The Social Life of DNA,” didn’t think that result had much historical significance about who left the South or why. She noted that in the study, the researchers only found 1 percent more European ancestry in the African Americans who left.
Dr. Nelson noted that historians are increasingly collaborating on genetic studies like these. But the new study does not include a historian among its authors. “The human intentions around escaping racial terror can’t possibly be reduced to genotype,” she said. “If you’re interested in understanding the Great Migration, it’s a tremendous lost opportunity.”
The authors of the new study said that the genetic variations of African Americans across the United States could be important for medical research. Researchers who want to study the influence of genes on diseases in African Americans must be aware of where their subjects live.
“If you’re drawing your cases from Chicago, can you use controls from South Carolina?” asked Eimear E. Kenny, a geneticist at the Icahn School of Medicine at Mount Sinai in New York and a co-author of the new study.
“I think this study would suggest you have to be very, very careful about that.”
Dr. Burchard of U.C.S.F. said that a better understanding of African-American genetics could also lead to discoveries that could benefit all people. Scientists found a rare genetic mutation in an African-American women for example, that lowered her cholesterol levels. That discovery led to a promising blockbuster drug for heart disease.
“Lo and behold it was relevant to all populations, regardless of race,” said Dr. Burchard. “It’s relevant if you’re European, if you’re African, if you’re Asian, if you’re pink, white, blue or green.”
Culled from New York Times
