The history of African-Americans has been shaped in part by two great journeys.
The first brought millions 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 look to leave the South, the scientists discovered.
The importance of that finding is not just historical, said Dr. Esteban G. Burchard, a physician and scientist at the University of California, San Francisco, who was not involved in the study.
A detailed map of genetic variations in African-Americans will help show how genes influence their risk for various 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 in Montreal.
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.
In the new study, Dr. Gravel and his colleagues analyzed the DNA of 3,726 African-Americans who participated 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 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. Dr. Gravel and his colleagues believe this variation is explained by European men and African women producing children — in other words, slave owners coercing sex from the women they held captive.
The databases that Dr. Gravel and his colleagues studied also included information about where the subjects now live. The scientists used this information to help trace the movements of African-Americans through the United States.
They found very strong genetic connections between African-Americans in the Deep South and those in the Northeast and Midwest.
The genetic similarities in African-Americans tend to cluster along the very train lines that their forebears took as they left the Jim Crow South: the Illinois Central to Chicago, for example, and the Atlantic Coast line up the East Coast.
The scientists were intrigued to find that European Americans who live in the South now are more closely related to African-Americans in the North or West than to present-day African-Americans in the South.
Dr. Gravel has proposed a surprising explanation: “The first people to migrate out of the South were the ones with the most European ancestry,” he said.
Alondra Nelson, the dean of social science at Columbia University and the author of “The Social Life of DNA: Race, Reparations, and Reconciliation after the Genome” said she did not think that result contributed much to understanding who left the South or why.
In the study, she noted, the researchers found just 1 percent more European ancestry in the African Americans who left the South.
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. San Francisco 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 woman, for example, that lowered her cholesterol levels. That discovery led to a promising 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.”