Our genetic make-up is the result of history. Historical events that influenced the patterns of migration and mating among our ancestors are reflected in our DNA — in our genetic relationships with each other and in our genetic risks for disease. This means that, to understand how genes affect our biology, geneticists often find it important to tease out how historical drivers of demographic change shaped present-day genetics.

Understanding the connection between history and DNA is especially important for African Americans, because slavery and discrimination caused profound and relatively rapid demographic change. A new study now offers a very broad look at African-American genetic history and shows how the DNA of present-day African Americans reflects their troubled history.

Slavery and its aftermath had a direct impact on two critical demographic factors that are especially important in genetics: migration and sex. The trans-Atlantic slave trade was a forced migration that carried nearly 400,000 Africans over to the colonies and, later, the United States. Once in North America, African slaves and their descendants mixed with whites of European ancestry, usually because enslaved black women were raped and exploited by white men. And, more recently, what’s known as the Great Migration dramatically re-shaped African-American demographics in the 20th century. Between 1915 and 1970, six million blacks left the South and settled in the Northern, Midwestern, and Western states, in hope of finding opportunities for a better life.

How this turbulent history shaped the genes of African Americans has been unclear because, until recently, most genetic studies have focused either on populations from different geographical regions around the world, or on Americans with European ancestry. Fortunately, African Americans are now being included in these studies on a larger scale, and several long-term studies have collected genetic data on thousands of African Americans, representing all areas of the country. In a recently published study, a team of researchers at McGill University in Montreal turned to this data to take a broad look at the genetic history of African Americans.

African Americans with a higher fraction of European ancestry, who often have lighter skin, had better social opportunities and were thus in a better position to migrate to northern and western states.

The researchers focused on nearly 4,000 African Americans who participated in two important studies, both sponsored by the National Institutes of Health. The Health and Retirement Study consists of older volunteers sampled from urban and rural areas across the U.S., while the Southern Community Cohort Study focuses on African Americans in the South, particularly areas that have a disproportionately high burden of disease. Together, these two studies are among the largest sources of genetic data on African Americans. Importantly, they represent a geographically broad sampling of the African-American population, which is critical for outlining the patterns of genetic history.

The researchers first looked at what fraction of African Americans’ genetic ancestry could be traced back to Africa. Not surprisingly, the data shows that, for most African Americans, the majority of their DNA comes from African ancestors. The results also show that essentially all African Americans have some European ancestry ancestry as well. The genetic mix of African and European DNA, however, follows a striking geographical trend: African Americans living in Southern states have more African DNA (83 percent) than those living in other areas of the country (80 percent). Conversely, African Americans outside the South have a larger fraction of European DNA. Even within the South, this trend holds: Blacks in Florida and South Carolina have more African DNA than those living in Kentucky and Virginia.

One explanation for this geographical bias could be that interracial marriages have been less frequent in Southern states. But this explanation appears to be wrong. The McGill researchers found that most of the European DNA among blacks today probably entered the African-American gene pool long before the Civil War, when the vast majority of blacks in the U.S. were slaves living in the South. The genetic patterns observed by the researchers suggest that, for at least a century before the Civil War, there was ongoing admixture between blacks and whites. After slavery ended, this interracial mixing dropped off steeply.

The implication of these findings won’t be surprising to anyone: Widespread sexual exploitation of slaves before the Civil War strongly influenced the genetic make-up of essentially all African Americans alive today.

But this poses a puzzle: If African Americans can trace most of their European ancestry to an era when America’s black population was overwhelmingly confined to the South, why is it that African Americans now living outside the South have more European DNA?

The researchers propose an interesting answer. They argue that the Great Migration of African Americans out of the South was genetically biased: African Americans with a higher fraction of European ancestry, who often have lighter skin, had better social opportunities and were thus in a better position to migrate to northern and Western states. Though it will take further evidence to show this definitively, the McGill researchers’ results imply that, even after the end of slavery, discrimination that varied with shades of skin color continued to influence the genetic history of African Americans.

Do these genetic findings matter to anyone other than historians and genealogists? The answers is yes — studies of genetic history like this one are important because they help explain why blacks and whites often have different genetic risk factors for the same diseases. African Americans are disproportionately affected by many common diseases, and while much of this is due to poverty and limited access to good health care, genetics plays a role as well. If African Americans are to fully benefit from modern health care, where diagnoses and treatments are increasingly tailored to a patient’s DNA, it is critical that we understand African Americans’ genetic history, and how it contributes to their health today. In other words, we need to understand not just the cultural and economic legacies of slavery and discrimination, but the genetic legacy as well.