American history isn’t just something we learn about in school—it’s also something we carry in our genes. That’s one insight offered by a new study in PLOS Genetics on how slavery and African-American immigration patterns have shaped the contemporary demographics of the US.

The study, which examined the genomes of 3,726 African-Americans, found differences in African-American genetic ancestry according to regional patterns. African-Americans living in the South had the highest percentage of African ancestors. Those living in the North and West regions had significantly lower percentages of African ancestors, and higher percentages of European ones. These genetic differences originate from the movements of people during the Great Migration—the massive exodus of over six million black people from the South to Northern cities between 1910 and 1970.

Geneticists use the term admixture to describe when two previously isolated genetic groups—such as Europeans and Africans—combine by interbreeding. Today, most African-Americans have both African and European ancestry. This can be traced back to the horrific legacies of slavery, during which European slave masters raped the African women they owned.

African-Americans living in the South had the highest percentage of African ancestors.

Simon Gravel, an assistant professor of human genetics, and his team at McGill University created a model of genetic data to approximate the point at which African and European genes had combined. Unsurprisingly, they found that admixture began before and around the time of the Civil War. Historically, the end of slavery largely meant the end of the systemic rape of enslaved black women, which also fit the end point for admixture in the genetic model.

Gravel and his team were even able to pinpoint evidence for this history of rape in their genetic model. They found a higher contribution of African ancestry from the X chromosome, signaling that women of African ancestry contributed more to the genetics of the African-Americans in the study than men of African ancestry. Another model of admixture in the study also showed a higher percentage of genetic contribution from male European ancestors.

Interestingly, researchers found that the people who had left the South during the Great Migration had less African ancestry than those who stayed. This difference became less pronounced over time: more recent migrants to areas outside the South had a higher percentage of African ancestry than those who were already living there.

These findings suggest that earlier waves of the Great Migration were comprised of black people with a greater percentage of European heritage than later waves. This makes sense, given the historical privileges accorded to lighter-skinned African-Americans. The fact that people with lighter skin were likely to face less job and housing discrimination may have enabled them to move away earlier than darker-skinned people.

The fact that people with lighter skin were likely to face less discrimination may have enabled them to move North earlier than darker-skinned people.

While the study looked at a fairly large sampling, it offers an incomplete picture of African-American genealogy. Many diverse groups of African-Americans live in the US, and some of their history and genes may not have been included.

The study’s authors suggest that these findings may help further genetic research on the diseases to which African-American communities are particularly susceptible, by improving the quality and amount of data on underrepresented populations, leading to better scientific models of risk. “If you’re not from a European background you have less information about your disease risk,” Gravel says. “It’s just because studies have the most focus on Europeans, so we understand the diversity of Europeans better.”

Dorothy Roberts, a professor of race, gender, and the law at the University of Pennsylvania, is critical of the notion that the study will help to reduce race-based health inequities. There are specific genetic diseases tied to ethnicity, such as Tay-Sachs disease. But Roberts notes that most racial disparities in health are attributable to socially-based differences, such as experiencing discrimination in the form of reduced opportunities and increased stress. “We can’t think … that knowing more about each group’s genetic makeup is going to necessarily lead to reducing the differences in health between these groups,” Roberts says.

However, Roberts says the study is quite useful in that it “helps to discredit the notion that there are genetically pure and distinct races of people.” We already know that racial classifications are a sociological rather than a biological construct. But studies like this one help to drive home this reality—and to help illuminate the ways in which history shapes our genetics.