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.