By Carl Zimmer, The New York Times December 24, 2014
In 1924, the State of Virginia attempted to define what it means to be white.
The state’s Racial Integrity Act, which barred marriages between whites and people of other races, defined whites as people “whose blood is entirely white, having no known, demonstrable or ascertainable admixture of the blood of another race.”
There was just one problem. As originally written, the law would have classified many of Virginia’s most prominent families as not white, because they claimed to be descended from Pocahontas.
So the Virginia legislature revised the act, establishing what came to be known as the “Pocahontas exception.” Virginians could be up to one-sixteenth Native American and still be white in the eyes of the law.
People who were one-sixteenth black, on the other hand, were still black.
In the United States, there is a long tradition of trying to draw sharp lines between ethnic groups, but our ancestry is a fluid and complex matter. In recent years geneticists have been uncovering new evidence about our shared heritage, and last week a team of scientists published the biggest genetic profile of the United States to date, based on a study of 160,000 people.
The researchers were able to trace variations in our genetic makeup from state to state, creating for the first time a sort of ancestry map.
“We use these terms — white, black, Indian, Latino — and they don’t really mean what we think they mean,” said Claudio Saunt, a historian at the University of Georgia who was not involved in the study.
The data for the new study were collected by 23andMe, the consumer DNA-testing company. When customers have their genes analyzed, the company asks them if they’d like to make their results available for study by staff scientists.
Over time the company has built a database that not only includes DNA, but also such details as a participant’s birthplace and the ethnic group with which he or she identifies. (23andMe strips the data of any information that might breach the privacy of participants.)
The scientists also have been developing software that learns to recognize the origins of the short segments of DNA that make up our genomes. Recently they used their program to calculate what percentage of each subject’s genomes was inherited from European, African or Native American forebears.
“This year we saw that we were in a great position to do the analysis,” said Joanna L. Mountain, senior director of research at 23andMe.
On average, the scientists found, people who identified as African-American had genes that were only 73.2 percent African.
European genes accounted for 24 percent of their DNA, while .8 percent came from Native Americans.
Latinos, on the other hand, had genes that were on average 65.1 percent European, 18 percent Native American, and 6.2 percent African. The researchers found that European-Americans had genomes that were on average 98.6 percent European, .19 percent African, and .18 Native American.
These broad estimates masked wide variation among individuals. Based on their sample, the resarchers estimated that over six million European-Americans have some African ancestry. As many as five million have genomes that are at least 1 percent Native American in origin. One in five African-Americans, too, has Native American roots.
Dr. Mountain and her colleagues also looked at how ancestry might influence ethnic identification.
Most Americans with less than 28 percent African-American ancestry say they are white, the researchers found. Above that threshold, people tended to describe themselves as African-American.
Katarzyna Bryc, a 23andMe researcher and co-author of the new study, didn’t want to speculate about why people’s sense of ethnic identity pivots at that point.
“We can only take it so far as geneticists,” she said.
The scientists also linked geographical patterns to their subjects’ ancestries. Latinos in the Southwest had high levels of Native American DNA, they found, while Latinos in the Southeast had high levels of African DNA.
The genes of African-Americans varied strikingly from state to state. In Oklahoma, the researchers estimated, 14 percent of African-Americans have genomes that are at least 2 percent Native American. This high percentage is probably due to the unique history of the state.
Some Native American tribes in the South, such as the Cherokee and Choctaw, kept African slaves. When they were expelled to Oklahoma in the 1830s, they brought the slaves with them. In some tribes, Native Americans and African slaves intermarried, and their descendants continue to live in Oklahoma today.
Dr. Saunt was fascinated in particular by the genetic findings among people in South Carolina. Dr. Mountain and her colleagues estimated that 13.3 percent of European-Americans in South Carolina have genes that are at least 1 percent African in origin.
But the researchers also found that African-Americans in South Carolina have among the lowest percentages of European DNA of any African-American population in the United States.
At one point, Dr. Saunt noted, the percentage of South Carolina residents who were slaves was greater than in any other state. But there was also a large population of freed slaves in Charleston permitted to interact with whites.
“We know lots of planters had mistresses in Charleston, and they obviously had children together,” said Dr. Saunt. “So what happened to those children? Some remained in the African-American community, and some moved into the white community when they were able to.”
Jeffrey C. Long, an anthropologist at the University of New Mexico who was not involved in the study, cautioned that the research was not based on a random sample of Americans.
Instead, Dr. Mountain and her colleagues studied only people who were curious enough about their DNA to pay for a test.
“Perhaps people who have mixed ancestry are more interested in their ancestry than people who don’t think they have mixed ancestry,” Dr. Long said.
David Reich, a geneticist at Harvard University and a co-author on the new study, acknowledged this was a reasonable concern. “It’s classic survey bias,” he said. But Dr. Reich also noted that the new results were consistent with smaller studies done in the past.
As genetic databases grow, Dr. Reich predicted it would be possible to get even more detailed insights into American history. DNA may be able to illuminate the movements of people across the United States, such as the Great Migration that took African-Americans from the South to Chicago.
“We’re in striking distance of that now,” Dr. Reich said.