Carl Zimmer, 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.

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The data for the new study were collected by 23andMe, the consumer DNA-testing company. {snip}

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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.

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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.

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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.

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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.

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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.

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