by Carl Zimmer … 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. … 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.

From the New York Times, their spin on the 23andMe racial admixture data that I’ve been writing about for a year or two.Actually, as the genome data has gotten more precise in the 21st Century, the big surprise has been how white are American whites. I wrote an article back in 2002 about some early Penn State data, but as more genetic markers have been analyzed, the picture has gotten less murky and white Americans have turned out to be extremely white.My guess is that most people who pay to have their ancestry analyzed are older. It’s not uncommon, for example, for the recently retired to get into genealogy.The usual estimate is that African-Americans are about 80% black. I’d suspect that this lower figure here may be an artifact of the selection process: paying to have your DNA analyzed probably appeals more to wealthier and whiter African-Americans, such as Henry Louis Gates.Once again, an artifact of the selection process. This sample is clearly not representative of the Mexican-American masses. Typically, studies of non-self-selected Hispanics in the Southwest, such as patients at a hospital, typically find the European and Native American ancestries to be of roughly comparable size.Of course, 98.6% white, 0.19% black, and 0.18% Indian only adds up to about 99%, so apparently there is some wiggle room in these numbers. But let’s just use the numbers as printed.

I don’t know which way the sample’s biases push this figure for whites, but in any case: whiteness in modern America turns out to be not very murky at all. These findings of 0.19% black and 0.18% American Indian are tiny numbers.

Think about your family tree back nine generations ago, which would mostly be in the 1700s. You have 512 slots in your family tree nine generations ago (two to the ninth power). The 23andMe numbers suggest that for the average white American, 1 of your 512 ancestors nine generations ago was black and 1 of 512 was Native American.

Here’s another way to think of it. If the average self-identified black is 73.2% black and the average self-identified white is 0.19% black, then the average black in America is 385 times blacker than the average white. That doesn’t seem very murky to me.

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.

There are about 200 million whites, so that means a little over 3% have any black ancestry that can be found by 23andMe.The sample size is quite small in this 1/4th black range. The traditional working of the one drop rule tended to push individuals away from 3/4th white / 1/4th black over the generations.

I suspect that this may also be an artifact of 23andMe appealing to genealogy hobbyists, whereas, say, elite African-Americans derive much of their eliteness from their ability to claim African ancestry so they aren’t in a hurry to pay money to find out how white they are: e.g., Professor Gates wasn’t all that excited to find out he’s about half white (if it had turned out he was all white, well, good-bye career).