These days, family secrets like this one are becoming harder to keep.
A growing number of companies now offer DNA tests that promise to pinpoint a customer’s heritage and, with permission, to identify genetic relatives. The firms include generalists like 23andMe and Ancestry.com and specialty companies like African Ancestry.
Millions of people have signed up for the tests, sending saliva samples to laboratories and paying $100 to $350 or more for an analysis.
The customers are eager to know where they came from, to find a familial context that may be lacking. The answers hidden in DNA can be revelatory, shedding light on hidden events occurring decades earlier and forever changing the family narrative.
But a new analysis of DNA test kits by The Wirecutter, a review site owned by The New York Times, finds that the services also have limitations that the providers do not always fully acknowledge.
Mr. Hutchinson decided to have his DNA analyzed by 23andMe. The report revealed he is one-quarter sub-Saharan African, which means that his mother was of mixed race. There was no indication of Italian or Swedish heritage.
Mr. Hutchinson also learned that his mother was not an only child, but had a brother and a sister. A genealogist helped him track down some first cousins in Alabama, who said they had been told never to contact Mr. Hutchinson or his family.
The cousins were delighted to hear from him. He plans to visit next year at Mardi Gras.
Mr. Hutchinson’s results were enlightening, but in other contexts ethnicity has posed particularly knotty problem for DNA testing firms. The very definitions of “race” and “ethnicity” are fuzzy, said Joseph Pickrell, a computational geneticist at the New York Genome Center laboratory, affiliated with Columbia University.
“Different people mean different things when they say ‘race,’” he said. In the United States, for example, a person with almost any African ancestry often is identified as black.
“That’s not necessarily the case in other parts of the world,” Dr. Pickrell said.
Researchers at 23andMe acknowledged the difficulty in a recent paper, writing, “It is important to note that ancestry, ethnicity, identity and race are complex labels that result both from visible traits, such as skin color, and from cultural, economic, geographical and social factors.”
In a recent study, the researchers decided to use Census Bureau definitions — black, white, Hispanic — to ask how often people who identify as one race actually have genetic markers indicating a mixed heritage.
After examining data from 160,000 customers who agreed to participate, the geneticists learned that 3.5 percent of those who said they were white actually had DNA that was 1 percent or more African in origin.
The chances of having African ancestry were highest in the South, and highest of all in South Carolina, where at least 13 percent of those who said they were white had African ancestors.
Among those who said they were black, genetic ancestry over all was 73.2 percent African, 0.8 percent Native American and 24 percent European — a profile that testifies to a history of violence against African-Americans, including rape.
The ancestry of those who said they were Hispanic was something of a hodgepodge. Some had no Native American ancestry; others had 50 percent or more.
Hispanics living in the South tended to have more African ancestry. As a group, their DNA was 6.2 percent African, 18 percent Native American and 65.1 percent European.
Jewish ancestry, on the other hand, is far easier to discern. Historically, these populations were small and Jews tended not to marry outsiders. As a result, they share telltale sequences of DNA, easily identified by testing.
But is this sort of ethnic categorization really instructive? Human beings share more than 99.9 percent of their DNA; what makes us different is vanishingly insignificant in terms of genetics.
If testing “tells me I’m 95 percent Ashkenazi Jewish and 5 percent Korean, is that really different from 100 percent Askenazi Jewish and zero percent Korean?” Jonathan Marks, an anthropology professor at the University of North Carolina at Charlotte, wondered in The Wirecutter.
The question of ethnicity is enmeshed with another difficult challenge for DNA testers: geography.
Genetics researchers generally know which DNA sequences originated on which continents. But pinpointing a particular country of origin, as many testing services claim to do, is far trickier.
Scientists simply do not have good data on the genetic characteristics of particular countries in, say, East Africa or East Asia. Even in more developed regions, distinguishing between Polish and, for instance, Russian heritage is inexact at best.
The precise numbers offered by some testing services raise eyebrows among genetics researchers. “It’s all privatized science, and the algorithms are not generally available for peer review,” Dr. Marks said.
“That’s why their ads always specify that this is for recreational purposes only: lawyer-speak for, ‘These results have no scientific standing.’”
For many, though, the point of DNA testing has nothing to do with ethnicity. Theresa Musumeci, 49, of Hockessin, Del., wanted to solve a longstanding mystery in her family. Who was her biological grandmother?