Developments in determining genetic ancestry have allowed people to determine their ethnic backgrounds, tracking down family through companies like Ancestry.com, 23andMe, and specialty companies like African Ancestry.

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Bob Hutchinson’s mother told him and his siblings almost nothing about her family, no matter how often they asked. “She was good at brushing people off,” said Hutchinson, 60.

When he was growing up, there were no photos of his mother as a child in the home, or of her own parents. She said that she was an only child, that her parents were dead. Her heritage, she claimed, was Italian and Swedish.

Hutchinson suspected that wasn’t the whole story. Then his sister-in-law, digging into the family past, found their mother’s childhood home listed in a 1930 census.

The family had lived in Montclair, New Jersey, and was described as “Negro.” Hutchinson, who runs an advertising agency and lives in Pacifica, California, had never been told he had African-American heritage.

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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. They include generalists like 23andMe and Ancestry.com and specialty companies like African Ancestry.

Millions of people have signed up with these firms, 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.

Hutchinson decided to have his DNA analyzed by 23andMe. The report revealed he is one-eighth sub-Saharan African, which means that his mother was of mixed race. There was some Italian and Swedish heritage.

Hutchinson also learned that his mother was not an only child, but had a brother. A genealogist helped him track down some first cousins in Alabama, who said they had been told never to contact Hutchinson or his family.

The cousins, Sandra Green and Eve Clark, were delighted to hear from him. He plans to visit next year at Mardi Gras.

Hutchinson’s results were enlightening, but in other contexts ethnicity has posed a 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,” 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 overall was 73.2 percent African, 0.8 percent Native American and 24 percent European. Experts say the large proportion of European DNA found in African Americans can be traced to before the Civil War, and the rape of enslaved African women.

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, Jewish populations were small and 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? Humans 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 Ashkenazi Jewish and zero percent Korean?” wondered Jonathan Marks, an anthropology professor at the University of North Carolina at Charlotte, in The Wirecutter.

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,” 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, Delaware, wanted the test to solve a long-standing mystery in her family. Who was her biological grandmother?

Decades ago, Musumeci’s mother had discovered she had been adopted after she overheard the nuns at her Catholic high school in Camden, New Jersey, talking about it.

For years, she searched for clues to her birth mother’s identity, eventually learning her name: Mary Culliton. But Musumeci’s mother died in 1995, at 55, without knowing anything more.

Musumeci decided to continue the quest by submitting a sample to Ancestry.com, which also will notify users of relatives in its database if they have given permission.

Among the matches were Musumeci’s half-brother and a few well-known cousins — but also a man she had never heard of. His great-aunt, it turned out, was her great-grandmother, Mary Culliton’s mother. He filled her in on the woman’s life.

Eventually Musumeci discovered other long-lost aunts and uncles. “I found five great family members,” she said. Yet there is one regret: Her mother did not survive long enough to learn the story of her own mother.

“I feel bad that the technology was not available for her,” Musumeci said.

If DNA testing has the potential to shed light on new family relationships, it can also muddy those that had seemed settled. While putting together The Wirecutter’s own report, one researcher learned that a grandparent was in fact not biologically related.

The news can be burdensome — or freeing.

Mark, a 43-year-old banker in Delaware, got his test results back from Ancestry.com along with a list of relatives in its database. Oddly, there was no one on the list from his father’s side of the family.

There was one name he recognized, though: his father’s best friend. Who, it turned out, actually was his biological father.

Mark, whose last name was withheld to protect his family’s privacy, is estranged from his mother, and the man he knew as his father died more than a decade ago. So he reached out to his father’s best friend, who confirmed an affair with his mother decades ago.

The two live near each other and have gotten together several times recently. The secret that the older man thought he would take to his grave is well out of the bag.

On one recent outing, Mark said, “He even showed me the place where I probably was conceived.”