The first surprise of Michèle’s DNA test was how much saliva it took. “It was daunting. It took us, like, 20 minutes. And the more we laughed, the harder it was to do.”

It was May 2016, and she and her now husband had ordered the kits as very early Christmas presents for themselves. They had been researching their family trees – Michèle had traced her father’s family back as far as the 1600s – and had wanted to test their DNA but had been put off by the cost. They were saving for a fertility treatment and had only recently moved to Florida from New York.

But Michèle was considering going back to study, and she had been told she had some Native American blood on her father’s side – maybe if she could say how much, she figured, she would be eligible for scholarships. And so she and her husband came to be cracking up, spitting into their individual vials. “We joked about sending them sweetener samples, to see if it still works.”

In bed one night six weeks later, they pulled up the results from AncestryDNA on the laptop. They were impressed by the news about Michèle’s husband, which mostly reflected what he had always been told about his family. Then they opened Michèle’s results. “The first thing that popped up is a pie chart, and one whole half was red and said ‘Italy’.

Facebook Twitter Pinterest Debbie Kennett, a genealogist at UCL. Photograph: Donald Michael Chambers/The Guardian

“I was dumbfounded. Like: ‘Well, that’s a huge mistake. They must have mixed mine up with someone else’s.’” She laughs, but you can hear it is hard-won. “My husband looks through it, then he says: ‘Honey, they didn’t make a mistake. This is you.’” That night, Michèle had her first ever anxiety attack.

Recently DNA testing – once only accessible to doctors and detectives – has been extended to anyone curious about where they came from and willing to spend the best part of £100 to find out. Since launching in May 2012, AncestryDNA says it has tested more than 10 million people in 30 countries. 23andMe says it has more than 5 million subscribers; FamilyTreeDNA claims 2 million.

All of them are racing to grow their databases and their accuracy. AncestryDNA is especially visible, sponsoring the television programme Long Lost Family and running expensive adverts inviting people to discover their inner Viking (“This sword is your history”) or their innate links to the EU post-Brexit.

It all sounds harmless, until you learn that as well as revealing customers’ ethnic origins, many DNA tests will also identify relatives in the companies’ databases. This “DNA-matching” can throw up previously unknown or unacknowledged brothers and sisters, cousins, uncles and aunts … or even reveal that the man you call dad is not your biological father. This is one reason that the UK’s Human Fertility and Embryology Authority is now calling on DNA testing services to warn customers about the possible emotional fallout.

What I learned from home DNA testing Read more

The kits that screen for genetic health risks can deliver equally devastating results. Yet all of these tests are typically taken in a spirit of casual curiosity. People are not prepared to have their lives rocked by what they may find out.

As her results sank in, Michèle says: “I just got very quiet. All these things from my past suddenly started going through my head: questions, feelings, things that couldn’t be explained, things that my mother would get angry or defensive about if I brought them up. I started to realise: ‘I think I’ve discovered a secret.’”

That night, she called her estranged mother, breaking their two-year silence. She strenuously denied any knowledge of Italian ancestry – and still Michèle thought there might have been a mistake. AncestryDNA’s database showed her as having first cousins in Syracuse, NY, where she had grown up, with an Italian surname she did not recognise.

The next morning Michèle called her aunt, who had only been 10 years old when Michèle’s mother had got pregnant at 18. Her DNA matches open on her laptop in front of her, Michèle asked her aunt if she remembered her mother dating an Italian boy in senior year. “I’m looking at the last name: 100% match, first cousin. And my aunt says: ‘The only guy I remember who comes to mind is her prom date.’ And she says that same surname.

“I stood up, my laptop went to the floor, I dropped my phone and I ran to the bathroom and started vomiting.”

In a series of text messages, Michèle’s mother furiously denied that her prom date was Michèle’s father, adding (Michèle says) that “it wouldn’t matter anyway”, since he had died in a motorcycle accident the previous year. “That’s how I found that out. It was very cruel.” Michèle found his obituary online, accompanied by a photo of a man with dark hair and olive skin. “It was literally the male version of me.”

A subsequent paternity test of the man she had thought was her father confirmed that there was no relation. “That was devastating for both of us,” says Michèle. He had not figured in her life when she was growing up, and they had only recently reconnected. She had moved to Florida in part to be closer to him.

Facebook Twitter Pinterest Kathy Piercy. Photograph: Kathy Piercy

Despite the absence of blood ties, he remains very much in Michèle’s life; paradoxically, she says, the discovery made their relationship stronger. But she has severed all ties with her mother who, three years on, continues to deny the results of three DNA tests.

Michèle’s story may sound dramatic, but it is not unique. Increasingly, DNA tests are bringing to light infidelities, adoptions, cover-ups and lies that have been concealed for decades.

There have been cases of people learning that they were conceived from donated sperm or even that they were switched at birth, says genealogist Debbie Kennett. “There have been a lot of secrets covered up in the past, and they are starting to come out.”

Last year, AncestryDNA made matches opt-in to comply with data retention legislation; keeping a “can of worms” shut may have been an added bonus, Kennett suggests. The company says that while almost every customer encounters surprises on their “self-discovery journey”, these are mostly “exciting and enriching”; for those with “more sensitive queries”, there is a dedicated team of experienced staff. Likewise, 23AndMe says it had specially trained customer-care representatives.

“When people get these unexpected findings, they tend to distrust the science at first,” says Kennett. “But even close matches can only reveal so much in isolation. The DNA on its own doesn’t give the science – you need the contextual family information as well.”

When, in June, Kathy Piercy was contacted by a woman claiming to be her first cousin, she was initially sceptical. “I thought: ‘Yeah, right – I know all my cousins.’ But there’s no doubting DNA.” She had joined AncestryDNA four months earlier to find out more about her ancestors’ journey from Ireland to New Zealand, where she lives “on a dusty road out the back of nowhere” in the rural Canterbury region. “In New Zealand and Australia, we’ve got different bits and pieces in us, so it’s more relevant. I wasn’t looking for anything in particular, because I didn’t know there was anything to find.”

But Judy Poole did. Raised by adoptive parents, as an adult she forged a tenuous relationship with her biological mother, who refused to discuss her father’s identity. She had sent her sample to AncestryDNA in April hoping that it might throw up clues, but was not at all expectant. Kathy was listed as her first cousin. After some back and forth and “a bit of maths”, says Kathy, they pieced together that her father was Kathy’s uncle, who at 19 had had a fling before his marriage. He died in 2012. “Whether he knew of Judy’s existence, we will never know,” says Kathy. But she was able to put Judy in touch with his son, her half-brother – “and he was over the moon to have a big sister”, she says.

Judy was taken aback by their instant connection – not to mention the physical resemblance. “I really do look like Kathy. I’ve got my father’s eyes.” She did not have an especially happy childhood, she says, so to have gained a new family has been “really special”. “If anything, you start losing relationships in later life – but I think what I really got out of spending time together was that I felt valued.” She adds: “That’s probably something that hasn’t been in my life before.”

Laura House, a genetic genealogist studying at the University of Strathclyde, had a more complicated encounter with DNA testing. In researching her own family, House learned her grandmother was illegitimate, the result of an extramarital affair that her great-grandmother had kept secret all her life. For her mother and aunt, “to learn something so significant about their mother, so long after she died, was quite moving”, she says. “There was a lot of pathos because my grandmother never knew the truth about who she really was, and [the man she thought was her father] was very important to everyone. His name was a part of our identity.”

The finding has caused tension within House’s family, and some members continue to doubt it. That is the risk with DNA tests, she says: not everyone is ready to learn that what they believe to be true, is not – especially secondhand. “You don’t have to have tested yourself to discover that your father is not your father.” And as more DNA is added to the database, you could be in store for more surprises.

Daily activity in an AncestryDNA community on Facebook highlights how easily they are uncovered. “Why does my sister show as a close family member or first cousin?” someone posted recently; “Half sisters,” was the reply, “sorry if u didn’t know.” Another user posted about finding her father in less than 24 hours of getting her results: “I’m the happiest girl in the world right now … He didn’t even know I existed.”

Facebook Twitter Pinterest Laura House at home in Gloucestershire with a photograph of her Spanish ancestors. Photograph: Adrian Sherratt/The Guardian

House says there needs to be more advice about how to approach those difficult conversations – it could even be included with the test kit. “People are being flung into these situations that are ethically extremely complicated and perhaps stressful, and they need to be able to manage it.

“Anything they find out will have implications for their family. The advert that says ‘I spat in a tube and discovered I was a Viking’ does not give any sense that that is the situation you may find yourself in.”

With tests that offer health screening, even surprises you have signed up for can have life-altering consequences. For example, 23andMe screens for genetic risk of diseases including Parkinson’s and late-onset Alzheimer’s, as well as whether you are a carrier for cystic fibrosis, among other conditions.

Next month, Sara Altschule, a 31-year-old writer based in Los Angeles, will undergo a preventative double mastectomy after 23andMe revealed that she had a roughly 70% chance of developing breast cancer. She had been given the test by her sister, although as a self-described “light hypochondriac”, she had paid for the health screening upgrade herself. Her greatest fear was that the results would show a risk of developing Alzheimer’s. “I was so excited that I only had one variant, I think for coeliac disease.”

Seven months later, in March this year, Altschule received an email from 23andMe, saying it had just been approved to test for three BRCA gene mutations linked to an increased hereditary risk of breast cancer. She opted in: “I believe that knowledge is power.” The results returned positive for the same BRCA 2 variant carried by her father’s cousin, who had gone on to develop cancer. The next working day she took a printout to a genetic counsellor.

To get that email, she says, “is really scary and overwhelming – it’s different from a person walking you through it”. But it was also accessible. Being Ashkenazi Jewish, her risk of having such a mutation was about one in 40, which she did not know before she was diagnosed. “At first it doesn’t feel as if it’s good news, but I look at it this way: now I can take charge of my health.”

She credits 23andMe’s testing with saving her life, although, she adds, it is hardly exhaustive, even for the conditions it does assess. “In reality, I probably have so many chances of developing so many things that the test doesn’t test for.” (A spokesperson for 23andMe stressed that it was not a diagnostic test, and that the many steps involved meant “the customer should be fully informed of what all the possible outcomes of the report might be, before they have even sent off their kit for testing”.)

Asked if she would have agreed to be tested for a condition that she was powerless to prevent or reduce her chances of developing, Altschule isn’t sure. “If there’s nothing you can do about it, I don’t think it would be helpful.” But she seems to reconsider mid-answer. “It would definitely be hard – but if you have that information, I think any person just wants to know. But you have to be ready for the answer.”

In August 2016, not three months after she had sent off her saliva sample, Michèle flew to New York to visit 80 family members she had never met. Michèle’s grandmother was delighted to meet her, her only granddaughter – and even more so to learn that she was pregnant. Her uncle took her to see her father’s grave.

She remembers her four-day visit as overwhelming, exhausting and surreal. “It took me at least a couple of weeks to recover,” she says. “But they welcomed me with open arms.”

Despite all that she has gained as a result – self-knowledge, a family – Michèle remains ambivalent about her “traumatic” discovery. “There have been times when I’m just kind of breaking down, and so angry and sad, that I say I wish I had never opened Pandora’s box,” she says, tearfully. Therapy, writing and, in particular, the birth of her daughter have helped her to move on.

A few weeks ago, she felt able to return to AncestryDNA, for the first time in two-and-a-half years. On logging in, she was told her ethnicity results had been updated. She was gripped by a sudden fear: what if it really had been all a mistake?

But, she says with a snort: “I was even more Italian than I’d thought I was.”