Last year, I did what 12 million people from all over the world have done and surrendered my spit to a home DNA-testing company. I hoped a MyHeritage test would bring me the peace I needed; my Irish mother had never been able to give me any information about my biological father. Raised by her and my white dad, I’d always longed for a country to attribute my blackness to, or for help answering the ubiquitous “Where are you from?” question. I’d spent years making up exotic-sounding combinations to justify my appearance (some days Jamaican-Spanish-Swedish; other days half Brazilian, or half Iranian). But, at 24, I was done with occupying a box of black ambiguity. Could I finally get a clear answer?

The results arrived by email on a summer’s day last year. I clicked on the “ethnicity estimate” link, which offers an analysis of DNA by country, my heart pounding as I scanned the digital map.

The test showed that my blackness comes from Nigeria; 43% of my DNA, in fact. Then there’s 1% from Kenya, and the rest from Great Britain and Ireland (55%), as well as eastern Europe (1%). I’d often been told I looked east African, or mixed with multiple countries, so I was surprised by what was nearly a 50:50 split.

I began to think my grandmother had had an affair, even my mother. My imagination ran riot. I wouldn’t do it again

I had no cultural knowledge of Nigeria; should I now start claiming it as my own? Did the results mean my very distant ancestors were Nigerian, or that my biological father was probably from there? Why did my features not resemble a typical west African? I felt more confused than ever.

This wasn’t quite what the adverts had promised. Targeted marketing for home-testing kits shows smiling (often mixed-race) models under the banner “find out your ethnicity”, or urges people to book holidays based on their “DNA story”. It’s estimated the industry will be worth a staggering £7.7bn by 2022; in the last year alone, market leader AncestryDNA pulled in $1bn in revenue.

While DNA home tests are more popular than ever, people are starting to raise questions about what happens after the results land. Concerns about the storage of sensitive genetic information were highlighted recently, when an open-source DNA testing site, GEDmatch, was used by the police to identify California’s Golden State Killer. As well as privacy concerns, there’s the emotional fallout of receiving confusing or life-changing results. Identities that have been cherished by families for generations can be dismantled overnight.

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Ayshah Blackman, in her 50s, is of Caribbean descent and lives in London. She had always known two things about her family: that they had Indian heritage, and that her father had another daughter he wasn’t in touch with. Last year, with his permission, she set about trying to track down her half-sister through the UKTV show The Secrets In My Family.

Blackman was encouraged to take the AncestryDNA test as part of the programme, and thrilled to eventually connect with her long-lost sibling, living on the other side of London. But she was shocked by the details of the results; according to the test, Ayshah had no Indian DNA at all. “It made me question my ancestry, the fact that I might not be what I thought I was. I began to think that my grandmother had had an affair, that my mother had an affair. My imagination ran riot,” she tells me.

Blackman’s AncestryDNA test traced her roots to west Africa. “That wasn’t a surprise,” she says, recalling the mix of Benin, Togo and other parts of west Africa that made up 43% of her DNA. She was also 13% Scandinavian, and parts Native American and British. “But Indian wasn’t on my chart – I spent months agonising about it,” she says.

For people of African descent, whose individual and collective histories are blurred by the legacies of colonialism, slavery and rape, what they know about their identities is particularly important. Blackman felt that one of the narratives woven through her family had been broken.

“That little thing of not having any Indian ancestry is now sitting on my shoulder – I may not be as much a part of this tribe as I thought I was,” she says. “If I had to do it again, I wouldn’t.”

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YouTube is full of videos of people revealing their DNA results, often with “click me” headlines such as What Am I? and I Was Lied To – My Shocking Results. They film themselves “unboxing” test kits like a new toy and taking cheek swabs, and then cut to footage in which they analyse their results. Many seem astonished by what they find, and begin to question whether their parents have been unfaithful, or whether they have been misled about their heritage; some clips are heartbreakingly difficult to watch.

Shana Dennis, 34, decided to make a YouTube video after taking her test. She was born in India but adopted at six weeks by a family in Australia. Wanting to find out more about her racial mix, she took a test from AncestryDNA, which analysed her as mainly central Asian in origin (44%), with links to Afghanistan, Azerbaijan, Kazakhstan, Kyrgyzstan and Uzbekistan. Dennis uploaded a video going through the geographical breakdown of her results. When commenters suggested she try other DNA companies to verify the findings, she decided to do just that; like several others, AncestryDNA’s website allows you to access your “raw DNA file” and send it to different companies for analysis.

Each site has produced wildly different results. DNA.Land suggests Dennis’s biggest country match is China, with 29%. WeGene puts that figure at closer to 58%, while MyHeritage suggests that most of her DNA comes from Mongolia, a 21% match. “The results caused even more confusion,” Dennis says. “Most think I’m Nepalese. Others have argued I’m not.”

For those who are already discriminated against​​, having their genome used against them​ may have serious implications

Rachel Nye, 30, from London, was also left without a clear answer. Nye’s mother has a black mother and a white father, but Nye has never known exactly where her grandmother was born.

“My nana died in 2008 but was always very vague about where she came from. She often gave different answers,” Nye says. “Some days she was British-born, other days she was from Barbados, some days she was African. I remember seeing two passports – one of them was Kenyan – but the names and dates of birth were different.”

Nye’s 23andMe test analysed her as 76.9% European, offering a breakdown that included the UK, France and Scandinavia. Her black heritage was less detailed; she’s 21.9% sub-Saharan African – 13.9% west African, 5.1% east African and 0.4% “African hunter-gatherer”.

Nye says she was frustrated by the lack of country breakdown within Africa; despite the fact the vast majority of the world’s genetic variation comes from the continent, DNA testing companies often have very few samples from Africa.

23andMe has launched a number of initiatives to redress this. In 2016, the company launched the African Genetics Project, offering free DNA kits to people with all four grandparents born in the same African country, or from the same ethnic or tribal group. Now it has launched the Populations Collaborations Program, which encourages researchers studying remote populations to submit their data to the website.

But questions have been raised about the ethics of European and American scientists harvesting genetic information from Africans and African scientists for economic gain. 23andMe has announced plans to share the test results of five million customers with GlaxoSmithKline, the drugs giant, in order to facilitate the design of new drugs. (Users are asked if they want to participate in scientific research when they sign up.)

Privacy is a major concern for everyone using these sites, but perhaps more so for those from minority backgrounds. For those who are already discriminated against, having their genome used against them – for example, in the criminal justice system – could have serious implications. AncestryDNA’s privacy agreement states it can only share a customer’s DNA with research partners with explicit consent; but it could disclose personal information to law enforcement if requested. (An internal report revealed that in 2017, AncestryDNA received 34 law enforcement requests, and provided information to 31.) MyHeritage asks customers to email if they want their sample removed from its database, though a representative tells me that the company does not sell or share DNA data with third parties. “We would need the explicit permission of our users – we do not own anyone’s DNA”, I am told on the phone. In 2010, to illustrate the privacy risks, researchers from the Whitehead Institute for Biomedical Research in Massachusetts identified nearly 50 people who had participated in an anonymous genomic study, based on publicly accessible information.

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There are many scientific limitations to the home DNA test. “These companies aren’t actually testing your ancestry at all,” says Mark Thomas, professor of evolutionary genetics at University College London. “They’re problematic in their claims to be able to infer an individual’s ancestry.”

There are a few reasons for this. First, the genetic information these DNA testing companies hold is based on living populations. When you send your spit off in a little tube, it is specific snippets, or markers, in your genome (the total collection of DNA that resides in your cells) that are being analysed, and then compared to the markers of others who are good representatives for distinct regions or ethnicities around the world. But as Thomas notes, the companies are only looking at very recent samples, from a relatively small group, in one specific database. “They are just saying: ‘If I wanted to make your genome, I could pull bits of your DNA from people all over the world who are around today. And this is just one way I could do it,’” he says.

The databases are skewed towards different parts of the world, too. “23andMe has more American customers, and AncestryDNA has more British and Australian,” Thomas explains. “And none of these companies asks: ‘What do we know about the genetics of the past, and which of those past inferred genetic clusters do we get our ancestry from?’ They are giving us what the market wants, not what the genetics tells us.”

There’s also the question of just how much information is passed down through a person’s DNA. Thomas explains that we probably inherit very few genes from our ancestors; DNA is inherited in “chunks” that break up the further back in time you go. “You start with two parents, then four grandparents, then eight great-grandparents, it goes to 16, 32 and so on. And by the time you go 10 generations back, there are ancestors from whom you inherit no DNA.”

I ask Dr Yaniv Erlich, who works for MyHeritage, how the company’s “ethnicity estimate” is created. He says they define good DNA “representatives” for English people as having “at least all of their eight great-grandparents born in England”. He believes you can “estimate that present-day individuals probably reflect populations from about 200 to 300 years ago, as they never got DNA of any other ethnicity. Evolution is not acting fast enough to create any substantial changes.”

Let’s be ­honest, these companies are using ­ethnicity as a nice, ­polished ­euphemism for race

But as the American academic Sheldon Krimsky and journalist David Cay Johnston explain in their online consumer guide, Ancestry DNA Testing And Privacy, markers maketh the result. “Today’s markers do not necessarily match the markers of 400 years ago, during the African colonisation and enslavement period,” they write. In other words, markers are inconsistent; sometimes they’re passed on and sometimes they’re not. There might be a lot of genetic markers Nigerians share, for example, but that are not necessarily exclusive to them.

Thomas tells me it is very possible that my birth father could be from anywhere, but have parents or grandparents who are Nigerian, or are from a “combination of countries with broad genetic similarities to Nigerians”.

The bigger question is, how much should we connect geography and identity anyway? An individual’s “ethnicity” is largely based on their own perception of cultural and social traits, not which geopolitical borders they were born between. And there aren’t universal genetic traits within certain groups, Thomas points out.

“Let’s be honest, these companies are using ethnicity as a nice, polished euphemism for race, and they’re trying to define biological races using this genetic data. That in itself is shifty,” he says. “If genetics has taught us one thing over 30 years or so, it’s that there are no clearcut biological racial categories. Everyone in the world is racialised in some way. But rather than overturning these outdated notions of race, these companies are servicing them instead – presumably because they get better profits.”

When I ask the home-testing sites about linking the language of ethnicity to science, they offer varying responses. An AncestryDNA spokesperson told me that “analysing DNA to determine a person’s ethnic breakdown is at the cutting edge of science”, explaining that they have “thousands” of DNA samples from around the world. “Each is from a specific location and most are accompanied by a documented family tree indicating deep heritage in a particular region.” A 23andMe spokesperson tells me the company does “not refer to ethnicity” in its analysis, instead calling it an “ancestry composition”. Yaniv Erlich of MyHeritage says: “Ethnicity is not encoded in someone’s genes, but this human-made construct can be in correlation with genetic variations. We use this correlation to infer the ethnicity.”

After I got over the initial shock of my own test results, I began to explore the rest of the MyHeritage site. I was amazed to find that I could contact a fourth cousin on my biological father’s side – my first black relative – and that she lived in the same city as me. We plan to meet up, and my quest to find out more about my biology continues.

But I know that decoding my DNA is only one chapter of my history. Ancestry is a legacy, not a bloodline. Our genetic script may be one of the most valuable things we own, but it’s never the whole story.

• This article was amended on 11 August 2018 to correct Golden Gate Killer to Golden State Killer; and to include a missing “not” in the sentence: Why did my features not resemble a typical west African?