On a lazy Sunday morning in May last year, Isobel Lloyd was at her boyfriend’s house, having coffee with his mum. The conversation had worked around to Lloyd’s grandma – her mother’s mother – who’d died in her 50s, when Lloyd was very young. Lloyd’s only memories of her had been hospice visits where her grandma lay bedbound, unable to talk or swallow, with no control over how her body moved. Lloyd had forgotten the name of her grandma’s disease, hadn’t thought about it in years. Like most 20-year-olds, she was future-focused – a student from Yorkshire, keen on her studies, in love with her boyfriend of four years.

Sitting in his family kitchen, they began reeling off degenerative diseases. Motor neurone. Multiple sclerosis. Parkinson’s. Alzheimer’s. Then finally Huntington’s disease (HD). In a flash of recognition, Lloyd knew that was the one her grandma had. “It just clicked,” she says. “I Googled it on my phone – and that’s when I read that it was genetic. My mum had a 50% risk of getting it – and if she did, I had a 50% risk, too.”

She didn’t tell her boyfriend’s mother what she’d just learned, “But I felt the colour rush out of my face,” says Lloyd, an only child. “I thought, ‘No way, that can’t be true.’ I was 20 years old and no one had told me?”

In fact, that’s not so unusual. Secrecy, evasion and lies are frequent features for families grappling with genetic disease. Whether it’s HD, a breast cancer gene, inheritable bowel cancer, early-onset Alzheimer’s, it’s not uncommon for younger generations to stumble upon their inheritance by noticing patterns, asking questions. By then, they’re faced not just with their frightening at-risk status, but also anger at all those years in the dark.

People start saying they don’t want their children to worry

This subject forms the heart of a new novel by Alice Peterson, who has a record of creating popular fiction from hard-hitting subjects like disability or addiction. In If You Were Here, 26-year-old Flo is newly engaged and about to emigrate when her grandma tells her that she might carry the HD gene. The book weaves three voices – Flo’s, her grandma’s and diaries of Flo’s mother, who had died years earlier. Together, they explore the reasons for keeping secrets, the constant agonising over when and how to pass on devastating information to the people we love most – and the cost of putting it off.

Peterson came to the subject after meeting a friend of her mother’s who had tested positive for the HD gene. “She was in her 50s, very brave, so stoic but the key thing was when she said, ‘I haven’t yet told my son,’” says Peterson. “She was a single mother of an only child – he was in his 20s. She said that telling him was the hardest thing she’d ever had to do.”

It’s an issue made more pertinent by a lawsuit due to be heard this autumn. In the first of its kind, an anonymous claimant is suing St George’s NHS Trust for failing to inform her of her HD risk. Her father, a St George’s patient, had HD and refused to let doctors tell his family – his daughter was pregnant and he feared she’d have a termination. She went on to have the child and has since tested positive for HD – so her daughter has a 50% chance of inheriting it, too. Although “doctor-patient confidentiality” has until now been sacrosanct, the claimant is arguing that her father’s doctors had a duty of care to inform her, regardless of his wishes.

Charlotte Tomlinson, a genetic counsellor with Guy’s and St Thomas’, has many concerns about its implications. There’s the practical question of how to locate and inform family members (and where do you stop? First generation? Second?) More importantly, what would the effect be on families? “Our role is to help families adjust and come to terms with genetic conditions together, not divide them up,” she says.

Genetic counsellors generally see patients when they’ve learned about a family disease, or are considering being tested for the gene. “We always encourage them to share information with family, to bring them to the counselling sessions, or help them practise how to tell them,” says Tomlinson. “It’s fairly common for someone to start by saying they don’t want their children to know because they don’t want them to ‘worry’. Unfortunately, not telling them isn’t going to prevent it from happening to them. Having a genetic condition is difficult and if you haven’t told your family, they can’t support you and you can’t support them.”

There are many practical reasons that make disclosure important. With certain cancers, there’s preventive action available – for example a mastectomy if you carry the BRCA1 or BRCA2 mutation, or regular colonoscopies if you have inherited bowel conditions. With HD, recent drug breakthroughs have given much hope to gene carriers who are still symptomless; and having a baby through pre-implantation genetic diagnosis, a type of IVF, can ensure your own children don’t inherit it. None of these can happen if you don’t know you’re at risk.

Putting it off to give them a happy childhood, then college, then their first job, never ends

Tomlinson says it’s best to pass on the information as early as possible. “The younger children are, the more normalised it becomes,” she says. “It’s having a conversation when they’re doing something – drawing, going for a walk – and saying, age-appropriately, ‘You know that granny has that condition x and we all share genes. That means one day I might get it and one day, you might get it…’ Children are very resilient and if you tell an eight-year-old they might get something when they’re 30, that’s very far away.” Putting it off to give them a happy childhood, then to get them through college, then settled into their first job, never ends. “When is the ‘right time’? When they tell you they’re 12 weeks pregnant?” asks Tomlinson. “I’ve had those cases.”

Lloyd believes it was her “human right” to know. When she confronted her parents, they were evasive, and assured her she didn’t need to worry – but she was piecing the picture together. She’d noticed jerky involuntary movements in her mother for years and realised that her granddad had been dropping hints. When her mum had written off a car, he’d been devastated. (Her grandma had once written a car off, too.) He’d told Lloyd, “I can tell you what will happen next at every stage…” She’d asked what he’d meant. He’d changed the subject.

“I drove to his house and asked if Mum had the HD gene,” says Lloyd. “He said, ‘Yes and your uncle does, too. They were tested 18 years ago.’” Lloyd was at risk and so were her cousins. Her mum and uncle had made a pact to tell the three children together and forbidden anyone else from doing so. “I asked my granddad who else knew,” says Lloyd. “He said, ‘Everyone.’ I thought of my cousin’s wedding a few months ago. She was starting married life, full of hope. So many guests present on that day knew she carried a 50% risk of having HD – but my cousin had no idea.”

Lloyd doesn’t know how she got home that night – she was crying so hard, she couldn’t see the road. “I felt every emotion,” she says. “Frightened to death, shock that no one had told me, worried for my mum, for myself, and for my boyfriend. I felt like a burden.”

One year on, she has coped incredibly. Her discovery forced the disease into the open, her cousins were told and they contacted the Huntington’s Disease Association, visited their local advisor and attended HD events. They also began seeing a genetic counsellor and Lloyd’s newly married cousin was tested for the gene because she wants to start a family. Fantastically, she doesn’t have it.

Lloyd has also had the test and is awaiting the result. “Most people don’t want to be tested – but I feel that if I find out, I’ve got time to accept it and learn to live with it,” she says. “There’ve been big breakthroughs, treatments to delay symptoms and I’m only 21 so I’m not petrified. I’ve met people with it who are so inspiring – they were happy, they were coping. They’re the opposite of my family where there’s so much denial, you can still hardly mention it. That’s not how I want to be.

“When I first found out, I blew my top with my parents, we had a big argument and I told them how angry I was and hurt at not being told. We’re OK now – I love them to bits – and I know they were trying to protect me. But I’d like to have known years ago in a calm conversation, an open chat, so it never hit me the way it did. Anything’s better than finding out through Google.”

Some names have been changed. If You Were Here by Alice Peterson (Simon & Schuster, £7.99) is published on 22 August; buy it from guardianbookshop.com for £7.03