One morning in 1987 Stephen Jones and his wife, Lesley, drove from their home in Osterley, west London, to Moorfields Eye Hospital in the City so their son Ben could have his eyes tested.

A nurse had picked up ‘some problem’ during a routine health check at Ben’s primary school. She was not sure exactly what it was, only that he needed to be seen by a specialist.

Stephen and Lesley were not overly worried because Ben’s eyesight was not noticeably bad. He had had some problems reading the blackboard, and occasionally tripped over his toys, but his parents put that down to clumsiness.

Otherwise Ben was a normal eight-year-old boy: he loved football, watching television and hanging out with his friends.

The consultant examined Ben’s eyes, then delivered the news.

He was consoling and deliberately used careful wording so as not to sensationalise the condition, but said that at some point in the future – maybe in two years or 20, it was impossible to predict exactly – Ben would start to lose his sight. Ben had retinitis pigmentosa, a group of inherited eye conditions that lead to incurable blindness.

‘It was a severe shock because it came out of the blue,’ Stephen recalls.

There was no history of blindness in their family.

‘I remember the assistant saying, with luck his eyesight will remain good throughout his education. So, in one visit you’re going from thinking, oh, he may have to wear glasses to being told he’ll probably lose his sight.’

Now they faced the dilemma of what to tell Ben. Although he had been sitting across the table from the consultant during the diagnosis, what had been said barely registered with him. (Ben says now he has no memory of that day; he says he was probably thinking about football – he was devoted to Everton.)

Should Stephen and Lesley tell their son what lay ahead and allow him to prepare for the future? Ben was obsessed with sport and his dream was to play competitive rugby or football. Or should they keep quiet about the timebomb in store? After all, how do you explain to a child that he has been sentenced to inevitable darkness?

The issue of disclosure, of what to reveal to whom and when, is increasingly being discussed as advances in genetic testing mean that people can be forewarned of their future. The success of the Human Genome Project, which mapped man’s entire genetic code, has made it possible to detect hereditary diseases that develop later in life. Huntington’s disease, for example, which is non-treatable and fatal, and which does not typically develop until middle age, can now be picked up in the womb and, in the case of IVF, even before with ‘pre-implantation genetic diagnosis’ – embryo screening; likewise, BRCA, the gene known to dramatically increase the risk of breast cancer.

Dr Ruth Newbury-Ecob, a consultant in clinical genetics at University Hospitals Bristol, points out that the dilemma facing Ben’s family ‘is where a child has a diagnosis and it’s a question of the age at which they’re made aware of that.’

What she sees in her clinic are parents with a strong family history of a genetic disease seeking a ‘predictive’ test: has their child inherited the disease or not? The technicalities of a genetic test are fairly simple, but its human implications are painful and complex and often lead to heart-rending decisions.

Individuals with a serious genetic disease in the family now have a choice. Do they live in hope? Or do they take the test, and then, if it is positive, spend the next 20 years knowing that death or incapacity approaches? How does knowing affect an individual’s decision about a job, a partner, having children? There is soul searching for the parents of children who test positive, without a doubt. But what of those who do the testing?

‘It can be difficult to stand back and observe a parent withholding information from their child, particularly in early teens when they are thinking about their future and you are aware they don’t know the full picture,’ Sophie Devery, a genetic counsellor at Moorfields, says. Most parents do tell their children, she adds, ‘but obviously we can’t get involved because that is the parents’ decision.’

‘It’s very difficult for parents to balance the difficulty of telling their child they have tested positive,’ Dr Newbury-Ecob acknowledges, ‘the distress that may cause, the anxiety for the child, and how it affects the parent/child relationship, against the long term, where if they choose not to discuss things and the child finds out through a third party, that can be very damaging for the relationship because of the breakdown of trust.’

Retinitis pigmentosa (RP) causes the slow disintegration of retinal cells: first the rods, which enable vision in low-light conditions (the first symptom of RP is typically night blindness); then the cones, which detect light and colour. As time goes on, many sufferers are left with narrow tunnel vision. There is a lot of individual variation in the disease. Some sufferers are blind by their early thirties; others can still have good eyesight into their forties and fifties. About 25,000 people in the UK suffer from RP (one in 3,000), and there is currently no cure.

There are three main hereditary patterns: ‘dominant’ requires only one abnormal gene from either parent; ‘recessive’ requires an abnormal gene from both parents; and X-linked, where only males are affected after inheriting one abnormal gene from their mother. Genetic testing for the disease was first developed in 2003, but still only about half of all RP cases can be genetically tested (the dominant and X-linked forms). And although it is inherited, half of all cases come as an abrupt shock.

‘Most of those are recessive but some are X-linked, where there hasn’t been a male born for some time with it,’ Anthony Moore, a professor of ophthalmology and an honorary consultant at Moorfields, explains. The other slightly unusual aspect of RP is that it is not always necessary to do a genetic test to get advance warning. It can be stumbled on during routine eye examinations and confirmed by an optician looking at the back of the eye (the tell-tale sign is changes in pigment).

Ben Jones is now 31 and in his second year training to be a vicar at Trinity College, Bristol. He is tall and slim with messy dark hair and an endearing boyish buoyancy. We meet in his light-filled flat in a residential suburb of the city, where he lives with his wife, Victoria, a stay-at-home mother, and their children, Evie, two and a half, and Caleb, four months. His father, Stephen, is visiting for the day (his parents now live in Newbury, where the family moved soon after Ben was diagnosed). Both father and son exude a wry humour, which suggests that this is their way of coping with the seriousness of the condition.

Ben was registered blind aged 17, and has used a white stick for the past five years. Yet he is not completely blind. He cannot see anything in his right eye, but has narrow tunnel vision in his left, as if seeing through a straw. Normal vision is 180 degrees; Ben’s is two degrees.

‘So from here, if I look at your right eye I can’t see your nose,’ he tells me. ‘In fact, if I look in your eye, I can’t see your eyebrow. So at any given moment I can only see a tiny area, and I make up the picture by not just looking at one place all the time.’

In good light, he can still read, work on a computer and watch television. It transpires that both of his parents were carriers of the RP gene (the recessive type), but did not know it. Consequently, Ben had a one in four chance of getting the disease. (His sister has been examined, and has no signs of RP on her retina, but she has a 50/50 chance of being a carrier.)

Stephen and Lesley decided to tell their son as soon as he was diagnosed. ‘I’m not very keen on secrets and holding things back from people,’ Stephen explains. ‘Obviously we didn’t spend our time saying to Ben, “You’re going to go blind.” You can protect your child by being entirely open, but not laying it on thick, and doing it as gently as possible. So when something arose like a forthcoming visit to Moorfields or if Ben trod on the cat, we would talk about it with him. The thing we did debate in the family was the extent to which we should tell his friends, and we all agreed from the outset that we should be very upfront about it, so when the need arose, people were aware.’ Stephen spoke about RP at Ben’s secondary school.

At first Ben’s only symptom was night blindness at the age of nine – in dim light, he struggled to find the door, for example. But by the age of 14 Ben had lost the sight in his right eye and the sight in his left eye was slowly closing in. At 17 he could no longer play in the school first team for rugby, basketball and football (he was in all three), and he was also noticeably stumbling into objects during the day. But he went on to study theology and computing at the University of Kent in Canterbury, and then trained as an RE teacher, which he did part time while also being a student worker at his church (where he met his wife).

‘The fact that I’ve always known [about the RP] has made it easier to bear,’ Ben says. ‘So at every stage I’ve known it’s going to get worse, but two years is the distant future when you’re 10. I can still see my kids and watch football on the telly, and at some point that is probably not going to be the case, but I don’t really think about that because I don’t know when that’s not going to be the case. I suspect that everything about this was much more disturbing for Mum and Dad than it was for me. And maybe that’s partly because I am relatively chilled out anyway, but also because I was told when I was eight and when you’re eight you just take stuff in your stride.’

Ben’s having RP also means he is a carrier and therefore has the potential, if Victoria is a carrier too (which they don’t know and don’t want to know at this stage), to have passed it on to their children. Does he want to get his children tested? ‘If one of them starts showing signs or even the vaguest problem, then I’d want to, but I’m not in a desperate rush – not because I don’t want to, but because it seems like an enormous amount of effort to go to for an off-chance.’

RP meant Ben abandoned his dream of being a professional footballer, and he regrets not being able to go to nightclubs – ‘not being able to see or hear is a very bad combination’. Walking without a white stick in an area he doesn’t know makes him feel vulnerable, but in familiar surroundings such as his home, which is unmodified, he moves freely – he puts a glass of water directly in my hand. When talking of the difficulties of RP, Ben is predisposed to focus on the comedy: bollards are, painfully, ‘always at knee height’; he walked up to his waist in a small lake at Kew Gardens; he fell down a manhole while on a school trip to the theatre. ‘I did the whole cartoon thing, legs twirling in the air, before plummeting.’

‘Ben has taken RP in his stride all his life,’ Stephen says, ‘even when he was going through his “Kevin” years’ – Ben’s teenage rebellion included riding motorbikes and quad bikes around a friend’s farm and subsequently doing badly in his A-levels – ‘but in terms of RP I’ve never heard him say, “It’s not fair” or “Why me?” or “This shouldn’t have happened.” He has just got on with life.’

Ben says his decision to become a vicar wasn’t dictated by his sight, but by the fact that he had always wanted to be a vicar. ‘In fact there will be issues – I won’t be able to be in a rural parish, for example, because I can’t drive and a lot of churches are quite dingy.’ Likewise his decision to marry young (he was 25 when he met Victoria) and have children was simply down to ‘finding the person I knew I wanted to spend the rest of my life with’.

Has knowing the future made Ben want to see as much as he can while he can?

‘I think so, but it’s hard to say what is me and what is the eyesight problem,’ he says. ‘I’ve always been someone who wanted to experience life.’ He aims to continue bungee jumping and white-water rafting no matter what. ‘But there has been some stuff I wanted to do before going blind. I will be forever grateful that whatever happens, I got to see my children.’

Not long after meeting Ben I went to see Tom Walker, a pharmacist, 39, in Newcastle. He is married to Elizabeth, a graphic designer, and they have three children, Charlie, 10, Hannah, eight, and Emily, four. Hannah was diagnosed with RP when she was three and a half. Both Tom and his wife are carriers of the recessive form of RP, and so have a one in four chance of having a child with RP.

‘I noticed Hannah didn’t see particularly well in the dark,’ Tom says. When he read her a bedtime story and the light was low, she couldn’t see the pictures on the page. If her teddy bear dropped off her bed, she couldn’t find it. Their GP referred them to a consultant at the eye hospital who diagnosed fast-acting RP, where the degeneration is rapid. ‘They told us she would be severely visually impaired by school age.’ Tom and Elizabeth were devastated. Hannah had just started nursery.

The vexing question was what to tell Hannah. ‘You carry the responsibility for a child at that age,’ Tom says. ‘She wasn’t old enough to understand.’ So they did not discuss it with her. And they still have not discussed it with her, partly because Hannah’s predicted deterioration has yet to happen. ‘She is long-sighted but no more or less than any other child in her school who wears glasses,’ Tom says. Hannah is the sort who loves to put on shinpads and kick footballs. ‘It’s only a matter of time, we know that,’ he says.

So will they discuss it with her? ‘What’s the trigger?’ Tom says. ‘Is it four and a half, five and a half, six? When would you consider a child to be old enough to cope with knowing that she will go blind?’ Tom’s eyes fill with tears. He is heartbroken and confused. ‘It’s hard for us to accept and that’s why I’d like to tell her on the day they find a cure, because that is easier to deal with.’

There are two treatments currently in clinical trials in Britain: gene therapy, where good copies of the gene that is faulty are injected into the retina; and artificial retinas or the ‘bionic eye’, where an electronic chip sits on the retina stimulating light-sensitive cells. Neither offers a complete cure and neither is imminently available on the market, but both are the focus of widespread hope.

Tom says that he and his wife rehearse telling Hannah in their minds. But not telling her has advantages. ‘The innocence of childhood,’ he says, ‘Protecting your child. Would you want to know that you were about to step out of that door and get killed by a bus? Most people wouldn’t want to know the future, you just want to live life to the full. That is what I want to do with my daughter. I want her to live life to the full.’

He adds that both sets of grandparents agree that not telling Hannah yet is the right thing to do. As do friends and other members of the family. The only person who disagrees is the consultant who diagnosed Hannah’s condition. ‘He’s advised what he would do in my situation – tell her – but respects my decision not to,’ Tom says.

Most parents explain in a simple way once their child is at school, Prof Moore says, but ‘there is a minority who do not like to discuss it. There are some families where even the grandparents do not want to talk about it. They feel very guilty.’

But not telling opens up other problems. Hannah has to be removed from the consulting-room during discussions at her yearly check-ups and has started to ask why ‘Daddy has gone back to talk with the man’. And the fact that they have confided in close family, friends and teachers (so Hannah can sit at the front of the class to see the blackboard) means that there is the potential for information to leak out.

‘It’s a bit of a tightrope we walk because we don’t want Hannah to find out in the playground from another child whose parents have discussed it with someone else,’ Tom says. And not telling requires ingenuity. Hannah has started to ask questions about why she cannot see as well as her friends at night. So far her parents have answered by saying everyone is different – ‘kind of skirting around the issue’.

‘It would be very easy to bury your head completely and that would not be the right thing,’ Tom says, ‘because when we do talk to Hannah about her eye condition she is going to want to know what we did. Did we try to look for a cure? Did we try to raise money to help with research? How far did we go on her behalf knowing what we knew? So we do sponsored events and charity things and I’m involved in the RP society for that reason.’ Tom swallows hard. ‘Just to explain to her when she’s older that we tried to do things… tried to help her, even though she didn’t know.’

RP Fighting Blindness (the British Retinitis Pigmentosa Society): 0845-123 2354; brps.org.uk. The Walker family’s names have been changed