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Like most 17-year-olds, Megan Smith has no idea what the future holds. But unlike her peers, she has more than just A-level exams and university choices on her mind.

Her father, Gary, has Huntington's disease - a neurodegenerative disorder - which is hereditary and has no cure.

Megan and her brother Ben have a 50% chance of inheriting the faulty gene which will one day trigger the disabling disease - but they have chosen not to find out.

"We both want to live our lives to the full. If we get it then we will - if we don't then it will be a lovely surprise," Megan says.

She knows that a simple blood test, which is available to those aged over 18, would confirm the presence of an expanded gene which causes Huntington's disease.

However, she has already decided not to live in the shadow of the illness.

Huntington's disease affects the cells in the brain, stopping the body and brain from working as well as they used to. It leads to problems with movement, thinking and behaviour and in most cases leads to a long, slow decline over a period of 15 to 20 years.

Big decision

It can start with twitchy movements and memory loss, lead to a gradual decline in the ability to manage day-to-day activities and often leave people needing care for all aspects of daily life.

Most people with the gene develop the disease in their early 40s. The Huntington's Disease Association estimates there are between 6,500 and 8,000 people in the UK with it.

Dr Elizabeth Howard, a specialist in genetic medicine at St Mary's Hospital, Manchester, says Megan's decision not to have the predictive test is not unusual.

"I would say around one in five come forward and choose to know. Most people carry on regardless."

Dr Howard runs weekly clinics for individuals affected by Huntington's disease and their at-risk relatives, and she says most want to talk about the genetic test.

Her job is to prepare them for all eventualities.

"We try to make sure people understand the implications of getting bad news. It's better to do that before the test, because it can have devastating consequences."

Genetic testing has also led to fears that those with a family history of the condition will have financial problems, such as getting insurance and mortgages.

Having children is often a big dilemma for Huntington's families too, knowing that they could pass on the faulty gene. Yet there is also the possibility of many years of normal life before offspring develop the disease - or get it at all.

A test called a pre-implantation genetic diagnosis is available to couples who have received a positive pre-symptomatic Huntington's disease result and want the chance of pregnancy that is unaffected by the disease.

The embryos are tested before they are implanted in the woman's womb.

But Dr Howard's experience is that few people take this route either, perhaps because they are unaware of their family history or because they are just prepared to take the chance.

Megan's family knew very little about the illness before Gary was diagnosed 10 years ago. Although Megan's grandfather also had Huntington's, they had no idea it could be passed down through the family.

'So guilty'

Megan's mother Carol wonders constantly what they would have done had they known.

"We might seriously have thought about whether to have children or not. We wouldn't have wanted to intentionally put them in danger.

"But they may not get it or pass it on."

Carol and Gary cling to the hope that their children will not have to suffer the same fate as their father and grandfather.

"I know he [Gary] feels so guilty that he could have passed it on to them. We hope every day they don't have it."

They told the children about Huntington's at the dinner table one evening when Gary had started to become unwell.

There were lots of tears and unanswered questions - but they do not regret being open about it.

Carol says: "Megan and Ben are so brave about it and they are such a great support to us. They're really good."

Watching her father's health deteriorate, seeing him change before her eyes, has been very difficult but Megan finds that a good way of coping is to raise funds and awareness about the disease, and to be bright and optimistic about the future.

Her father would expect nothing less.

Megan's family are helping to raise awareness for , which raises money for Genetic Disorders UK - the UK charity raising money for children with genetic disorders.