Trevor Lawrence tries not to fret, but often he can't help himself. "It's always in the back of your mind that it could happen to you," he says. "It's a continual worry because I wouldn't want to live like that. I wouldn't wish it on anyone."

Lawrence, 36, is talking about Alzheimer's disease. As a child he watched his father's mother die - bedridden and distressed - of the disease. Two years ago his father was diagnosed with Alzheimer's. He must now watch his father, who is 69, degenerate rapidly from the same illness. "I grew up with my grandmother being ill," he says, "and it was tragic. Now my father's going the same way. It was upsetting when it was my grandmother, but it's a lot more upsetting when it's your father. He has completely changed. He's nothing like the parent I grew up with."

Patrick Nethercot's mother, Joan Elliott, died in 1999, aged 81. She had been suffering from lung cancer and Alzheimer's. "Here was this woman who was not only my mother, but a woman with a powerful intellect," says Nethercot, 55, who lives in Durham. "She was a journalist and broadcaster who had done so much in her life and then she changed.

"My stepfather carried the burden. He looked after and cared for her, and I saw her once or twice a week. We were close and it was very distressing. Sometimes she'd ring me up and ask me to take her home, even though she was at home. But she meant the home she lived in as a child. At other times I would be with her and she would ask where I was."

Alzheimer's disease usually begins with lapses of memory and mood swings. Over time, a person often becomes increasingly confused until their personality seems to disintegrate.

The disease is physical and occurs when brain cells, along with the nerves and transmitters which carry instructions around the brain, are attacked. The cells die and the brain shrinks, making it more difficult to think, to remember things, and even to speak.

No one is quite sure why Alzheimer's happens, although the greatest risk factor is age (one in five 80-year-olds will be affected by the disease). However, it is known that some people are at risk because of their genetic make-up. That's what worries Lawrence.

Earlier this week, researchers at the University of Pittsburgh announced that they had identified a new genetic marker which indicates a markedly increased danger of contracting the disease. It was already known that a gene named APOE 4 was associated with increased risk, but now a small area of DNA on chromosome 10 seems to be linked to Alzheimer's as well. For people who have both of these genetic risks, the likelihood of Alzheimer's is increased 16-fold. Your chances of getting the disease, if you are a carrier, are higher than a smoker's risk of contracting cancer.

The researchers spent a decade studying 300 relatives of Alzheimer's patients and found that 18 of these people later developed the disease themselves. "People should understand that most Alzheimer's is not genetic," says Dr Richard Harvey, the director of research at the British Alzheimer's Society. "It's rare for there to be a genetic cause, but your own personal risk is in creased if you have these genetic factors.

"Up until now we only knew of APOE 4," says Harvey. "But finding something on chromosome 10 confirms a lot of suspicions. Now we need to determine exactly what it is."

Harvey is keen to stress that an increased risk factor does not mean that someone will definitely develop Alzheimer's. But with no test available for the genetic marker, relatives have no way of knowing if their risk is increased.

For Lawrence, the new research is both good and bad news. "I'm keen for progress to be made," he says. "But that doesn't mean I won't worry. Every time I get a headache, I'm nervous in case that's the start of it. I worry for myself, for my wife and for my two young daughters, although hopefully by the time they have grown up, they'll have come up with something." Trevor Lawrence is not, in fact, his real name. He is so worried about insurance companies discriminating against him because of his family history that he asked to remain anonymous.

"I can't think of a worse disease. Your body is fine, but your mind has gone. You end up a living cabbage. My father used to be a wonderful sportsman, and now all that has gone. He can't speak, he doesn't recognise us. He can just about feed himself. It's hard when you remember what he used to be like."

Nethercot says he tries not to worry about the risk of contracting the illness himself. "My father kept his faculties until he died at 89, so you have to put it into context," he says. "In any case, there isn't much you can do about it."

But he admits that he is concerned about those who might have to care for him.

"I worry about the problems the family would face. I saw what my stepfather went through, and the pain, anguish and distress, not to mention the sheer fatigue and desperation, that he suffered. I didn't know much about Alzheimer's before, but now I know how difficult it is to deal with. I wonder how my wife and family would cope."