Tom Barton wasn't ill, but he had his thyroid gland removed. A gene test meant doctors were able to predict that he would get the same cancer as his mother - so he opted for surgery

It's a Tuesday evening at the beginning of September, and tomorrow I'm due to go into Addenbrooke's Hospital in Cambridge for an operation to remove my thyroid gland. I know precisely what to expect because six weeks ago my brother, Owen, had exactly the same operation. Curiously, neither Owen nor I are at all ill.

My doctors say that if I don't have this operation, I will almost certainly get a very rare form of cancer called medullary thyroid cancer. The chances of me developing this disease are around 95 per cent. It would be 100 per cent, except that I may be run over, or fall out of a tree, before I'm diagnosed.

They know this because I have a mutation in one of my genes, which causes a syndrome called Multiple Endocrine Neoplasia 2A (MEN2A). I was tested after my mother developed thyroid cancer three years ago. There was a 50 per cent chance of each of her children carrying the gene defect. After tests, both my brother and I, her only children, were found to be carriers.

The upshot of this is threefold: unless my thyroid gland is removed I will develop thyroid cancer; I stand a high chance of developing serious problems with my adrenal gland; and I'm likely to experience a growth in my parathyroid glands, which control the body's calcium levels.

Genetic screening for MEN2A has only been available since 1994, and, in many ways, my operation is an early indicator of an increasing revolution in health care. Instead of waiting for me to become ill, doctors are taking pre-emptive action based on the results of a genetic test.

The man to blame for the fact that I'm having this operation is Professor Bruce Ponder of the Cancer Research Institute at Cambridge University. He led the group which discovered the defect, in a gene called the RET gene on chromosome 10, which causes MEN2A. He tells me he became interested in inherited cancers after working at a thyroid clinic at the Royal Marsden Hospital in Surrey in 1981.

"The next two case notes in the pile were both for families who had inherited forms of thyroid cancer. All the family members were being screened for the disease, even though only half of them stood any chance of getting ill," he says. "I realised that, by collecting samples of families with a history of this cancer, it may be possible to find the gene defect which causes the illness."

Over the next few years, assisted by his wife, a nurse, Professor Ponder collected blood samples and data from over 150 people in 40 different families. He then used a technique called "linkage analysis" to compare DNA sequences within individual families, looking for particular sequences which were shared only by those who had had the cancer. This helped him to roughly map the problem to chromosome 10. But that was just the start of six years' hard graft.

"Chromosome 10 was an unmapped chromosome," he explains. "The search was something like looking at a map of Australia in the 1600s."

It's now the day after my operation, and I'm reading the get well card my brother sent me. He says he's looking forward to using our matching scars to explain our "head-swap" story. I now have that scar, running three inches across my throat. And, importantly, I no longer have a thyroid gland. I'm in less pain than I'd expected, but moving my head is extremely uncomfortable.

It turns out that, just before I went into theatre yesterday, Owen had the result of the biopsy of the gland they removed from his neck six weeks ago. It shows that, even though he is only 28 years old, there were signs of cancer in his thyroid and surrounding lymph nodes. On the one hand, this is great news: without the gene test and surgery he wouldn't have discovered this tumour until it was causing him problems. He would have needed highly invasive surgery and crippling radiotherapy, similar to that which my mother went through three years ago. In effect, our mum's dodgy gene has saved his life.

The downside, though, is that if cancerous cells have passed into his lymph system, he could end up needing further treatment. Moreover, it doesn't bode well for me. Owen is just two-and-a-half years older than I am. If he has already developed a tumour, it's possible I will have, too. I won't get my path-lab results for another few weeks, but there is a chance that they will carry bad news.

Around one in every 30,000 people is thought to have the RET gene mutation which causes MEN2A. A large number of those people are likely to be related. In my immediate family there are three of us out of a possible three. By my estimation, we are one of less than 500 British families to have the syndrome.

In a way, we are all lucky. Not only is there a clear-cut genetic test for the syndrome, but there is a relatively simple course of action to deal with the potential problem: remove the thyroid. This is in stark contrast to many other conditions that can be linked to genetics.

Take breast cancer, for example. Around 5 to 10 per cent of breast cancer cases are thought to be familial, caused by inherited factors. Of these, however, only around 20 per cent can be traced to one particular gene. For the other 80 per cent, doctors are left to assess what Professor Ponder calls a genetic "hand of cards".

It's a much less exact science. A number of genes, working together, may or may not increase a particular woman's likelihood of developing the disease. Moreover, for any woman who is told she's at risk of developing breast cancer, the preventative option - a mastectomy - is much more complex.

Where, then, does the future of genetic diagnosis lie? In the film Gattaca, when Vincent (Ethan Hawke) is born, his parents are given a genetic reading. It shows the likelihood of him developing a clutch of diseases. Because he has a 99 per cent chance of carrying a heart defect, his life expectancy is estimated at 30.2 years. It seems possible that what I've just experienced could be a precursor to just such a reading. Professor Ponder isn't so sure.

"At the moment, I don't think telling people that they stand an increased chance of getting an untreatable and unpreventable disease would have any constructive results," he says.

Already, however, there are internet sites which will give you a genetic profile based on DNA fingerprinting techniques. The validity of their results is questionable. But there is clearly a growing market for this sort of information.

Professor Ponder accepts that assessing patients' genetic profiles may be used by the health service in the future. "It is likely that, for some common conditions, we will find that there are some people who can be better treated with certain medicines, and the health service could save money by matching treatments with individuals better. I suppose, if you could determine a genetic predisposition to conditions like diabetes, then you could advise people better on leading lifestyles which can help them avoid it."

As I look to the future, I know that genetic testing is going to play a role in the rest of my life. I will be screened for conditions related to MEN2A. And, at some point, I will be posed with a difficult question. I know I have a genetic condition, and I know that when I have children each will stand a 50 per cent chance of inheriting that same defect. If I decide to become a father, I will be facing issues that, just over 10 years ago, I couldn't have known about. Then, as Professor Ponder says, I will have to become much more sophisticated about how I evaluate those probabilities.

Thyroid cancer: the facts

* There are 1,500 cases of thyroid cancer a year. In most cases the cause is unknown. The first sign is a nodule in the neck, which may be painless.

* One in every 30,000 people has the genetic defect MEN2A which causes thyroid cancer. The defect is responsible for 2 per cent of cases of the cancer.

* Thyroid cancer caused by MEN2A develops in people around the age of 25. It can also lead to growths in the parathyroid and adrenal glands.

* Once diagnosed, people with thyroid cancer have an operation to remove the gland - thyroidectomy - followed by treatment to kill any remaining cancer cells.

* After surgery, patients need to take thyroxine therapy, to replace the hormone made by the gland, for the rest of their lives.