Kampala — When Nancy Smith's younger sister collapsed and died during a game of squash at the age of 26, it seemed like a terrible but isolated tragedy. That was what the doctors said, and for 12 years Smith and her family believed it.

Then, in 2003, Smith fell ill. She was admitted to hospital, where tests revealed that she had car-diomyopathy, a heart disorder which affects generations. "Getting that diagnosis was like the end of my world," said Smith. "I realised I could never go back to being the person I had been."

Smith, now 47, had always prided herself on keeping active but realised that her lifestyle, far from keeping her fit, had simply speeded up the beginnings of the disease, which had killed her sister.

"My biggest thought was 'if only we'd known,we could have done something to stop this happening to us,"she said.

Nancy's generation is, however, likely to be the last of her family to be left in such ignorance. In 1953, James Watson worked out the structure of DNA, an acid that contains the genetic instructions for the development and functioning of living organisms. Watson won a Nobel Prize for his discovery. Now, analysis of his genetic code could lead to medical breakthroughs.

In 2003, scientists published the first transcripts of the human genome. The human genome refers to the DNA found in body cells. Every bodily function, from memory to sperm or egg formation is controlled by DNA, so the genome holds the code for thousands of different proteins and other molecules needed by the body.

Recently, Watson's genome, containing a list of the genes that controlled his body, was published. The publication creates hope that, in the near future, people may know what genetic diseases they have and how best they should be treated. Researchers have used genome publication technology to identify some of the genes linked to cardiomyopathy, the disease that killed Nancy's sister, giving her and her family a deep insight into their medical destinies.

It was the kind of knowledge many people might shy away from, but not Nancy. "I saw my sister die, and if I could stop that happening to one of my children, then I would move heaven and earth," she said.

Her children were tested; two were okay, but her eldest son, Robert, was positive for the disease-causing gene. "It was terrible to have it confirmed but we all felt it was better to know what we were dealing with," said Smith.

However, the costs of publishing an individual's genome are still high. Watson's genome was completed in two months for about £500,000 (sh1.7b).

Jonathan Rothberg, of 454 Life Sciences, the company that analysed Watson's DNA said , "We did Watson's genome months ago, but since then the cost has gone down to £100,000(sh350m). I'm confident it will have fallen to £1,000 (sh3.5m) within six years."

Recently, two other companies announced plans for a £50 (sh175,000) test, that could sequence an entire human genome for "less than the price of a nice pair of jeans". This would open up the prospect of mapping everybody's genome, a process that scientists hope would lead to rapid progress in preventing or curing diseases.

Since 1983, about 1,400 disease-causing genes have been found. However, such conditions that arise from a single gene represent only about 5% of diseases. The other 95%, scientists have realised, are more complex: several genes are involved in causing disease and the lifestyle and environment of those afflicted also play a huge part. This connection means that scientists have trouble working out which genes affect which diseases. That is the problem that whole-genome scanning might help to resolve.

Professor Kay Davies, of the Medical Research Council said: "In complex conditions like schizophrenia, we have to sequence the genomes of thousands of sufferers to find out how they differ from those without the disease so as to locate the genes that are to blame."

Researchers at the Wellcome Trust Sanger Institute, in Cambridgeshire, are targeting cancer diagnosis. Don Powell, a Sanger spokesman said: "In the past we could only study a few genes but if we can study the whole genome of a cancer cell and compare it with normal ones we can really see what is happening and potentially find ways of stopping it."

Carlos Caldas, professor of cancer medicine at Cambridge University and a specialist in breast tumours, believes such research will eventually lead to personalised medicine.

"Soon we will be able to sequence every gene in a cancer cell. That could allow us to work out which therapies will work best for each patient."

Outwardly healthy people could also have their genome analysed and informed of hereditary risk factors and the precautions, such as exercise and diet, they could take.

However, researchers such as Caldas and Davies are concerned about the way breakthroughs in genomics are being exploited by the private sector and offered to the public not necessarily for medical benefit. There are now at least 20 firms offering various kinds of gene testing.

Some scientists also believe the information generated by the genetic testing offers nothing new. John Harris, professor of bioethics at the University of Manchester said: "The utility of genetic testing is being much-hyped. The main advice that comes from all these tests is that we should get lots of exercise and sleep, avoid being overweight and not drink too much and we all know that anyway."

Mari Baker of Navigenics believes such views are outdated. "We recently tested a woman and found she had a high risk of colon cancer," she said. "When she visited the doctor they found a growth and removed it. Our test may have saved her life."

Whether the public will use the results of gene testing to take precautions is not known. People often have unreasonable attitudes towards risk, as was shown by a 1980s study by the National Institutes of Health.

Smith found a similar reluctance to heed the warnings in her son Robert. "He knows he could have cardiomyopathy in his genes but he won't change," she said. "He still drinks alcohol and lives a student lifestyle."

Smith worries about him but does not regret the decision to undergo genetic testing. It gave us control," she said. "It's better to know the worst and make your decisions accordingly rather than just stay in ignorance, but there's no doubt some kinds of knowledge are hard to bear."