FOR Carole Kushnir, a test that she took out of simple curiosity about her genetic make-up revealed a double risk of cancer. And for Karen Durrett, it led to her meeting a father she had never known - and brought to light a potentially fatal condition.

The women are among 180,000 people who have paid to have their DNA analysed by 23andMe, the world's biggest private genomics company, to assess more than 200 genetic traits and health risks.

That number of customers is expected to soar after the Californian company - which takes its name from the 23 pairs of chromosomes in a normal human cell - cut the cost of a testing kit to $US99 ($94) last month.

The company, co-founded by Anne Wojcicki, the wife of the Google entrepreneur Sergey Brin, plans to reach 1 million clients by the end of this year.

But the boom in business and interest in genomes and genetics has led to controversy about the commercial use of one the most crucial medical breakthroughs of recent years.

For proponents such as Ms Wojcicki, whose husband has a genetic mutation that significantly increases the risk of Parkinson's disease, this is the dawn of a new era.

For the price of a night out, individuals can learn key elements of their genetic composition and take treatment, or protect their children from hereditary health risks.

Critics, including doctors, bioethicists and geneticists, are concerned people will be overloaded with information that is difficult for them to interpret, and that the test will raise unnecessary health fears or false reassurances, lead to unneeded procedures, or cause stress about conditions that cannot be cured.

''It is unregulated. Much of the information people get is misleading or unreliable and genetic tests are a poor predictor of the big killer diseases,'' said Helen Wallace, of GeneWatch UK, a genetic watchdog group. ''There is also the danger of people receiving scary surprises, without the presence of a doctor, for which they are not prepared.''

Mrs Kushnir, 69, from Silicon Valley, is scornful of such concerns. ''It's patronising in the extreme, particularly in the era of the internet, when people go to their computer to check the symptoms if they just get a sore throat, to argue that only experts can handle this sort of information,'' she said.

To take a test, a client spits into a test tube or swabs the inside of their cheek, then sends the sample for analysis. There

was no family medical history to alarm Mrs Kushnir but the test identified a mutation in the BRCA2 gene, which puts her at risk of breast and ovarian cancer.

''I was obviously not happy to discover that I had the mutation but relieved to have the information so that I could do something about it,'' she said.

After tests, a specialist advised that the threat of breast cancer could be monitored by regular examinations. Because ovarian cancer is difficult to detect early, he recommended she have her ovaries removed - which she did.

She encouraged other family members to take tests. Several, including one of her sons, discovered they had a similar genetic mutation that heightened their cancer risk.

But recent focus group findings and professional surveys illustrate the diverging opinions about how to use these new weapons.

Almost all the parents surveyed said they wanted to be informed of every risk of disease for their children, even if the condition was untreatable. Most doctors, geneticists and bioethicists believe that only information that could lead to action should be shared.

In an effort to provide guidelines the American College of Medical Genetics and Genomics is drawing up a list of major conditions to search for.

Mrs Durrett, 53, from Georgia, took the test to determine the cause of some minor health problems but it illustrated another side effect: the exposure of secrets that can tear families apart.

She learnt she was at risk of breast cancer but also that the man she had called ''dad'' for 50 years was not her father. She tracked down her biological father and a half-sister, who had breast cancer. A biopsy found Mrs Durrett had cancer.

Telegraph, London