It happened one morning 20 years ago. A British scientist Alec Jeffrey came across DNA fingerprinting: He identified the patterns of genetic material that are unique to almost every individual. His discovery changed everything from the way we do criminal investigations to the way we decide family law. But the professor of genetics at the University of Leicester, UK, is still surprised, and a bit worried, by the power of the technology he released upon the world.

The patterns within DNA are unique to each individual, except identical twins, who share the same pattern. The ability to identify these patterns has been used to convict murderers and to clear people who are wrongly accused. It is also used to identify the victims of war and settle disputes over who is the father of a child.

Jeffrey said he and his colleagues made the discovery by accident while tracking genetic variations. But, within six months of the discovery, genetic fingerprinting had been used in an immigration case, to prove that an African boy really was his parents' son. In 1986, it was used for the first time in a British criminal case: It cleared one suspect after being accused of two murders and helped convict another man.

DNA testing is now very common. In Britain, a national criminal database established in 1995 now contains 2.5 million DNA samples. The U.S. and Canada are developing similar systems. But there are fears about the stored DNA samples and how they could be used to harm a person's privacy. That includes a person's medical history, racial origin or psychological profile. "There is the long-term risk that people can get into these samples and start getting additional information about a person's paternity or risk of disease," Jeffrey said.

DNA testing is not an unfailing proof of identity. Still, it is considered a reasonably reliable system for determining the things it is used for. Jeffrey estimates the probability of two individuals' DNA profiles matching in the most commonly used tests at one in a billion.