It's a very good way of identifying people, for example, committing serial burglaries as well as those committing what are regarded as the more serious crimes

Jack Straw

Current DNA tests show that, for example, sperm in a rape case is "exceptionally unlikely" to have come from a source other than the perpetrator.

Which is why, in criminal trials, forensic experts talk about the probability that a sample came from the defendant, rather than answering "yes" or "no" when asked if it matches.

Professor Shaw said the problem with this is we will never really know how many had been locked in prison because of DNA matches who are innocent. "People do regard DNA evidence as being very strong."

In his keynote speech to the Labour Party conference in September, Prime Minister Tony Blair said the Government wanted to set up a national DNA library.

DNA tests can be strong evidence in criminal trials

He announced his plan that anyone arrested and taken to a police station for all but minor offences would face compulsory DNA testing.

Their "genetic fingerprint" would be stored, in order to pinpoint suspects whose genetic make-up matched forensic evidence found at crime scenes.

Under current legislation, if a suspect is tested and later found to be above suspicion, their DNA sample is destroyed once someone is convicted.

A Home Office consultation paper said the samples would only be stored with the donor's consent, and would be kept on a separate database to the National DNA Database, which contains the genetic fingerprints of 600,000 offenders - the biggest in the world.

Guilty until proven innocent

Professor Shaw said the proposed library could mean that innocent people become suspects, just because their DNA is close to that of the offender.

"If I was suspected of some crime and a DNA sample was taken and a profile stored, it's possible that at some future date, my DNA might be matched to the DNA of a suspect in a crime.

"Therefore the police would come knocking at my door and it would be very difficult to argue that I had nothing to do with that crime."

Although the chances of that happening were remote, "the more samples you get, the greater the chance that you find two samples that look the same but are not actually from the same person".

Miniscule sample needed

DNA fingerprinting involves isolating DNA from a cell, which is then treated with enzymes to break it into pieces. These are separated using an electric current, and the pattern of the separated fragments is used to compare samples.

All that is needed to test DNA is one cell. But a stronger match is made when a larger sample is available - at least one-thousandth the size of a salt grain, invisible to the naked eye yet able to be separated and matched.

I'm very sorry to say that a very high proportion of people who will commit crimes in the future are people who have committed crimes in the past

Jack Straw

Although a strand of hair contains no genetic material, the skin cells clinging to the base of the hair can be tested.

If a suspect drank from a glass left at the crime scene, scientists can test for skin cells from their lips.

Even a fingerprint can be tested if skin cells from the offender's finger are left behind.

"But it could have been a cell left lounging around on the surface before the offender put their finger on the same place," Professor Shaw said.

"So there is always the question 'did it really come from that person?'. You can never be 100% certain."