Guilt and innocence put under the microscope
DNA has become a central factor in the case of Madeleine McCann. Chris Thornton looks at the molecular evidence that is hard to argue against.
After all the appeals and the fruitless sightings, attention has now turned to what may be the last traces of Madeleine McCann. Because Portuguese police may have discovered her DNA in a car rented after her disappearance, the four-year-old's parents, Kate and Gerry McCann, have formally been declared suspects in their daughter's disappearance.
The police backed away from reports yesterday that they had made a 100% match between Madeleine and the forensic material found in the rental car - a statement, which like so much in this case, begged more questions.
DNA matching is a precise science. When experts present DNA evidence in court, they tend to talk about the chances of one person's DNA being mistaken for another's as one in a range of billions.
Last night in Portugal there were reports of an "88% match" of fluid from the boot of the car to Madeleine, a range that is considerably less than is generally relied on in court. Much is unexplained and filtered through third parties, so the exact nature of the DNA evidence remains obscure.
But DNA could still be the determining factor in this case, not least because when it is properly established it is virtually irrefutable.
DNA evidence is still relatively new. Fingerprint evidence has been accepted in courts for over a century, but it has only been 20 years since the first conviction in a UK court on the basis of DNA evidence.
But in those two decades it has become an important feature of the criminal justice system around the world - so much so that when there is no DNA evidence at the scene of a crime, defence lawyers will sometimes use that to persuade juries of their client's innocence. If he didn't leave any DNA, he couldn't have done it.
For investigators, one of the most useful aspects of DNA is that it is available from virtually any bodily material left behind at a crime scene. For a determined criminal, leaving fingerprints is as easily avoided as slipping on a pair of gloves.
But if there is blood, spit, hair, skin - even fingernails - left behind, there is DNA evidence to pursue.
DNA is often described as the molecular blueprint for the human body. Every human cell has DNA wrapped around its chromosomes.
Most of it looks exactly like the DNA that comes from every other human being. But enough - roughly three million "base pairs" - varies from person to person so that scientists can match a hair to the person who left it behind.
Apart from identical twins, who have the same DNA profile, scientists now say they can identify a person with "absolute certainty" if they are given a sound sample of bodily material.
Improvements in scientific techniques over the past 20 years have made it harder for the criminals. Spots of blood that were too small to yield evidence even a few years ago may now give up a DNA profile.
The innocent have also benefited. In America, hundreds of people have been freed from prison as a result of DNA evidence, some of them from Death Row. The first UK case to secure a conviction - double murderer Colin Pitchfork - also freed a teenager who had confessed to one of the killings.
In Northern Ireland, DNA evidence has been used in hundreds of cases - notably catching the killer and rapist of an elderly woman in Bushmills, more than 20 years after the murder. Steven Shepherd was only convicted of murdering Vera Waring when his DNA matched a pubic hair found on the victim and blood found at the scene. Expert testimony said there was a one in a billion chance that he was not at the crime scene and - despite his protests of innocence - Shepherd was convicted.
In the relatively short time that it's been available, DNA evidence has become so established that defence lawyers rarely challenge the results themselves. Instead, they tend to concentrate on the circumstances in which the evidence was gathered, trying to establish that police may have contaminated or mishandled the material from which the DNA was extracted. It's harder to argue against the DNA itself.