Spare pairs of genes

Observations on DNA

In the Grand Chamber of the European Court of Human Rights in Strasbourg on 27 February, two British lawyers began their attempt to overturn one of the most controversial police practices of recent years: the retention of DNA profiles and biological samples taken from individuals subsequently cleared of crimes.

More than 200,000 people with no criminal record are kept on the national DNA database. Attempts to have their details removed have been met with blanket refusal by senior police officers and the British courts.

Now the solicitor Peter Mahy and barrister Stephen Cragg are asking the European court to intervene. Mahy says it is "the most important case on the human rights implication of retaining biometric data and will probably be one of the most important human rights cases of all time".

He has a point. Britain today is obsessed with the DNA profiling of its citizens. Our database is, per head of population, the biggest in the world and contains more than 4.25 million entries. Every month, police take 60,000 new DNA samples and once on the database, that entry is linked, by computer, to their criminal records or - for those not subsequently convicted - to the incidents that first required them to be sampled. It is the first time that UK records of this depth and permanence have been kept.

We should not be surprised by this obsession. If DNA is left behind at a burglary, detection rates rise from 15 to 45 per cent, says Tony Lake, head of forensics for the Association of Chief Police Officers, while for car crime it rises from 9 to 30 per cent. In recent cases, such as those of the murderers Steve Wright and Mark Dixie, DNA has played a key role in obtaining convictions.

Profiling exploits a small section of DNA that contains pieces that repeat themselves over and over. Different people have different numbers of these genetic stutters, which can be counted to produce a digital readout. The system used by police, SGM Plus, provides a 20-number readout. Crucially, we each inherit ten of these DNA stretches from one parent, and ten from the other. Thus, it is possible to trace family connections through the DNA database. The South Yorkshire businessman James Lloyd - the "shoe rapist" who kept his victims' footwear as souvenirs - was only caught when his sister was arrested for drink-driving and profiled. Ten numbers on her readout matched crime-scene DNA and led police to Lloyd.

It is easy to see why police are upset at the prospect that the European court may remove hundreds of thousands of profiles if it backs Mahy and Cragg (who are appealing on behalf of an unnamed juvenile and another man, Michael Marper, who have their DNA profiles stored although they were never found guilty of any crimes). "That would have major implications for our ability to carry out serious crime investigations," says Lake.

But as Helen Wallace of GeneWatch UK, which monitors genetic technologies in Britain, points out, there is also a clear issue of proportionality. "Keeping a person's DNA profile and their biological samples in a permanent database is a very serious issue. Yet these are innocent people. And in future, when DNA data is shared between more and more countries, the risks of its abuse and loss will increase dramatically," she says.

But then there is the example of Scotland's police, who have their own DNA database but who routinely discard DNA taken from those subsequently acquitted or whose cases are dropped. And yet, with its more enlightened attitude, Scotland has a slightly better rate of bringing DNA-linked cases to court compared to England.

Robin McKie is science editor of the Observer