Hardly a week goes by without a mention of DNA’s contribution to criminal justice. Last week Sandip Patel was convicted of killing a prostitute near Baker Street 30 years ago: DNA belatedly proved that his hairs were caught in her ring. A few days before, a double murderer, Colin Pitchfork, was controversially granted a parole hearing 36 years after being the defendant in ‘the first case of its kind in Britain to use DNA profiling’, as the Times put it.

Only that’s wrong. Colin Pitchfork was not the first person in Britain to be convicted with DNA; he was the first person in the world. The story of how DNA profiling was invented and then applied to solving crimes is a truly remarkable, entirely British success story. We seem to have forgotten this, perhaps because of the modesty of the inventor, Sir Alec Jeffreys.

Pitchfork was not the first person in Britain to be convicted with DNA: he was the first person in the world

Without Jeffreys’s insight, it might not have happened until years later. What’s more, almost nothing but good has come from it. DNA is even better at exonerating the innocent than convicting the guilty. In tracing lost relatives and sorting out paternity it usually – not always – brings joy as well as justice.

I first met Jeffreys at a British Association meeting in Brighton in 1983, in my very first week as a science reporter for the Economist. As a young geneticist he electrified the audience with a magical talk about the emerging glimpses of the structure of genes. He described a world of coded messages, repetitive deserts of so-called ‘junk DNA’ and ‘rusting hulks’ of disused pseudogenes. Over the next year I called him regularly as a source for new stories. Then one day he very politely brushed me off: ‘I’m too busy to talk.’ I later found out why.

At 9.05 a.m. on 10 September 1984, Jeffreys had a genuine eureka moment in his laboratory at Leicester University. He fished an X-ray film out of the developing tank in his dark room, turned on the light and saw something striking. His technician’s DNA had a pattern of repeats of a particular sequence that was similar to both her parents’ patterns but not the same. Suddenly he realised that every individual must have a unique profile in the repetitive DNA sequences between the genes.

Over the next two days he cut himself on purpose to leave spots of blood around the laboratory, returning to them later to see if the DNA was intact enough to be tested. Nobody expected that DNA could be used to identify people even decades after being deposited in tiny amounts. It’s remarkably fortunate: an organic molecule that in the dry, crystalline state resists decomposition. I also still find it baffling that in a sample replete with lots of copies of a book 800 times as long as the Bible (i.e., the full genome that is in every cell), you can pull out a handful of particular sentences with speed and certainty. But you can.

Jeffreys first applied the technique to help a Ghanaian immigrant prove to the Home Office that her son was her son. The mother’s delight fired him on. Then out of the blue in 1986 he had a call from the local police. They were investigating the rape and murder of Dawn Ashworth near the Leicestershire village of Narborough. They had their man: Richard Buckland, who had learning difficulties, had confessed to the crime under interrogation. But a very similar rape and murder, of Lynda Mann, had happened nearby three years before and Buckland denied committing that crime. Since sperm samples were available from both victims, could Jeffreys prove the two schoolgirls were raped by the same man?

He agreed to try, while being privately unsure that his technique would work for such samples. To his surprise, he got a clear result: the same man had committed both crimes. But it was not Buckland. The police unleashed some Anglo-Saxon down the phone and sought a second opinion from another lab, which agreed with Jeffreys. Reluctantly, they dropped the case against Buckland. Thus DNA exonerated an innocent man before it ever convicted a guilty one.

The police then asked more than 5,000 men aged between 17 and 34 in the surrounding area to give blood for Jeffreys to test. None matched the samples on the victims. A dead end. Then one day somebody told the police they had overheard a man in a pub saying he had been persuaded to give blood in the place of a work colleague, because the colleague did not trust the police. They tracked down Colin Pitchfork, tested his DNA and found a perfect match. He confessed to both crimes.

Without DNA, Buckland would have gone to jail and Pitchfork would have been free to kill again, leaving more families destroyed by grief. In the years that followed, DNA profiling solved many new cases as well as old and cold ones. Hundreds of people lived because murderers were convicted by DNA evidence before they could kill again. Used properly, DNA profiling became highly reliable, rendering the mythical Holmes-Poirot genius less necessary. 

Better still, the technique allowed the Innocence Project in the US to exonerate 202 innocent people (and counting) who were on death row or serving long prison sentences for crimes they did not commit. Many are black. It was started by O.J. Simpson’s lawyer, Barry Scheck, who had successfully cast doubt on the accuracy of DNA profiling in that case, but then championed it.

To everybody’s surprise, therefore, the biggest use of the new knowledge of the coded messages in DNA was not in medicine but in forensic science – thanks to Alec Jeffreys. DNA became famous for the first time. As James Watson, the co-discoverer of the structure of DNA, joked to me at the turn of the century, the two people who truly made DNA a household name were not himself and Francis Crick, but O.J. Simpson and Monica Lewinsky.