Today, we know that a blood stain, or single hair, recovered from a crime scene can identify the perpetrator. It's remarkable that, just 30 years ago, we hadn't a clue. But who cracked the DNA killer code, and how?

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Telly shows filled with crime dramas showing white-coated forensic experts searching for  evidence, or Jeremy Kyle offering paternity tests to guests. Everyone’s now known to have a unique DNA code, even identical twins.

But, not so long ago, the idea that we’d be able to pinpoint an individual from a trace of sweat or blood was implausible.

Who cracked the DNA code and how?

On 10 September 1984, a geneticist at Leicester University, Professor Alec Jeffreys, was running tests to trace genes within family groups.

Professor Alec Jeffreys. Photo: PA

Professor Sir Alec Jeffreys pictured on the 20th anniversary of his discovery. (Photo: PA Photos)

He’d hit on a fragment of DNA that was repeated on different chromosomes in cells belonging to men and women. Realising that this could be unique to an individual, Jeffreys devised an experiment testing people, and their relatives. The results were staggering.

Every individual in the sample had a different barcode, which could be identified with precision. Jeffreys could even clearly see links in DNA between parents and their children.

‘It was an absolute Eureka moment,’ Jeffreys recalls.

But, in 1984, not even Jeffreys knew whether this DNA was stable, or whether it’d break apart when a cell died.

So, Jeffreys spend the next two days leaving his own blood marks around the laboratory. Later, testing those bloodstains, the DNA remained intact. It raised an obvious question…

Could DNA identify criminals who’d left behind blood at a crime scene?

It was several years before the theory would be put to the test. In 1987, Leicestershire police were investigating the rape and murder of two schoolgirls.

On 22 November, 1983, the body of 15-year-old Lynda Mann had been found raped and strangled on a footpath in the village of Narborough.

15-year-old schoolgirl Lynda Mann who was raped and murdered. Photo: PA

15-year-old schoolgirl Lynda Mann who was raped and murdered. (Photo: PA Photos)

Nearly three years later, in July 1986, the body of another girl, Dawn Ashworth, also 15, was found.

She’d also been strangled and dumped in woodland, less than a mile from the scene of Lynda’s murder.

Initially, a local man, Richard Buckland, 17, confessed to the second murder, but not Lynda’s. Police believed the two crimes were undoubtedly linked, and turned to Dr Jeffreys to help prove it.

In January 1987, police asked all Narborough men aged 17-34 to submit blood for DNA testing. Jeffreys set about using his DNA profiling technique to test the samples.

DNA for profiling can be extracted from samples of human cells including blood, semen, skin, saliva, mucus, perspiration and the roots of hair. Dr Jeffreys compared two semen samples, recovered from Lynda and Dawn’s crime scenes, with Buckland’s blood sample.

Jeffreys conclusively proved that both girls had been killed by the same man…

But it wasn’t Buckland. The police had the wrong man, and were clueless as to the real murderer’s identity.

Until, weeks later, a local man was overheard in a pub admitting to friends that he’d provided blood on behalf of a friend, Colin Pitchfork, then 25.

Pitchfork was a baker, a family man…and a convicted flasher.

Colin Pitchfork. Photo: REX/Shutterstock

Colin Pitchfork (Photo: REX/Shutterstock)

He was arrested, and tests proved that Pitchfork’s DNA matched both semen samples.

In 1988, Pitchfork admitted two counts of murder, two of rape, two of indecent assault and one of conspiracy to pervert the course of justice.

He was jailed for life, with a minimum of 30 years, at Leicester Crown Court.

Without the DNA evidence, it’s highly likely that Buckland would have taken the rap while Pitchfork remained free. In 2009, Pitchfork’s sentence was cut to 28 years, and is now eligible for parole.

It was the first of thousands of murder cases worldwide to use DNA profiling to secure the guilty, and free the innocent.

Sir Alec Jeffreys received a knighthood in 1994 for his services to science and technology.

‘I was just lucky that I got to discover DNA fingerprinting. If I hadn’t, someone else would have done it by now,’ Jeffreys concluded, modestly.

Currently, the UK national DNA database is the largest of any country in the world. Many debate whether DNA profiling is an invasion of privacy.