It's a little after nine o'clock on the morning of Monday the 10th of September 1984, and a young research scientist is in the darkroom at the University of Leicester's genetics department, developing x-ray films for a project looking at patterns of genetic variance between three members of the same family.
As he pulls the first negative from the developing tank and inspects it, the penny drops. Alec Jeffreys - now Sir Alec, the Royal Society's Wolfson Professor of Genetics - was looking at the first DNA fingerprint, the unique "supermarket bar-code" that sets out each individual's genetic profile.
"Within seconds it was obvious that we had stumbled upon a DNA based method not only for biological identification, but also for sorting out family relationships," he says.
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It's a good story, and from the self-deprecating way he tells it, you might be fooled into thinking this was just another one of those days: An incremental advance along the road to a better understanding of the biology that makes us all tick. But what Sir Alec had discovered was truly revolutionary. One of those "eureka!" moments that are actually quite rare in science, and open the floodgates on a whole new area of research.
Although he says he immediately appreciated the implications, Sir Alec admits he was ill-prepared for what happened next.
The first case came in March 1985 when the technique was used to decide an immigration case. The first paternity dispute followed soon after, and later that year the team was asked to help with the investigation of the rape and murder of two young girls in nearby Enderby. When Colin Pitchfork was convicted of both crimes in 1988 it was largely on the basis of DNA evidence.
The Forensic Science Service's Alison Fendley says that's lead to a step change in the investigation of crime, "The biggest breakthrough in this or the last century".
Speaking on the programme this morning the chief executive of the National Policing Improvement Agency, Peter Neyroud said DNA profiling was helping to solve nearly 400 murders every year, 800 rapes and serious sexual assaults, and some 8,000 burglaries. By next year he hopes further improvements in the technique will allow samples to be processed within an hour or two at the scene of a crime.
Sir Alec has no problem with that, or with the authorities keeping a DNA database of convicted criminals to help in future investigations. But he is very worried by what he describes as "mission creep" - the retention of samples from innocent people caught up in an investigation but subsequently cleared.
We're getting close to 5 million people on the DNA database now, he says, and something like 800, 000 of them are entirely innocent. That raises all sorts of questions about stigmatisation and discrimination.
"My genome is my property, not the state's," he says.
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