As a boy in Luton, Alec Jeffreys dissected a dead cat from his paper route on the family dining table, made explosions with a modified chemistry set, and was left with a sulfuric acid scar on his chin that he still hides under his beard. Born in Oxford in 1950, he earned first class honors in biochemistry at Merton College, Oxford, and moved to the University of Leicester in 1977. On September 10, 1984, while studying an X-ray film of DNA from his lab technician’s family, he saw both shared patterns and differences, and within about 30 minutes grasped that minisatellite regions could identify individuals like a barcode.

The first real case in 1985 was an immigration dispute in which his test proved a British boy of Ghanaian family was his mother’s son. Then came the Narborough murders of Lynda Mann and Dawn Ashworth: the DNA cleared Richard Buckland, a teenager who had confessed to one killing, and led to the conviction of Colin Pitchfork. In 1992 Jeffreys’ team confirmed the remains of Josef Mengele using DNA from his mother and son. The episode ends with the shift to PCR and STR profiling, the UK National DNA Database launched in 1995, and Jeffreys’ objection to storing the profiles of people who were never convicted.

• The Victorian brass microscope from his father that started his bumblebee dissections
• How radioactive probes turned minisatellites into dark bands on X-ray film
• Why his Leicester lab was the only place doing DNA fingerprinting until 1987
• About 5.6 million people held on the UK database by 2020, arrested or not
• His proposal for a universal database run by an independent third party and searched only by warrant