Jeffreys was born into a middle-class family in Oxford, where he spent the first six years of his life until 1956, when the family moved to Luton, Bedfordshire. He attributes his curiosity and inventiveness to having been gained from his father, as well as his paternal grandfather, who held a number of patents. When he was eight, his father gave him a chemistry set, which he enhanced over the next few years with extra chemicals, even including a small bottle of sulfuric acid. He says he liked making small explosions, but an accidental splash of the sulfuric acid caused a burn, which left a permanent scar on his chin (now under his beard). His father also bought him a Victorian-era brass microscope, which he used to examine biological specimens. At about 12, he made a small dissecting kit (including a scalpel, crafted from a flattened pin) which he used to dissect a bumblebee, but he got into trouble with his parents when he progressed to dissecting a larger specimen. One Sunday morning he found a dead cat on the road while doing his paper round and took it home in his bag. He relates that he started to dissect it on the dining room table before Sunday lunch, causing a foul smell throughout the house after he ruptured its intestines.
Jeffreys was a pupil at Luton Grammar School and then Luton Sixth Form College. He won a scholarship to study at Merton College, Oxford on a four-year course, where he graduated in 1972 with first-class honours in biochemistry. Jeffreys completed his doctorate, or DPhil as it is known at Oxford, on the mitochondria of cultured mammalian cells, as a postgraduate student at the Genetics Laboratory at the University of Oxford.
After finishing his doctorate, he moved to the University of Amsterdam, where he worked on mammalian genes as a research fellow, and then to the University of Leicester in 1977, where in 1984 he discovered a method of showing variations between individuals' DNA, inventing and developing genetic fingerprinting.
Jeffreys says he had a "eureka moment" in his lab in Leicester after looking at the X-ray film image of a DNA experiment on 10 September 1984, which unexpectedly showed both similarities and differences between the DNA of different members of his technician's family. Within about half an hour, he continued, he realized the possible scope of DNA fingerprinting, which uses variations in the genetic code to identify individuals. The method has become important in forensic science to assist police detective work, and it has also proved useful in resolving paternity and immigration disputes. The method can also be applied to non-human species, for example in wildlife population genetics studies. Before his methods were commercialised in 1987, his laboratory was the only centre in the world that carried out DNA fingerprinting, and was consequently very busy, receiving inquiries from all over the globe.
Jeffreys's DNA method was first put to use in 1985 when he was asked to help in a disputed immigration case to confirm the identity of a British boy whose family was originally from Ghana. The case was resolved when the DNA results proved that the boy was closely related to the other members of the family, and Jeffreys saw the relief in the mother's face when she heard the results. DNA fingerprinting was first used in a police forensic test to identify the killer of two teenagers, Lynda Mann and Dawn Ashworth, who had been raped and murdered in Narborough, Leicestershire, in 1983 and 1986 respectively. Colin Pitchfork was identified and convicted of their murders after samples taken from him matched semen samples taken from the two dead girls. This turned out to be a specifically important identification; British authorities believe that without it an innocent man would have inevitably been convicted. Not only did Jeffreys's work in this case prove who the real killer was, but it exonerated Richard Buckland, initially a prime suspect, who likely would have spent his life in prison otherwise. In 1992, Jeffreys' methods were used to confirm the identity for German prosecutors of the Nazi Dr. Josef Mengele, who had died in 1979, by comparing DNA obtained from a femur bone of his exhumed skeleton, with DNA from his widow and son, in a similar way to paternity testing.
DNA profiling, based on typing individual highly variable minisatellites in the human genome, was also developed by Alec Jeffreys and his team in 1985, with the term (DNA fingerprinting) being retained for the initial test that types many minisatellites simultaneously. By focusing on just a few of these highly variable minisatellites, DNA profiling made the system more sensitive, more reproducible and amenable to computer databases. It soon became the standard forensic DNA system used in criminal case work and paternity testing worldwide.
The development of DNA amplification by the polymerase chain reaction (PCR) opened up new approaches to forensic DNA testing, allowing automation, greatly increased sensitivity and a move to alternative marker systems. The most commonly used markers are now variable microsatellites, also known as short tandem repeats (STRs), which Jeffreys first exploited in 1990 in the Mengele case. STR profiling was further refined by a team of scientists led by Peter Gill at the Forensic Science Service in the 1990s, allowing the launch of the UK National DNA Database (NDNAD) in 1995. With highly automated and sophisticated equipment, modern-day DNA profiling can process hundreds of samples each day. Sixteen micro satellites, plus a marker for sex determination, are used with the current system developed for the NDNAD, giving a discrimination power of one in over a billion. Under British law, anyone arrested in England, Wales or Northern Ireland has their DNA profile taken and stored on the database whether or not they are convicted (different rules apply in Scotland). The national database now contains the DNA information of nearly five million people. Jeffreys has opposed the current use of DNA profiling, where the government has access to that database, and has instead proposed a database of all people's DNA, access to which would be controlled by an independent third party.
Jeffreys met his future wife, Sue Miles, in a youth club in the centre of Luton, Bedfordshire, before he became a university student, and married her on 28 August 1971. Jeffreys has one brother and one sister; he and his wife have two daughters, born in 1979 and 1983.