Jeffreys was born at Luton in Bedforshire and educated at Oxford, where he completed his PhD in 1975. After spending two years at the University of Amsterdam as a research fellow he joined the genetics department of the University of Leicester. He was appointed professor of genetics in 1987 and knighted in 1994.
Jeffreys is noted as the discoverer of the technique known as 'genetic (or DNA) fingerprinting'. In 1984 he was working on the gene that codes for the protein, myoglobin. Part of the gene consisted of short sequences repeated a number of times. The number of repeats was found to vary between individuals and became known as VNTRs ('variable number tandem repeats'). Initially Jeffreys saw the VNTRs as no more than useful gene markers of the myoglobin gene. Later he came to the conclusion that they were unique to the individual - they could act like a fingerprint.
The marker sequences can be identified by cleaving the DNA with restriction enzymes and using a gene probe - a single-strand fragment of DNA or RNA with a base sequence complementary to that of the marker. If the bases are labeled with a radioactive tracer, they can be identified on separation by electrophoresis.
Very small samples of DNA can be used, obtained, for example, from blood, semen, saliva, etc., and the technique has been exploited in forensic science and in the investigation of paternity and other family relationships.