How was a murderer traced through blood samples?

Within a period of three years, the small English village of Narborough was shocked by the rape and murder of two 15-year-old high school girls. Both girls disappeared on a country lane as they were making their way home. Shortly after the second murder, in 1986, the local police arrested a worker from the local mental institution who confessed to the second murder, but denied any knowledge of the first. Because the police were fairly certain that both murders had been committed by the same man, they needed to find some way of establishing the suspect's guilt.

Alec Jeffreys, professor at Leicester University, had discovered that a small piece of DNA that resides in each of us is located on many different chromosomes and may be present in variable numbers at any of these sites. When the DNA of an individual is examined on a gel and this specific variable piece of DNA is detected with radioactivity, the result is a distinctive pattern. The pattern is so distinctive that no two individuals, except for identical twins, have the same pattern. Because of the uniqueness of each pattern, the technique is called DNA fingerprinting.
The police sent forensic samples from the victims and a blood sample from the suspect to Dr. Jeffreys for analysis. DNA fingerprinting showed that the police were correct, and that both girls had been raped and murdered by the same man. However, the assailant's DNA fingerprint was totally different from that of the suspect who had confessed to one of the murders. As a result, the police realized that the confession was false, and the man was released from custody, the first person to be proved innocent by DNA fingerprinting.
Armed with the DNA fingerprint of the true assailant, the police launched the world's first DNA-based manhunt, and requested, on a voluntary basis, a small sample of blood from all men in the vicinity between the ages of 18 and 35. Over 5,000 blood samples were sent to the Home Office Forensic Laboratories for DNA analysis. The true assailant, Colin Pitchfork, almost escaped the manhunt by getting a friend to give blood in his name. However, the substitute, while drinking with friends in a local pub, mentioned the switch and was overheard. The police were notified, Pitchfork was arrested, his blood was tested and found to be a perfect match to the samples obtained from the two murdered girls. He was convicted and is now serving two life sentences.
Since this important test case, many criminals have been brought to justice through Dr. Alec Jeffreys' technique of DNA fingerprinting.

Career Profile

Alec Jeffreys

My interest in genetics and biochemistry began at the age of seven when my father presented me with a microscope and a remarkably lethal chemistry set. I followed this growing interest in chemistry and biology throughout my school days and became particularly interested in the interface between the two subjects, namely biochemistry.

I went on to study biochemistry at Oxford and became increasingly interested in the application of biochemical techniques to the study of genetics. In 1975, I was given the opportunity of working on a project, in Amsterdam, on the biochemical isolation of a mammalian gene, something which, at the time, had never been successfully attempted.

These early studies of mammalian gene structure led to an interest in trying to detect inherited variation within genes between individuals, and also to unravel the evolutionary origin of contemporary genes. It was these combined interests that led me to study highly variable regions of DNA, in part from academic interest and in part to provide better genetic markers for use in medical genetics. This work culminated in the generation of the first DNA fingerprint and the realization that these highly individual-specific patterns could be used for identification and the determination of parentage.