On this page
Every NEET student learns that DNA fingerprinting compares repeated DNA sequences between people. Fewer know that the method was found almost by accident, on a Monday morning, by a geneticist who was looking for something else. This post covers who Alec Jeffreys is, what his probe detected, how the first cases worked, and a worked example of reading a paternity pattern the way his team did.
Who is Alec Jeffreys?
The University of Leicester's biography of Sir Alec Jeffreys says he studied biochemistry and genetics at Merton College, Oxford. As a postdoctoral fellow in Amsterdam, working with Richard Flavell, he was one of the first to discover split genes. He moved to the Department of Genetics at Leicester in 1977. He was elected to the Royal Society in 1986 and knighted in 1994, and he retired in September 2012. In 2014 the Royal Society gave him its Copley Medal "for his pioneering work on variation and mutation in the human genome".
What was the eureka moment?
Leicester's history of genetic fingerprinting quotes him: "My life changed on Monday morning at 9.05 am, 10 September 1984. What emerged was the world's first genetic fingerprint." The X-ray film showed extraordinarily variable patterns of DNA from a technician and her mother and father, and from non-human samples. His first reaction, he says, was "this is too complicated", and then he realised what he had.
The 2005 Lasker Award citation explains the science. While studying the human myoglobin gene, Jeffreys found a minisatellite: a short DNA sequence repeated in tandem, with different people carrying different numbers of repeats. The repeats at several such sites shared a common core. He built a radioactive probe from copies of that core and used Edwin Southern's blotting method to find many minisatellites at once. Each person's set of fragment lengths formed a fingerprint.
He published the method in Nature in March 1985 as Hypervariable 'minisatellite' regions in human DNA, describing a shared 10 to 15 base-pair core. A second paper that July, Individual-specific 'fingerprints' of human DNA, showed the patterns were specific to one person, or to identical twins, and could be used for parenthood testing.
What were the first real cases?
The first came in March 1985. The Lasker citation describes a boy, a UK citizen, returning from Ghana to join his mother and siblings. Officials suspected his passport was forged and that he was a nephew or unrelated. DNA fingerprints showed that every one of his bands matched either his mother's or those of her undisputed children, and the family was reunited. Jeffreys described the case in Nature as a positive identification in an immigration test-case. "Of all the cases this is the one that means most to me," he told Leicester.
The forensic test followed. Leicester's history recounts that after two girls were murdered near Enderby in Leicestershire, a man who had confessed to one killing was shown by DNA profiling to be innocent of both. Police then sampled men across the area. Colin Pitchfork was caught after he was overheard saying he had persuaded a friend to give a sample for him.
How do you read a DNA fingerprint? A worked example
The Lasker citation states the rule Jeffreys showed: each child's bands come from the parents, roughly half from the mother and half from the father, except where a new mutation appears.
Here are sample numbers for a made-up case. A child's fingerprint has 16 bands. The mother is known.
Compare child and mother. Suppose 8 of the child's bands are also in the mother's pattern. These are maternal.
The other 8 bands must have come from the biological father.
Alleged father A has 7 of those 8 bands. Alleged father B has 2.
B is excluded: he lacks 6 bands the father must carry. A matches all but one.
Is A excluded by the one missing band? Not on its own. Jeffreys's group measured mutation rates at minisatellites and found new length alleles arising at up to 5% per gamete at the most unstable site. One unexplained band can be a mutation; several cannot.
Common mistakes:
Expecting a perfect half-and-half split. It is roughly half, because a parent may share a band with the other parent by chance.
Excluding on one mismatch. Allow for mutation, as Jeffreys warned in 1988.
Confusing the probe with the gel. The gel separates fragments by size; the probe reveals which ones contain the repeat.
Our NEET guide to DNA fingerprinting and the Human Genome Project covers the steps as NCERT lists them, and the post on gel electrophoresis explains why shorter fragments run further.
What can a student take from Jeffreys's way of working?
Look again at the result that seems too messy. His first thought was that the film was too complicated. The complexity was the point.
Ask what else a finding could do. On the same day, his team listed crime, paternity and twins, and his wife added immigration.
Keep testing your own method. He went on to measure how often minisatellites mutate, the very thing that could cause a false exclusion.
If band-matching problems keep tripping you up, the Fermi Problem Library has worked problems with a step-by-step solver that explains the reasoning rather than printing an answer.
Try it now: draw five bands for a mother, a child and two alleged fathers, and decide which father fits.
