DNA fingerprinting

About 99.9% of the human genome is the same in everyone. The remaining 0.1% holds the inherited differences that make each person unique. DNA fingerprinting compares this part, without reading the whole sequence. It was developed by Alec Jeffreys in 1984, at the University of Leicester.

More than 90% of our DNA does not code for proteins. Within it are short sequences repeated many times in a row, called VNTRs (variable number tandem repeats):

• microsatellites: repeats of 1-9 base pairs;
• minisatellites: repeats of 10-100 base pairs.

The number of repeats at each place varies from person to person, and is inherited: we carry one copy from each parent.

The steps:

• Isolate DNA from a sample: blood, hair, skin, semen, or a cheek (buccal) swab. If there is very little, PCR can amplify it first.
• Cut it with restriction enzymes.
• Separate the fragments by agarose gel electrophoresis.
• Transfer them to a nylon or nitrocellulose membrane and fix them (by UV, or baking at 80 °C).
• Hybridise with labelled VNTR probes (Southern hybridisation).
• Detect the bands.

Because people differ in their repeats, the fragment lengths differ too. This RFLP (restriction fragment length polymorphism) pattern is specific to each person. In a child's pattern, every band comes from either the mother or the father.

Uses: settling paternity and maternity cases; forensics (matching evidence from a crime scene); comparing the DNA of fossils with modern relatives to study evolution; studying inherited disorders; and perhaps one day a 'genetic barcode' of personal identity.