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Devised by Alex Jeffreys at Leicester University and now widely used in forensic science and other uses of identification.
Different bands, resembling a supermarket bar code, are produced, which are unique to any one individual, except identical twins.
DNA is obtained from the white blood cells, mixed with restriction endonuclease, which 'cuts' the DNA into millions of fragments, but not the repeated regions, which retain their original length.
The DNA fragments are then loaded into a slot at the end of an agarose gel.
An electric current is passed across the gel, with the positive electrode at the furthest end from the fragments.
Since DNA molecules have a negative charge, they will migrate through the gel towards the positive electrode. Larger fragments move more slowly than the smaller ones. The result is a series of bands down the gel, but these are invisible at this stage.
Since the gel is difficult to keep, the DNA bands are then transferred to a nylon membrane, which is incubated overnight with radioactive probes.
The radioactive probes bind with the DNA that has repeating regions.
A sheet of X-Ray photographic film is laid over the membrane in total darkness. The radiation will affect the film, which is later developed, as a photograph.
The developed film reveals the series of DNA bands, which are unique to an individual.
Many illnesses are now being traced back to particular defective genes. Some, like Huntington's disease, only appear later in life - between 30 and 50 years of age.
While pinpointing the genes responsible for these terrible diseases is a breakthrough, it also poses a dilemma...
In the past, children who have a 50:50 chance of inheriting the disease faced a long wait to find out if they have been affected. Now they can choose to be tested, and then cope with the devastating news.
Screening tests can reveal whether an unborn child has Down's syndrome or cystic fibrosis for example.
Termination of pregnancy may be a decision that parents have to face, and this is where counselling is required, for such a difficult decision.
With high-risk parents, embryos may be produced outside the body (so called ''test-tube babies'') and after screening for defective genes, only normal embryos are implanted.
Transplanting foreign tissue carried great risk of rejection by the body's immune system. The patient faces the rest of their life with a cocktail of anti-suppressant drugs.
There is a great shortage in organs suitable for transplantation, resulting in may patients suffering or even dying before they get a chance to have the transplant operation.
Organs from other animals can be used in human transplantation, but they pose a potentially greater risk of rejection.
One way to avoid this is to change the chemical signature of the surface of the organ. This is achieved by adding the human gene that produces the correct chemical signals onto the surface of the organ so that the recipient's antibodies recognise it as 'self'.