PARIS, Jan 3 (AFP) - Invented 20 years ago by British scientist Sir Alec Jeffreys, forensic DNA has been used to identify rapists and descendants of kings and to settle paternity suits.
Now it faces its most demanding test, in helping to name thousands of victims in Asia's tsunami calamity.
"Genetic fingerprinting," as the technique is often called, is based on the individuality of the human genetic code, the chemical recipe for life contained in the DNA molecule.
DNA sleuths look at specific locations in the code, sifting through highly variable repeating sequences called microsatellites, which account for about one percent of the overall genome.
People who are related have the same markers, inherited from their mother or father, in these microsatellites.
People who are unrelated can, theoretically, also have the same match. But the probability is extraordinarily remote -- it is one chance in 100 billion provided there is no lab error, the sample has not been contaminated and more than two dozen microsatellites are checked.
DNA can be taken from cells in tissue, blood, semen, saliva or hair. The sample is purified in the lab and then amplified by polymerase chain reaction (PCR). This is like a biological photocopier, producing millions of the original copy.
The amplified sample is then separated into 11 specific areas of interest, using an electrified gel and a cutting enzyme to separate the DNA strand.
X-ray film is used to detect the radioactive pattern from the sample. The image is then organised by computer into coloured bands, thus creating an individual's genetic profile that can be easily compared against another.
Looking for matches is done by taking a sample from a presumed relative, and seeing if there are correlations.
Hundreds of experts have been sent to Thailand and Sri Lanka to compile DNA material, in the quest of ending uncertainty for people who are missing a loved-one.
Many of them have been sent by police forces from rich countries, trying to identify tourists who have gone missing at beach resorts.
Janine Kimpton, spokeswoman for Forensic Science Service, a brance of the Britain's Home Office which is helping the search, told AFP the biggest problem facing the collectors is dampness and heat.
This could affect a special kind of blotting paper on which fluid samples of DNA are stored and then analysed when it is brought back to the laboratory.
"The sample can be kept on this paper for weeks, provided it is dried and relatively stable," she said.
If the body is decomposing, that is not a problem per se, as tissue and bone can also be taken for DNA testing, she said.
The problem with those samples, however, is in ensuring that they are kept cool so that they do not degrade, and this may be difficult in remote locations where there is no electricity supply for refrigeration, she said.
"They need to be kept cool, or frozen ideally," she said.
"We take numerous samples from every body to get the best chance of getting a result and then the question is really about getting them back to Britain and getting them profiled as soon as possible."
01/03/2005 13:17 GMT - AFP