ORDINARILY, the sex-offender who attacked a woman in Birmingham, UK, would never have been caught. But thanks to a novel twist on DNA profiling a man is now awaiting sentence for the crime.
DNA profiling has become such a successful forensic tool it is easy to forget it has serious limitations. One kind of DNA sample that has until now defied useful analysis is a mixed one, containing genetic material from more than one person. If two people have been smoking the same cigarette, for example, or drinking from the same glass, any attempt to produce a DNA profile yields only a confusing muddle that cannot be interpreted, says Kate Jones, an analyst at the FIB, part of the UK government鈥檚 Forensic Science Service. She says that up to 4 per cent of DNA-containing samples are sullied in this way, and are therefore useless for detective work.
She and her colleagues have now worked out a computerised way of identifying a few hundred probable combinations of DNA profiles from within a mixed sample, which can then be used to search for matches among the profiles from 4.3 million people held in the UK鈥檚 National DNA Database. Although a match to one of these probable DNA profiles could not be used to convict someone in court, it could provide detectives with a valuable lead on which to base their investigations, Jones says.
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During standard DNA profiling, DNA is extracted from a sample and the polymerase chain reaction is used to make multiple copies of several short tandem repeats 鈥� non-coding areas of DNA a couple of letters long 鈥� at 11 signature locations on the human genome. The number of repeats at each location varies between individuals and can be estimated by the technique known as electrophoresis. This yields a trace showing one or two distinct peaks for each location, whose size relates to the number of repeats on the pair of chromosomes on which they arise.
In a mixed sample from two people there can be up to four peaks at each location, all of a similar size, and the problem is knowing which peaks belong to which person. To narrow down the possibilities, the FIB team has developed a computer program called Pendulum that pairs off these peaks on the basis of the amount of DNA present in the sample from each person. In this way it can eliminate most of the many possible pairs of DNA profiles and come up with the 500 most likely pairs to form the mix.
In the Birmingham case, sunglasses left behind by the attacker yielded a mixed sample of skin cells. There was just one person on the national database who matched a DNA profile from one of the 500 most likely pairs predicted by Pendulum. He later pleaded guilty to the crime.
GeneWatch, a group that promotes ethical use of genetic technologies, warns of dangers in the technique. 鈥淎 computer-generated statistical DNA profiling technique like this is open to false matches and could contribute to miscarriages of justice,鈥� says deputy director Helen Wallace. 鈥淭here is scope for errors to creep in so we would like to see independent oversight of such technology.鈥�
