SOME types of DNA testing being presented in courtrooms were unreliable, and could make innocent defendants appear guilty, an expert on the "genetic fingerprinting" system told delegates.
Prof Lawrence Kobilinsky of the John Jay College of Criminal Justice in New York said that some PCR (Polymerase Chain Reaction) testing methods could give juries the impression that a defendant was guilty, when a more thorough test would show that they could not have committed the crime.
Prof Kobilinsky said the basic PCR method was able to identify six genes in a DNA sample. However, the World Health Organisation had now stated that there were 16 identifiable genes, divided into sub groups of those six original genes. Prof Kobilinsky said he was very concerned that in some cases juries were being told of a DNA "match" when, if a more sensitive DNA testing method was used, Aft could be shown that no match actually existed.
For example, if a defendant's DNA had gene types 3 and 4, and a crime scene sample also had types 3 and 4, this had been used to indicate guilt. However, it was now apparent that the type 4 gene had many subdivisions. A defendant could have types 3 and 4(a) while the fl scene sample had types 3 and 4(b).
Results obtained by testing methods which could not make the distinction were being used "in courtrooms all over the world" as evidence of guilt, whereas more thorough testing would show the distinction between the type 4 genes in the samples, and prove that the defendant was innocent.
Mr Tom McEwan of the Institute for Law and Justice in the US told delegates that 28 prisoners had been freed from US jails after DNA evidence showed that they did not commit the crimes of which they were convicted.
Mr McEwan was one of the authors of a report, "Convicted by Juries - Exonerated by Science", for the US Department of Justice, which examined how these innocent people had come to be imprisoned.
The report reinforced the view that eyewitness evidence was often unreliable. Many of the cases also showed malpractice by the police or withholding of evidence from the defence.
The report concluded that DNA testing should be carried out as early as possible in the criminal justice process, even before a person is formally arrested.