Interesting point. According to the BBC, in respect of the evidence against David Norris, But the forensic scientists found two small hairs in the evidence bag that had held the jeans. One hair was 1mm in length and the second was 2mm in length. The longer hair was tested for a mitochondrial DNA match with Stephen. MtDNA is only passed down through the maternal line. It does not provide a full profile of an individual, but is a very powerful scientific tool, often used to investigate family ties. A geneticist examined MtDNA databases and told the court the chance of it not coming from Stephen was one in 1,000. The cold case team that found the evidence say that the hair was "close to the limit" of what could be examined for DNA. R. v. Watters, Court of Appeal (Criminal Division), October 19, 2000, established the precident that a probability of 1 in 267 could not be considered 'beyond reasonable doubt' and should not have been left to a jury to decide. If the Judge failed to direct the jury adequately on this point (I've seen no reports that he did but I've not read the summing up) then it would serve as strong grounds for Appeal. ETA: R. v. Watters suggested that even odds as high as 1 in 29,000 could be unsafe.