DNA Profiling - "DNA In Context"

Article taken from Issue 1 of the Forensic Access Newsletter "Benchmark"

A significant part of the Forensic Access caseload concerns DNA profiling. Initial instructions generally request us to check all relevant laboratory records of the tests themselves and perhaps to repeat some of the more critical analyses if sufficient material remains. While all this is important and our investigations do sometimes throw up weaknesses in aspects of the technical work, much more commonly it is the relevance of the results in the circumstances of the case that might more properly be questioned.

Take, for example, the case of a man who came home to find the body of a woman friend of his there. He had left her there with her boyfriend some hours earlier. The woman had been strangled and had lost a small amount of blood from the mouth, and some blood - which looked as though it might be mixed with saliva, was discovered on the man's shoe. This was analysed and found to contain DNA of the same sort as the dead woman's. The man was charged with her murder.

One of our DNA scientists was called on to check the technical work at the laboratory where it had been carried out. This she did and she found nothing in it with which to disagree. Nor did she find any fault with the statistical interpretation of the results. However, what she did question was the relevance of the work in that there was no dispute that the defendant had found the body or that he had moved it. Rather the question here was whether or not this sort of contact with the body could explain the type of staining that had been found. In Sue's opinion it might.

A case dealt with by another of our DNA experts concerned a man charged with supplying drugs to undercover police officers. The drugs had been contained in cling-film wraps and, at some stage, these were supposed to have been concealed in the man's mouth. Tests carried out on swabbings from the surface of the cling-film confirmed that DNA in these could have come from the defendant.

However, our investigations confirmed that the link with the defendant in particular was relatively weak since the DNA could alternatively have come from any of 1 in 24 of the rest of the population. But beyond that, no tests had ever been carried out on the cling- film, or on the DNA extracts made from it, to establish whether or not there had been any saliva on it. To this extent, there was nothing to support the view that the wraps had necessarily ever been in anyone's mouth.