All the evidence

Forensic science has recently gained a high profile worldwide, owing in part to publicity surrounding criminal cases in which advanced methods have been used to determine guilt or innocence, sometimes even overturning previous verdicts. In particular, advances in DNA sampling and testing have spawned powerful methods that police investigators increasingly rely on in serious crime cases. As DNA testing is not always sufficient—or relevant—to solve a crime, there has been equally impressive progress in other fields of forensic biology, notably entomology and palynology— pollen analysis. As a result, criminal investigation and prosecution are being transformed. There is now so much confidence in forensic science that opponents of the death penalty in the USA had hoped to use DNA testing to demonstrate that an executed murderer was wrongly convicted, thus proving wrong the system of capital punishment. The case in question involved coal miner Roger Coleman, convicted and sentenced to death in 1982 for the murder of his wife’s sister, Wanda McCoy. His articulate pleas of innocence while on death row sustained a vigorous worldwide campaign to clear him or at least to commute his sentence to life imprisonment. In 1990, DNA testing was deemed to have reached the point where it could settle the matter, and a private laboratory, Forensic Science Associates (Richmond, CA, USA), was commissioned to analyse semen samples preserved from the crime scene. Polymerase chain reaction was used to target and amplify the DQA1 region in the human leukocyte antigen class II gene. This region has six alleles, making 21 possible genotypes that occur with varying probability in the human male population. The laboratory found that the DNA sample had the same profile as Roger Coleman’s DNA and that this was shared by 2% of the male population. This, combined with the existing evidence and Coleman’s failure to pass a liedetector test, was considered sufficient to confirm his guilt. Coleman was executed on 20 May 1992. But with 2% of males sharing Coleman’s DQA1 genotype, the DNA test was not sufficient to prove the case beyond all reasonable doubt, and over time the campaign mounted to re-test the sample with higherprecision techniques. Virginia Governor Mark R. Warner conceded to the pressure late in 2005 and ordered new tests, which were conducted by the Centre of Forensic Sciences in Toronto, Canada. The tests found a one in 19 million probability that the DNA sample came from an individual unrelated to Coleman, establishing his guilt even to the satisfaction of his supporters.