The Australian High Court has ruled that it is OK to state as - a
percentage - the probability of a person NOT sharing a given DNA profile
with a defendant. The case in question involved Yusuf Aytugrul, who was
convicted of murdering his ex-girlfriend. The Appeal Court
ruled that he was not unfairly prejudiced by the way DNA evidence was
presented to the jury. Expert evidence was presented at Aytugrul's trial
in 2009 of a match between
the accused's DNA and mitochondrial DNA taken from a hair found on
Bayrak's thumbnail. Specifically the exter stated that 99.9 percent of
people do not share a DNA profile with an accused. Full report here.
Unfortunately, what was not discussed in the Appeal was whether the 99.9% number was correct or
meaningful. Since mitochondrial DNA is known special
problems, the 99.9% figure is almost certainly not correct. People are beginning to realise that not all DNA evidence is especially probabative. 'Matches' involving mixture DNA and/or low template DNA may not tell you very much at all, especially given the possibility of cross contamination and subjectivity in analysis. I am currently working on a case where this is a particular concern. Itiel Dror's recent paper Subjectivity and bias in forensic DNA mixture interpretation is a must read as it highlights the extent to which the experts' results are subjective and influenced by contextual bias.
More general concerns about the validity of a range of forensic evidence is covered in this very interesting programme.