David Balding of the University of Melbourne and Mikkel Andersen of Aalborg University have developed an open-source software that can help the court understand how many people in a population will match a single Y-chromosome profile detected at a crime scene. The details of their study can be found in last week's PLOS Genetics.
Forensic Y-chromosome DNA can sometimes be hard to use in cases because when passed down from fathers to sons, there is little change. Therefore, a single Y-chromosome profile can be shared by dozens of men in a population.
Balding and Andersen propose that courts be notified about the likely number of matching males in the population and the possible consequences of their relatedness. They also show how the distribution of matching males can be affected by database information and suggest that it needs to be made clear to the court that Y-chromosome evidence cannot definitively identify the culprit.
The new software could be used to help courts improve the accuracy of Y chromosome evidence and increase the understanding by jurors and judges.
"We think this work is going to make a big improvement to how Y profile evidence is presented in courts. We will soon extend this work to mixtures of Y-chromosome profiles from multiple males, and also address the corresponding problem for the maternally-inherited mtDNA profiles. Our approach also allows us to include information from any relatives of the suspect whose profile is already available, and we will be working to develop that aspect," says Balding.
Image: A simulated population of males, with lines indicating father-son links. The suspected source of the DNA, whose profile matches that from the crime scene, is shown in red and other males with matching Y profiles, who are often close relatives, are yellow. The dashed line separates the last three generations, those further back in time will typically be already dead or otherwise unlikely to be of interest (depending on the circumstances of the crime). Image courtesy of Mikkel Andersen.