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Among How Many Others?

A pair of researchers from Australia and Denmark has developed a software program to help forensic investigators gauge to how many people a Y-chromosome profile uncovered at a crime scene might match. As Y chromosomes are passed from father to son with few changes, there could be multiple individuals in a population with the same profile.

The University of Copenhagen's Mikkel Andersen and the University of Melbourne's David Balding describe their simulation model, which uses locus mutation rates, variance in reproductive success, and the population growth rate to determine the number of individuals that might harbor that profile, in PLOS Genetics. Other approaches, they note, determine the match probability.

The pair says this approach could be easier to present in court and suggest that it could be presented like this:

A Y-chromosome profile was recovered from the crime scene. Mr Q has a matching Y profile and so is not excluded as a contributor of DNA. Using population genetics theory and data, we conclude that the number of males in the population with a matching Y profile is probably less than 20, and is very unlikely (probability < 5%) to exceed 40. 

Balding adds in a statement that he and Andersen will soon apply this same approach to mixtures of Y-chromosome profiles and mitochondrial DNA profiles.