Relevant propositions for Y chromosome interpretation

The Y chromosomal haplotype is expected to be identical (or close to, depending on the mutation rate) among a male and many of his paternal relatives. This means that often the same evidential value for the DNA evidence is obtained, whether the true donor or one of his close paternal relatives is compared to a crime sample. Commentators (see for example the UK Forensic Science Regulator or Amorim) have suggested to change the proposition pair to compare the probability of the evidence if the Person of Interest (POI) or one of his close paternal relatives left the DNA to the probability of the evidence if an unrelated male from the population left the DNA. We argue that this is problematic because there is no clear definition of close paternal relatives and truly unrelated males do not exist. Instead, we take a starting point in the traditional proposition pair “The source of the male DNA is the POI” versus “The source of the male DNA is not the POI” and make the latter one operational by suggesting that it is formulated as “The source of the male DNA is a random man from the population”. The issue of matching males in the POI's lineage is then addressed either in a comment in the statement or directly through a probability model.