Match probabilities for Y-chromosomal profiles: A paradigm shift

Abstract

Calculating match probabilities for genetic profiles under the hypothesis that a person of interest (henceforth the 'suspect') is not the donor of a particular forensic trace is challenging because the reference population is usually not clear-defined in such cases. Hence, the concept of 'suspect population' was introduced to allow for the fact that possible alternative trace donors often resemble the suspect in terms of their geographic, ethnic and social affiliation. In fact, they may even be genetically related to the suspect. Although the possibility of relatedness affects the size of the match probability in general, this is not a practically relevant issue for autosomal profiles owing to the stochastic independence of the markers included. The situation is, however, different for Y-chromosomal short tandem repeat (STR) profiles. Here, almost all profile matches are likely to be identical by descent, and profile identity may be common even among distantly related males. The degree of relatedness between suspect and alternative trace donors is thus a critical aspect of the evaluation of Y-chromosomal STR profile matches. All methods hitherto proposed to derive match probabilities for such cases, including our own, were anti-conservative in that they equated the suspect population to a database population (of virtually unrelated individuals). The nature of the true suspect population, however, is usually unknown and hard, if not impossible, to determine so that calculation of proper match probabilities remains a difficult task. Therefore, since lower Y-chromosomal match probabilities would result from higher mutation rates, even for close relatives, future research should rather focus upon the characterization of novel and more mutable Y-chromosomal STRs.

Keywords: Haplotype frequency; Likelihood ratio; Rapidly mutating markers; Short tandem repeat; Suspect population.