MatchY in practice: Factors influencing pedigree-based Y-STR match probabilities

While widely applied in forensic practice, equating Y-STR match probabilities with haplotype frequencies derived from population databases has been considered inappropriate, as these databases cannot be representative of each and every case-specific suspect population. To address this inherent limitation of the forensic application of Y-STRs, we previously introduced a mathematical framework for calculating Y-STR match probabilities based on the suspect's male pedigree. Recently, we extended and implemented this framework in the publicly available MatchY software tool, which allows users to calculate match probabilities for any Y-STRs with known mutation rates and for pedigrees of varying size and complexity. Here, we present a comprehensive exploration of marker and pedigree-based factors impacting the Y-STR match probability outcomes obtained with MatchY. Moreover, we introduce another application of MatchY, the trace mode, aimed at forensic intelligence rather than evidential value computation. These insights provide valuable guidance on the practical use of MatchY for calculating Y-STR match probabilities in routine forensic casework for cases both with and without suspects.