Probabilities of finding trace profile donors and their paternal relatives in Y-STR reference databases

Forensic investigative genetic genealogy using Y-chromosome short tandem repeat (Y-STR) DNA profiles can give investigative leads in criminal cases by searching for the Y-STR trace profile or similar but not identical Y-STR profiles in relevant Y-STR databases. We conducted a simulation study with Yfiler^TM Plus and PowerPlex® Y23 Y-STR profiles to estimate the probabilities of finding matches and near-matches in Y-STR databases. The success rate of finding the trace profile donors or their close relatives was quantified. We used the malan R software package to simulate the populations based on the Wright-Fisher model with the YHRD Y-STR mutation rates where uncertainties were incorporated in a Bayesian manner, a variance in reproductive success of 0.2, and a constant size for 100 generations followed by a 2% growth for 150 generations. Y-STR databases were generated by randomly drawing Y-STR profiles from a Yfiler^TM Plus and PowerPlex® Y23 Y-STR population data set, respectively. In a population of 500,066 individuals, a database size of 0.5% of the population resulted in a Y-STR database match probability of ca. 6% and 10% for Yfiler^TM Plus and PowerPlex® Y23, respectively. Increasing the database size to 5% of the population resulted in a Y-STR match probability of ca. 41% and 54% for Yfiler^TM Plus and PowerPlex® Y23, respectively. When a Y-STR match was found in the database, the probability of one of the individuals with the matching profiles being related within five meioses to the trace donor was ca. 64% and 56% for Yfiler^TM Plus and PowerPlex® Y23, respectively, including the cases where the Y-STR profile originated from the donor. In this case, the closest relative in the database was found among the matching individuals with a probability of ca. 91%.