Male DNA on underwear: Can Y- STR results help discriminate activities?

This research is based on a case in which the complainant alleged that the defendant had removed her underwear, while the defendant claimed they had only danced together. To assist in resolving this issue, DNA was collected from the sides of the complainant's underwear and Y-chromosome STR (Y-STR) analysis performed using the PowerPlex® Y23 System. As no published research were available on DNA recovered from the sides of underwear given the two alleged activities, 60 experiments were conducted to generate specific data on the expected results given underwear removal and given dancing. The underwear removal was simulated using a dummy, while the dancing experiments involved real participants. The paper discusses a principled approach to representing prior knowledge and combining it with the experimental observations when assigning probabilities to DNA recovery outcomes under the competing activity level propositions. To explore whether our LRs were calibrated, we applied our model to published ground truth studies. For the presence of a full Y-STR profile PP23, we show that our LRs in the order of 50 are robust. The prevalence of DNA from cohabitants was also studied. In our experiments, taking the cohabitant's Y-STR profile into account did not impact the value of the observations given activity level propositions. However, having access to the cohabitant reference sample increases the number of interpretable DNA mixtures and can provide relevant information. We therefore recommend requesting the cohabitant DNA reference samples, particularly in cases involving balanced Y-STR mixtures.