Trace DNA refers to latent biological material deposited on an object or individual through direct or indirect contact, and it is often characterized by low template amounts and mixed samples. This study investigated the persistence and two-way transfer of DNA under fingernails and on forearms following simulated physical aggression scenarios. Three-time intervals were selected, and nine pairs of male and female participants were recruited to act as the mock offender or mock victim. The mock victim scratched the mock offender's forearm 10 times to simulate a physical struggle. Samples were collected under the mock victim’s fingernails and on the mock offender’s forearm immediately after the event (T0), and at 3 hours (T3) and 5 hours (T5) post-assault. Subungual samples were collected using 4N6FLOQSwabs® (subungual shape), while forearm samples were recovered using the cotton double-swab technique. DNA extraction was performed using the QIAamp DNA Investigator kit in combination with QIAshredder columns, followed by quantification with the Quantifiler™ Trio DNA kit. PCR amplification was performed using the GlobalFiler™ and Yfiler™ Plus PCR kits, and amplicons separation was performed on the Applied Biosystems SeqStudio platform. STR profiles were analyzed with the GeneMapper™ ID-X v1.6, and mixture DNA proportions were estimated manually and using DNAStatistX probabilistic genotyping software. A total of 168 trace samples were processed. In fingernail samples, the average DNA contribution of the mock offender significantly decreased over time, dropping from over 20% at T0 to 8% at T3 and 4% at T5. Conversely, the mock victim was the major contributor with an average profile proportion increasing from approximately 76% to 90% over the five-hour period. In forearm samples, the mock offender was the major DNA donor, while the victim’s average DNA contribution dropped from approximately 17% at T0 to 2% at T5. Across the three-time intervals, DNA contributions from unknown sources accounted for approximately 4% to 9% of the total STR profile. Probabilistic genotyping analyses using DNAStatistX are in progress. Overall, this study provides valuable insights into DNA persistence and transfer dynamics in physical assault scenarios, supporting the evaluation of forensic biology findings at the activity level.

Authors

• Fabio Oldoni (Arcadia University, United States of America)
• Jean Poulson (Arcadia University, United States of America)
• Anna Franzoni (Copan Italia, Italy)
• Sarah Gino (Università degli Studi del Piemonte Orientale and Università di Torino, Italy)
• Daniele Podini (The George Washington University, United States of America)