Contacting surfaces are rarely DNA free: Another look at transfer when both surfaces have DNA

Understanding DNA transfer, persistence, prevalence, and recovery (TPPR) has become increasingly crucial in forensic investigations. Most DNA transfer studies have focused on one-way transfer, neglecting bi-directional exchange. This study examines two-way transfer of blood and touch DNA between plastic and cotton substrates under passive and friction conditions, using methods from previous unidirectional transfer experiments. Our investigation established statistically significant correlations between bi-directional DNA transfer rates and manner of contact but not substrate type and type of biological material. While, in general, no significant differences were noted between bi-directional and unidirectional transfer rates, significant differences were observed for certain variable combinations where unidirectional transfer resulted in higher transfer rates. This research provides baseline data on bi-directional DNA transfer under semi-controlled conditions, complementing existing unidirectional transfer knowledge. Understanding bi-directional transfer is crucial for accurately modelling DNA transfer events in forensic scenarios, especially for activity level evaluations.