Forensic analysis of fingernail debris after a scratch experiment and its applications in violence against women investigation

Violent acts against women, mainly perpetrated by males, comprise close and forceful physical contact between the victim and the assailant, which may lead to the accumulation of genetic material under the protected fingernail hyponychium area of the victim. Thus, the collection of fingernail debris may provide biological evidence for the evaluation of the criminal circumstances. The aim of the study was to explore the mechanisms of male DNA transfer, persistence, and recovery under fingernails of women following a mock physical assault, as well as the prevalence of male cellular material under normal, non-criminal circumstances. Forty-eight female volunteers were enrolled and asked to scratch male volunteers’ forearms, mimicking a defensive action during a physical assault. Scratching took place after the cleaning of the hyponychial area. Targeted sampling of fingernail debris was performed at regular time intervals after the scratching to explore the persistence of male cellular material over time. The prevalence of male DNA under volunteers’ fingernails in normal daily life was investigated by collecting volunteers’ fingernail samples two weeks before the scratch. For the recovery study, two different sampling strategies were employed, involving either cumulative or finger by finger sample collection. The transfer, persistence, prevalence and recovery of male DNA under volunteers’ fingernails were investigated through a Y-STR typing approach and complemented by autosomal STR profiling of relevant samples. The results confirm that the amount of male DNA under fingernails after scratching rapidly decreases over time. Although an increased time lag between scratching and sampling increases the chances to observe genetic profiles acquired from casual, non-crime related transfers, relevant profiles were obtained from samples collected even 24 h after scratching. These findings suggest that such data need be interpreted with caution and an alternative, non-crime related scenario should be considered.