Detection of offender DNA following skin-to-skin contact with a victim

Abstract

Forensic medical examiners are frequently asked to examine persons who claim to have been assaulted. If the suspect is unknown and there has been contact between his or her skin and the alleged victim, there is an expectation that DNA can be collected from the victim's skin. In this way, the retrieved suspect DNA from the skin of the victim can be used to support the proposition that places the suspect at the scene. This study investigated the transfer and persistence of offender DNA on a victim following a mock physical assault and further transfer of the offender DNA onto clothing worn over the assaulted area. Mock assault scenarios were conducted with the offender using medium pressure without friction and heavy pressure with friction on the wrist and upper arm of the victim. Samples were taken at either 0 h, 3 h or 24 h post assault, with 18 assault scenarios conducted at each time point. Samples from the victim's skin where the assault had taken place and, where applicable, clothing worn over the assaulted area were collected using the double swabbing method. Offender DNA was observed on the victim's skin at 0 h in the majority of samples with a higher transfer rate observed where heavy pressure and friction was used. The presence of the offender profile was detected in the samples collected at 3 h and 24 h, with 25% and 12% of samples respectively producing a LR in support of Hp. Transfer of offender DNA to clothing worn over the assaulted area was demonstrated, with 19% of samples producing a LR in support of Hp. Samples taken from clothing were complex mixtures, with 23% of samples producing four or more person mixtures. Indirect transfer of other DNA was also observed, with background DNA on the offender's skin observed in the clothing swabs of the victim on a number of occasions. Our data suggests that sampling from clothing worn over the assaulted area may be an additional or better avenue for the recovery of offender DNA post assault where there has been significant time between assault and sampling.

Keywords: Background DNA; Forensic DNA analysis; Persistence; Skin; Trace DNA; Transfer.