An investigation into the background levels of DNA and saliva typically detectable on penises

Sexual assault offences are a global issue prominent across various communities, often involving vaginal, oral or anal penetration with a penis. Challenges are often presented to forensic scientists in court regarding how DNA got to the collection point, when evaluating penile samples taken in association with sexual assault offences. These challenges are the result of a lack of research addressing prevalence and persistence of self and non-self-DNA on a penis following everyday activities. This research investigated the prevalence of self- and non-self-DNA on penises following everyday activities, with no imposed conditions. Forty-one adult participants self-collected samples from the head and shaft of their penis, using wet and dry swabbing. Reference samples from the donors and volunteering co-habitants or partners were collected to establish the source of self- and non-self-DNA detected. The RSID^TM-Saliva immunochromatographic test was used to assess the prevalence of saliva on the shaft samples prior to DNA analysis. Profiles were produced from all samples using standard methodologies. Non-self-DNA was detected in 36% of head samples and 64% of shaft samples. All mixed profiles revealed two contributors, except one three-person mixture from a shaft sample. The donor was the major contributor in all but thirteen samples. Donor age, the presence of a co-habitant or partner, sample timing, time since sexual activity and the presence of saliva were found to significantly affect the total, self- and non-self-DNA amounts detected. These findings produce valuable data for the evaluation of self- and non-self-DNA amounts detectable on penises following everyday activities, considering both intimate and non-intimate contact.