Background levels of male DNA in the vaginal cavity

Abstract

In alleged sexual assault investigations, the detection of male DNA on female intimate swabs can be established through Y-STR profiling. However, in cases where the defendant lives in the same dwelling as the complainant, the defendant may argue that his DNA was transferred through normal social contact, and dispute that intimate contact occurred. This study aimed to determine whether or not male DNA could be detected in samples collected from the vaginal cavity after everyday activities. A total of 300 samples from eight individuals were subjected to DNA quantification and Y-STR amplification. Three donors reported sexual contact during the time of sampling, hence the detection of male-specific DNA in post-coital samples was concomitantly evaluated. The majority of samples (n=237, 79%) were undetermined for male DNA by real-time PCR quantification. No Y-STR profiles having three or more alleles were obtained in these samples, with the exception of three samples which displayed three alleles each. One sample was collected six days post-coital, one sample likely presented artefacts, and one exhibited two alleles at the same locus. Male-specific quantification values were obtained for the remaining 63 samples (21%), however only 31 samples displayed alleles at three or more loci. Twenty-three of the 63 samples showed inhibition during real-time PCR quantification, and the majority of these exhibited no Y-STRs. A further three samples were excluded from the dataset due to possible contamination. The maximum confirmed time frame for obtaining a full Y-STR profile was six days post-coital, but may be longer in some donors. Overall concordance between DNA quantification and Y-STR amplification was 85.8%. This study demonstrates that obtaining a Y-STR profile comprising three or more alleles from a vaginal swab collected from a woman who has not participated in recent intimate sexual activities is not indicated. This suggests that there is unlikely to be detectable adventitious transfer of DNA from male individuals living in the same household as a female donor. However, approximately one in seven samples may display 1-2 unexpected allelic peaks (<400 RFU). These are unlikely to originate from the male partner or cohabitant of the donor and should be interpreted with extreme caution, and with every effort attempted to replicate results before a conclusion is derived about their relevance. These findings support the significance of male DNA profiles consisting of more than three alleles obtained from female intimate samples during investigations of alleged sexual assault.

Keywords: Background DNA; DNA transfer; Forensic; Vagina; Y-STR.