Transfer and persistence factors of male DNA following heterosexual intercourse

Abstract

The detection and identification of spermatozoa is highly sought in sexual assault casework. Presence irrefutably demonstrates a sexual element to the alleged incident, establishing connections between individuals, providing source and sub-source level information that can holistically contextualise the facts of the case. However, the recovery of spermatozoa is inherently variable, with factors including transfer and persistence paramount to understanding and qualifying its presence or absence. This research aimed to realistically mimic factors that could influence the transfer and persistence of DNA from male to female following heterosexual intercourse with ejaculation, in the absence of a condom. Both novel and current conventional strategies for body fluid identification were employed, focussing on time since intercourse (TSI) and natural vaginal drainage mechanisms. For the purpose of informing forensic treatment of sexual assault casework in the future. Specifically, to provide guidance for the interpretation of DNA profiling results obtained from sexual assault scenarios routinely encountered during a criminal investigation, and best exhibit selection and strategy of samples collected in early evidence and medical examinations kits. The first objective investigated the value of a vulval gauze wipe as a sample collection method in an early evidence collection capacity. Male DNA was detected on vulval gauze wipes up to three days following a coital event. Demonstrating that vulval gauze wipes can be a good noninvasive source of evidence and have an attractive appeal for inclusion in an early evidence kit. Particularly, in instances where the individual for logistical, cultural, religious or alternative personal reasons are unable to undertake a full medical examination. The second objective explored natural drainage mechanisms of the vagina, and whether male DNA would be detectable on unlaundered underwear at extended time intervals following sexual intercourse. Male DNA was detectable before and after wear on underwear samples for up to 21 days post sexual intercourse. However, the results demonstrated variability. An increased likelihood of obtaining mixed profiles with the action of minitaping, advocates caution in the interpretability of results obtained for this sample type. Albeit, predominantly the corresponding male partner could qualify the majority of samples, depending on the person. Suggesting in cases where the assailant is not known to the individual, and no contact can be established, extended underwear samples would be profitable. The final objective evaluated time since intercourse (TSI) in the form of self-collected vaginal swabs to actively challenge the applicability of currently accepted time frames for medical examination exhibits to be routinely tested and used as evidence. Probative Y STR results were obtained at 14 days post-coitus. Whilst the sample size was limited, no other study has investigated such an extended TSI. With the sensitivity of contemporary technologies, currently held presumptions appear to be outdated and beg revision. A novel and exploratory focus considered the feasibility of coupling Y-STR profiling with LCN methodology (Y-LCN). With further research, optimisation and validation, this concept demonstrates real promise as a candidate for recovering the typical low and trace level DNA associated with sexual assault casework samples.