Is it possible to predict the origin of epithelial cells? – A comparison of secondary transfer of skin epithelial cells versus vaginal mucous membrane cells by direct contact

Abstract

In alleged sexual assault and rape cases, the focus has often been to collect samples from the victim's body, for detection of body fluids or skin cells from the offender. But in many cases intimate body samples from the perpetrator(s) can also be informative. However, in cases where the female victim claims vaginal penetration, the defendant may display an alternative explanation to the DNA findings, i.e. that the victim's skin cells has been secondarily transferred to his penis. We hypothesized that female DNA will be detected in a significantly greater amount on swabs from penis after intercourse than after secondary transfer by skin contact.Fourteen male-female couples were recruited to test the above hypothesis, by collecting penile swabs from 3 specified anatomical locations: Glans, shaft, and the coronal sulcus, after two different situations: Vaginal intercourse and secondary transfer of epithelial cells by skin contact. The results show that penile swabs following intercourse produce significantly higher DNA concentrations than after secondary transfer by skin contact. Our results, indicates which of the anatomical regions is best suited for sampling. The DNA profiling results show a preponderance of female profiles over male profiles following intercourse compared to secondary skin contact.Based on these data, it is possible to make a statistical model to distinguish between samples taken after intercourse and samples taken after secondary transfer by skin contact based on the amount of female DNA and mixture proportion (Mx) between female and male DNA in samples collected from penis swabs.

AnLRframework incorporating sensitivity analysis to model multiple direct and secondary transfer events on skin surface

“…Various methods of comparison have been used in the literature: including counting self vs non-self alleles, mixture proportions and DNA quantities recovered [2,9,[19][20][21][22][23]. However, mixtures commonly include background DNA from unknown contributors which interfere with positive identification of POI alleles.…”

(Draft) DNA Mixture Interpretation

“…Fourteen couples were recruited to test the 4258 ______________________________________________________________________________________________________ This publication is available free of charge from: https://doi.org/10.6028/NIST.IR.8351-draft hypotheses that female DNA was more likely to be detected following intercourse than social contact. The authors report the possibility of using their data to make a statistical model to distinguish "between samples taken after intercourse and samples taken after secondary transfer by skin contact" (Bouzga et al 2020).…”

Evaluating forensic DNA evidence: Connecting the dots

“…Second, such variation, specifically differences in the shedder status of individuals, has been shown to impact the probabilities of DNA transfer to worn T‐shirts in a simulated attack scenario (Fonneløp et al, 2017). These limitations therefore suggest that DNA quantity alone may not be sufficiently informative to be relied upon for evaluations given activity level propositions, though it may be useful to consider alongside other case specific details (Bouzga et al, 2020; Samie et al, 2020; Taylor et al, 2017). However, it has been proposed that the relative proportions within mixed DNA profiles may be more informative, for example, with respect to distinguishing handlers of an item (Butcher et al, 2019).…”