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Probabilistically determining the cellular source of DNA derived from differential extractions in sexual assault scenarios

  • Duncan Taylor
    Correspondence
    Correspondence to: Forensic Science South Australia, 21 Divett Place, Adelaide, SA 5000, Australia.
    Affiliations
    Forensic Science South Australia, 21 Divett Place, Adelaide, SA 5000, Australia

    School of Biological Sciences, Flinders University, GPO Box 2100, Adelaide, SA 5001, Australia
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      Highlights

      • A Bayesian network is described to help address questions regarding the cellular source of DNA from differential extractions.
      • Sperm and epithelial scoring from microscopy and DNA profiling results are considered within the Bayesian Network.
      • Sensitivity analysis shows that in many instances it is possible to provide strong levels of support to an individual having donating sperm (or not) to a trace.
      • Further experimental work is suggested that will assist in assigning probabilities to nodes within the network, or to refine the network structure.

      Abstract

      Sexual assault cases are the type of case that often produces questions about the cellular source of DNA. In these cases multiple findings of microscopy, DNA profiling and presumptive testing need to be considered when addressing source level propositions. In this work, I consider a line of questioning that has been raised a number of times in the recent past, where in court it was disputed that low levels of sperm seen on a microscope slide were the cellular source of the male DNA profile component generated from the sperm fraction of a differential DNA extraction. I demonstrate how the cell scoring results and DNA profiling results can be considered together, in helping address this source level question through the use of Bayesian Networks.

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