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A mixed DNA profile controversy revisited |
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| Authors: | Tim Kalafut Simone Pugh Peter Gill Sarah Abbas Marie Semaan Issam Mansour James Curran Jo-Anne Bright Tacha Hicks Richard Wivell John Buckleton |
| Affiliation: | 1. California Department of Justice, Redding, California, USASearch for more papers by this authorPeter Gill PhD, Sarah Abbas MSc, Marie Semaan MSc, Issam Mansour PhD, James Curran PhD, Jo-Anne Bright PhD, Tacha Hicks PhD, Richard Wivell BSc (hons), John Buckleton DSc,
First published: 14 October 2021
Previous to this submission, the problem described in this paper was presented by three of the authors (Semaan, Abbas, Mansour) in the Journal of Forensic Research (2020);2. at the 21st Triannual Meeting of the International Association of Forensic Sciences, August 21–25, 2017, in Toronto, Canada;3. and at the 73rd Annual Scientific Meeting of the American Academy of Forensic Sciences, February 15–19, 2021, held virtually. Semaan, Abbas, and Mansour have agreed to join the rest of the authors in acknowledging the initial presentations were incorrect, and this paper describes a more appropriate solution. |
| Abstract: | Semaan et al. (J Forensic Res, 2020, 11, 453) discuss a mock case “where eight different individuals P1 through P8] could not be excluded in a mixed DNA analysis. Even though … expert DNA mixture analysis software was used.” Two of these are the true donors. The LRs reported are incorrect due to the incorrect entry of propositions into LRmix Studio. This forced the software to account for most of the alleles as drop-in, resulting in LRs 60–70 orders of magnitude larger than expected. P1, P2, P4, P5, and P8 can be manually excluded using peak heights. This has relevance when using LRmix which does not use peak heights. We extend the work using the same two reference genotypes who were the true contributors as Semaan et al. (J Forensic Res, 2020, 11, 453). We simulate three two-donor mixtures with peak heights using these two genotypes and analyze using STRmix?. For the simulated 1:1 mixture, one of the non-donors’ LRs supported him being a contributor when no conditioning was used. When considered in combination with any other potential donors (i.e., with conditioning), this non-donor was correctly eliminated. For the 3:1 mixture, all results correctly supported that the non-donors were not contributors. The low-template 4:1 mixture LRs with no conditioning showed support for all eight profiles as donors. However, the results from pair-wise conditioning showed that only the two ground truth donors had LRs supporting that they were contributors to the mixture. We recommend the use of peak heights and conditioning profiles, as this allows better sensitivity and specificity even when the persons share many alleles. |
| Keywords: | DNA mixtures exhaustive propositions forensic DNA analysis LRmix probabilistic genotyping relatives STRmix™ |
