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A mixed DNA profile controversy revisited

NCJ Number
Journal of Forensic Sciences Volume: Online Dated: October 2021
Tim Kalafut ; Simone Pugh ; Peter Gill ; Sarah Abbas ; Marie Semaan ; Issam Mansour; James Curran ; Jo-Anne Bright ; Tacha Hicks; Richard Wivell ; John Buckleton
Date Published
October 2021

This article reports on an extension of the authors previous work, using the same two reference genotypes who were the true contributors as Semaan et al. (J Forensic Res, 2020, 11, 453), simulating three two-donor mixtures with peak heights, using these two genotypes and using STRmix™ in the analysis. 


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. For the simulated 1:1 mixture in the current study, 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. The authors recommend the use of peak heights and conditioning profiles, since this enables better sensitivity and specificity even when the persons share many alleles. (publisher abstract modified)

Date Published: October 1, 2021