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Exploring the Capabilities of Mixture Interpretation using TrueAllele software

M.D. Coble and J.M. Butler, "Exploring the capabilities of mixture interpretation using TrueAllele software", International Society for Forensic Genetics, Vienna, Austria, 3-Sep-2011.


Slide presentation of Michael Coble's International Society for Forensic Genetics 2011 talk.

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DNA mixtures from sexual assault evidence or high volume crimes such as burglaries can be challenging for the forensic scientist to interpret. The problem is exacerbated when the evidence contains more than two contributors or is highly compromised due to DNA degradation. ISFG guidelines for mixture interpretation developed by Gill et al. (2006) have been widely accepted by the global forensic DNA community.

Laboratories have developed in-house spreadsheets or have purchased commercial software to rapidly calculate the multiple parameters necessary for mixture interpretation using the Clayton et al. (1998) method (e.g. peak height ratio, mixture ratio, etc.). Additionally, mixture software can be used to calculate statistics using either Random Man Not Excluded (e.g., combined probability of inclusion, CPI) or Combined Likelihood Ratio (CLR) to evaluate the data.

We have explored the capabilities of the TrueAllele software (Cybergenetics, Pittsburgh, PA, USA) by analyzing an assortment of two-, three-, and four-person mixtures. The software uses quantitative probabilistic genotype modeling of the data to form a joint LR statistic for the weight of the evidence. We examined a series of controlled two-person mixtures with differing contributor ratios and a broad range of allele sharing between the samples to determine the efficacy and reproducibility of the software. For complex mixtures, we examined the gain in information (measured by the log LR) compared to data evaluated with CPI and CLR statistics. We also examined the benefits of analyzing multiple replicate samples of low template DNA mixtures amplified with enhanced techniques (increased cycles and increased polymerase).