Estimating the number of contributors to two-, three-, and four-person mixtures containing DNA in high template and low template amounts

Abstract

AimTo develop guidelines to estimate the number of contributors to two-, three-, and four-person mixtures containing either high template DNA (HT-DNA) or low template DNA (LT-DNA) amounts.MethodsSeven hundred and twenty-eight purposeful two-, three-, and four-person mixtures composed of 85 individuals of various ethnicities with template amounts ranging from 10 to 500 pg were examined. The number of alleles labeled at each locus and the number of labeled different and repeating alleles at each locus as well over all loci for 2 HT-DNA or 3 LT-DNA replicates were determined. Guidelines based on these data were then evaluated with 117 mixtures generated from items handled by known individuals.ResultsThe number of different alleles over all loci and replicates was used to initially categorize mixtures. Ranges were established based on the averages plus and minus 2 standard deviations, and to encompass all observations, the maximum and the minimum values. To differentiate samples that could be classified in more than one grouping, the number of loci with 4 or more repeating or different alleles, which were specific to three- and four-person mixtures, were verified. Misclassified samples showed an extraordinary amount of allele sharing or stutter.ConclusionsThese guidelines proved to be useful tools to distinguish low template and high template two-, three-, and four-person mixtures. Due to the inherent higher probability of allele sharing, four-person mixtures were more challenging. Because of allelic drop-out, this was also the case for samples with very low amounts of template DNA or extreme mixture ratios.

Likelihood ratio statistics for DNA mixtures allowing for drop-out and drop-in

“…The expected drop-out of a minor contributor to a three-person mixture with dissimilar ratios, as seen in the first case example, is higher than that of one of the contributors to a two-person mixture with similar ratios, as observed in the second case. These differences highlight the importance of determining the number of contributors to a mixture for both high and low template DNA samples [4,5].…”

Humans Are Animals, Too: Critical Commonalities and Differences Between Human and Wildlife Forensic Genetics

“…STRs are also used to determine the minimum number of individuals/contributors present in submitted evidence . In human forensics, the number of contributors is primarily of importance in calculation of statistical weight for mixed profiles.…”

Forensic DNA Analysis

“…While it is possible to identify each allele manually, it is more efficient uted to this DNA, but there will always be a level of uncertainty about how many individuals' DNA is present. It is possible to state that for up to four alleles at least two people must have contributed, and three people for up to six alleles, but the fact that alleles are shared amongst the human population can cause low allele counts especially for less informative loci (Paoletti et al 2005;Buckleton et al 2007;Perez et al 2011). Assumptions about the number of contributors to an unknown mixture are an important part of mixture interpretation and the statistical evaluation of a mixture comparison.…”