title: An assessment of the information content of likelihood ratios derived from complex mixtures.
abstract: With the increasing sensitivity of DNA typing methodologies, as well as increasing awareness by law enforcement of the perceived capabilities of DNA typing, complex mixtures consisting of DNA from two or more contributors are increasingly being encountered. However, insufficient research has been conducted to characterize the ability to distinguish a true contributor (TC) from a known non-contributor (KNC) in these complex samples, and under what specific conditions. In order to investigate this question, sets of six 15-locus Caucasian genotype profiles were simulated and used to create mixtures containing 2-5 contributors. Likelihood ratios were computed for various situations, including varying numbers of contributors and unknowns in the evidence profile, as well as comparisons of the evidence profile to TCs and KNCs. This work was intended to illustrate the best-case scenario, in which all alleles from the TC were detected in the simulated evidence samples. Therefore the possibility of drop-out was not modeled in this study. The computer program DNAMIX was then used to compute LRs comparing the evidence profile to TCs and KNCs. This resulted in 140,000 LRs for each of the two scenarios. These complex mixture simulations show that, even when all alleles are detected (i.e. no drop-out), TCs can generate LRs less than 1 across a 15-locus profile. However, this outcome was rare, 7 of 140,000 replicates (0.005%), and associated only with mixtures comprising 5 contributors in which the numerator hypothesis includes one or more unknown contributors. For KNCs, LRs were found to be greater than 1 in a small number of replicates (75 of 140,000 replicates, or 0.05%). These replicates were limited to 4 and 5 person mixtures with 1 or more unknowns in the numerator. Only 5 of these 75 replicates (0.004%) yielded an LR greater than 1,000. Thus, overall, these results imply that the weight of evidence that can be derived from complex mixtures containing up to 5 contributors, under a scenario in which no drop-out is required to explain any of the contributors, is remarkably high. This is a useful benchmark result on top of which to layer the effects of additional factors, such as drop-out, peak height, and other variables.
authors: Department of Ecology and Evolutionary Biology, University of California, Los Angeles, 621 Charles E. Young Dr. South, Los Angeles, CA 90095-1606, USA.
authors: Clare D
authors: CD
authors: Marsden
authors: Forensic DNA Consultant, 650 Castro Street, Suite 120-404, Mountain View, CA, 94041, USA.
authors: Norah
authors: N
authors: Rudin
authors: Department of Criminal Justice Administration, California State University, East Bay, 4069 Meiklejohn Hall, 25800 Carlos Bee Boulevard, Hayward, CA 94542, USA.
authors: Keith
authors: K
authors: Inman
authors: Department of Ecology and Evolutionary Biology, University of California, Los Angeles, 621 Charles E. Young Dr. South, Los Angeles, CA 90095-1606, USA. Electronic address: klohmueller@ucla.edu.
authors: Kirk E
authors: KE
authors: Lohmueller
mesh_terms: D000483:Alleles; D045424:Complex Mixtures / Q000032:analysis* / Q000235:genetics; D003198:Computer Simulation; D004247:DNA / Q000032:analysis* / Q000235:genetics; D016172:DNA Fingerprinting / Q000379:methods* / Q000706:statistics & numerical data; D053803:Forensic Genetics / Q000379:methods* / Q000706:statistics & numerical data; D005838:Genotype; D006801:Humans; D016013:Likelihood Functions; D018895:Microsatellite Repeats
chemical_list: D045424:Complex Mixtures; D004247:DNA
keywords: Complex mixture; Forensic DNA; Known non-contributor; Likelihood ratio; Simulation; Statistics