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DNA Mixture Genotyping by Probabilistic Computer Interpretation of Binomially-Sampled Laser Captured Cell Populations: Combining Quantitative Data for Greater Identification Information

NCJ Number
Sci Justice Dated: 2012
Date Published
0 pages

This study tested the hypothesis that LCM-mediated isolation of multiple groups of cells (‘binomial sampling’) from two-person DNA admixtures would create separate cell sub-populations with differing constituent weight ratios.


The project also predicted that interpreting the resulting DNA profiling data by the quantitative computer-based TrueAllele® interpretation system would result in an efficient recovery of the constituent genotypes due to newfound abilities to compute a maximum LR from sub-samples with skewed weight ratios, and to jointly interpret all possible pairings of sub-samples using a joint likelihood function. Two-person DNA admixtures are frequently encountered in criminal cases and their interpretation can be challenging, particularly if the amount of DNA contributed by both individuals is approximately equal. Due to an inevitable degree of uncertainty in the constituent genotypes, reduced statistical weight is given to the mixture evidence compared to that expected from the constituent single source contributors. The goal of mixture analysis is to precisely discern the constituent genotypes. The current study posits a novel strategy to accomplish this. As a proof of concept, 10 separate cell samplings of size 20 recovered by LCM from each of two 1:1 buccal cell mixtures were DNA-STR profiled using a specifically developed LCN methodology, with the data analyzed by the TrueAllele® Casework system. In accordance with the binomial sampling hypothesis, the sub-samples exhibited weight ratios that were well dispersed from the 50-percent center value (50 ± 35 percent at the 95-percent level). The maximum log(LR) information for a genotype inferred from a single 20 cell sample was 18.5 ban, with an average log(LR) information of 11.7 ban. Co-inferring genotypes using a joint likelihood function with two sub-samples essentially recovered the full genotype information. This research demonstrated that a similar gain in genotype information can be obtained with standard (28-cycle) PCR conditions, using the same joint interpretation methods. The article concludes with a discussion of the implications of this work for routine forensic practice. (publisher abstract modified)

Date Published: January 1, 2012