Mark W Perlin, PhD, MD, PhD Cybergenetics, Pittsburgh, PA, USA

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Presentation transcript:

Scientific Combination of DNA Evidence: A Handgun Mixture in Eight Parts Mark W Perlin, PhD, MD, PhD Cybergenetics, Pittsburgh, PA, USA Matt Greenhalgh Orchid Cellmark, Abingdon, UK Australia and New Zealand Forensic Science Society September, 2010 Cybergenetics © 2003-2010 Cybergenetics © 2007-2010

Handgun Evidence Cybergenetics © 2007-2010 2

DNA Swabs 6 5 4 3 Cybergenetics © 2007-2010 3

Duplicate Amplifications 6 5 4 3 Cybergenetics © 2007-2010 4

Data: 8 amplifications 3 4 5 6

Data: Locus D18 3 4 5 6

Human Review: Major 3 CPI: LR = 17,000 log(LR) = 4.23

Quantitative Likelihood Function Locus D18 85% major [15 19] 15% minor [13 13] 3

Quantitative Likelihood Function Locus D18 85% major [15 19] 15% minor [13 13] 3 Data Model

Computer Infers Major Genotype Definite genotype at every locus 100% probability at one allele pair Full RMP log(LR) = 16.72 trillion-fold increase 4.23  16.72 over CPI

Computer Infers Minor Genotype Uncertain genotype at every locus < 100% probability at multiple allele pairs Finds match strength log(LR) = 4.99 100,000-fold increase 0  4.99 over no result

Joint Likelihood Function 85% major [15 19] 15% minor [13 13] Locus D18 3

Greater match strength Infer Minor Genotype More genotype certainty with two amplifications of one PCR template Greater match strength log(LR) = 5.25 some increase 0  4.99  5.25

Mixture Weight: 4 templates 3 4 5 6

Joint Likelihood Function 3 Locus D18 major [15 19] minor [13 13] 4

Greater match strength Infer Minor Genotype Higher genotype certainty with two amplifications of two PCR templates Greater match strength log(LR) = 9.73 large increase 0  4.99  5.25  9.73

Joint Likelihood Function 3 Locus D18 4 major [15 19] minor [13 13] 5 6

Infer Minor Genotype Definite genotype at every locus two amplifications of four PCR templates Full match strength log(LR) = 12.39 reaches RMP 0  4.99  5.25  9.73  12.39

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Conclusions Two mixture genotype examples Major contributor Quantitative likelihood modeling that uses all of the data is more informative than qualitative threshold methods Minor contributor Combining DNA evidence using a joint likelihood function is more informative than separately examining data in isolation Trillion-fold increase in identification information