DNA Identification: Mixture Interpretation

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Presentation transcript:

DNA Identification: Mixture Interpretation Mark W Perlin, PhD, MD, PhD Cybergenetics, Pittsburgh, PA Continuing Legal Education Allegheny County Courthouse March, 2011 Cybergenetics © 2003-2011 Cybergenetics © 2007-2010

Fingernail DNA Evidence 93.3% victim + 6.7% DNA component Cybergenetics © 2007-2010

DNA Evidence • DNA from under victim's fingernails (Q83) • two contributors to DNA mixture • 93.3% victim & 6.7% unknown • 1,000 pg DNA in 25 ul • STR analysis with ProfilerPlus®, Cofiler® • know victim contributor genotype (K53) • TrueAllele® computer interpretation (using genotype addition method) infer unknown contributor genotype • only after having inferred unknown, compare with suspect genotype (K2) Cybergenetics © 2007-2010

Three DNA Match Statistics Score Method 13 thousand inclusion 23 million subtraction 189 billion addition • Why are there different match results? • How do mixture interpretation methods differ? • What results should be presented in court? Cybergenetics © 2007-2010

DNA Mixture Data Some amount of contributor A genotype Mixture data with genotypes of contributors A & B + PCR Other amount of contributor B genotype Cybergenetics © 2007-2010

D7S820 victim other Cybergenetics © 2007-2010

Quantitative Mixture Interpretation Step 1: infer genotype • consider every possible allele pair • compare pattern with DNA data • Rule: better fit's more likely it high likelihood genotype allele pair probability a,b a,c ? low likelihood b,d c,d … ? Cybergenetics © 2007-2010

Quantitative Genotype victim genotype data 8 12 other genotype + 10, 13 ? 10 13 victim other genotype pattern = 8 10 12 13 Cybergenetics © 2007-2010

Quantitative Information At the suspect's genotype, identification vs. coincidence? after (evidence) Prob(suspect matches evidence) 100% data = Prob(suspect matches population) 1.72% before (population) = 58 Step 2: match genotype high probability retains LR information Cybergenetics © 2007-2010

Qualitative Manual Review Step 1: infer genotype Rule: every pair gets equal share genotype allele pair likelihood listed allele pairs are all assigned the same likelihood a,a a,b ? a,c a,d … ? Cybergenetics © 2007-2010

Qualitative Genotype • apply threshold • discard peak data • make all the same • 10 possible pairs • equal likelihood • diffuse probability • lose match strength cutoff 8 10 12 13 8 10 12 13 8 10 Cybergenetics © 2007-2010

Qualitative Information At the suspect's genotype, identification vs. coincidence? after (evidence) Prob(suspect matches evidence) 4.42% data = Prob(suspect matches population) 1.72% before (population) = 2.57 Step 2: match genotype lower probability loses LR information Cybergenetics © 2007-2010

DNA Match Comparison Joint genotype product rule, combines loci 13 thousand (4) 23 million (7) 189 billion (11) Cybergenetics © 2007-2010

Computer Preserves Information 13.26 Cybergenetics © 2007-2010

Human Review Loses Information 13.26 6.24 7.03 Cybergenetics © 2007-2010