Commonwealth v. Lyons. Cybergenetics How TrueAllele Works (Part 2) Degraded DNA and Allele Dropout. Cybergenetics Webinar November, 2014

Transcription

1 How TrueAllele Works (Part 2) Degraded DNA and Allele Dropout Cybergenetics Webinar November, 2014 Mark W Perlin, PhD, MD, PhD Cybergenetics, Pittsburgh, PA Cybergenetics Commonwealth v. Lyons homicide: DNA mixture evidence 70% victim 30% other Victim's blood spatter pulsating spray from severed carotid artery Cybergenetics

2 Copy DNA with PCR Copy intact DNA Degraded DNA break Can t copy broken DNA Longer molecules copy less With degradation, a longer DNA molecule has a greater chance of having a break Cybergenetics

3 Break density & copy probability the break density p = Pr(break at a nucleotide) q = Pr(no break at a nucleotide) = 1 p q n = Pr(no break in n nucleotides) the copy probability Decay factor λ q = e lnq = e λ q n = e λn break density copy probability p q λ n 1/ DNA decay curve With degradation, a longer DNA molecule makes fewer DNA copies Observed DNA Pr( copy) = e λn signal decay λ =.0035 n 1/2 = 200 half length DNA size (100 bp) Cybergenetics

4 Reduced match information Degraded DNA Lower peak height Greater data variation More genotype uncertainty Diffused genotype probability Reduced match information Differential mixture degradation computer separates decay curves Affects mixture weight, genotypes Cybergenetics

5 posterior likelihood prior Pr( λ = v data) Pr( data λ = v) Pr( λ = v) Degraded mixture separation Victim DNA λ 1 Degraded other DNA λ 2 posterior ( ) Pr( data λ = v,w = w) Pr W = w Mixture weight separation Pr W = w data likelihood prior ( ) w 2 = 30% w 1 = 70% 30:70 vs 15:85 posterior ( ) Pr( data λ = v,w = w, X = x) Pr( X = x) Genotype separation Pr X = x data likelihood prior Cybergenetics

6 Information comparison Combined probability of inclusion of 42 thousand. (Human review with thresholds) A match between the suspect and the evidence is a billion times more probable than coincidence. (TrueAllele not solving for degradation) A match between the suspect and the evidence is a trillion times more probable than coincidence. (TrueAllele solving for degradation) Likelihood ratio 9 ban 12 ban Hierarchical mixture modeling k th contributor experiment data d k,1 d k,2 d k,n locus weights W k,1 W k,2 W k,n template weight W k prior probability W 0 Cybergenetics

7 Allele dropout 7 13? An allele present in a contributor genotype is not seen as a peak in the STR data. Genotype modeling Compare patterns: fixed data vs possible explanations Cybergenetics

8 Faithful data Genotype & LR 22 Imbalanced data Cybergenetics

9 Genotype & LR 1.5 Drop out data 7 13 Genotype & LR.35 Cybergenetics

10 How inclusion interpretation of DNA mixture evidence reduces identification information. Perlin MW, Dormer K, Hornyak J, Meyers T, Lorenz, W, American Academy of Forensic Sciences, Washington, DC, TrueAllele result, but no CPI Locus free of drop-out or imbalance? NO (0.346) N = 29 YES (0.664) N = 68 Count log(lr) An investigation of software programs using semi-continuous and continuous methods for complex DNA mixture interpretation. Coble M, Myers S, Klaver J, Kloosterman A, Leiden University, The Netherlands, 9th International Conference on Forensic Inference and Statistics, Separating truth from noise Simple two person mixture, 10% minor contributor log(lr) 6 3 TrueAllele ng (2 drop) 1/2 ng (8 drop) 1/4 ng (14 drop) An investigation of software programs using semi-continuous and continuous methods for complex DNA mixture interpretation. Coble M, Myers S, Klaver J, Kloosterman A, Leiden University, The Netherlands, 9th International Conference on Forensic Inference and Statistics, Separating truth from noise Simple two person mixture, 10% minor contributor log(lr) ng (2 drop) 1/2 ng (8 drop) 1/4 ng (14 drop) TrueAllele STRmix LRmix LabRetriever Threshold and drop parameter Cybergenetics

11 How TrueAllele Works Part 1, 16-Oct-2014 Genotype modeling and the likelihood ratio Part 2, 20-Nov-2014 Degraded DNA and allele dropout Part 3, 18-Dec-2014 Kinship, paternity and missing persons Cybergenetics