Murder in McKeesport October 25, 2008 Tamir Thomas.

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Presentation transcript:

Murder in McKeesport October 25, 2008 Tamir Thomas

Biological evidence

DNA analysis PowerPlex ® 16 STR Partial DNA profiles obtained for both the gun and the cap

Human review results Black420 Caucasian500 Hispanic470 Black5.7 quadrillion Caucasian9.3 quadrillion Hispanic1.8 quadrillion Match to Leland Davis

Prosecutor question What is the true match information of the evidence to the suspect?

TrueAllele operator Replicate computer runs for each item Three unknown mixture contributors Degraded DNA was considered STR evidence data.fsa genetic analyzer files Evidence genotypes probability distributions

TrueAllele report Genotype probability distributions Evidence genotypeSuspect genotype Population genotype Likelihood ratio (LR) DNA match statistic Perlin MW. Explaining the likelihood ratio in DNA mixture interpretation. Promega's Twenty First International Symposium on Human Identification, 2010; San Antonio, TX.

TrueAllele DNA match Black18.6 billion Caucasian12.1 billion Hispanic3.37 billion Black89 quadrillion Caucasian420 quadrillion Hispanic73.5 quadrillion LR match to Leland Davis

Trial preparation case report direct examination PowerPoint slides background reading other questions

TrueAllele reports 2 & 3 2. Is Dominick Haynes in the DNA evidence? Answer: No – million factor against. 3. Is anyone else in both DNA evidence items? Answer: No – Leland Davis is the only one.

TrueAllele precedent Commonwealth of Pennsylvania v. Kevin James Foley Superior Court, 2012 No pretrial admissibility hearing

Computer Interpretation of Quantitative DNA Evidence Commonwealth v Leland Davis August, 2012 Pittsburgh, Pennsylvania Mark W Perlin, PhD, MD, PhD Cybergenetics, Pittsburgh, PA Cybergenetics ©

DNA genotype 8, ACGT A genetic locus has two DNA sentences, one from each parent. 9 locus Many alleles allow for many many allele pairs. A person's genotype is relatively unique. mother allele father allele repeated word An allele is the number of repeated words. A genotype at a locus is a pair of alleles.

DNA evidence interpretation Evidence item Evidence data LabInfer Evidence genotype Known genotype 10, 50% 11, 30% 12, 20% 10, 12 Compare

Computers can use all the data Quantitative peak heights at locus vWA peak size peak height

People may use less of the data Over threshold, peaks are labeled as allele events Threshold Under threshold, alleles vanish All-or-none allele peaks, each given equal status

How the computer thinks Consider every possible genotype solution Explain the peak pattern Better explanation has a higher likelihood One person's allele pair Another person's allele pair A third person’s allele pair

Evidence genotype Objective genotype determined solely from the DNA data. Never sees a suspect. 2% 91% 3% 1%

DNA match information Probability(evidence match) Probability(coincidental match) How much more does the suspect match the evidence than a random person? 8x 11% 91%

Match information at 15 loci

Is the suspect in the evidence? A match between the handgun and Leland Davis is: 18.6 billion times more probable than a coincidental match to an unrelated Black person 12.1 billion times more probable than a coincidental match to an unrelated Caucasian person 3.37 billion times more probable than a coincidental match to an unrelated Hispanic person

Is the suspect in the evidence? A match between the baseball cap and Leland Davis is: 89 quadrillion times more probable than a coincidental match to an unrelated Black person 420 quadrillion times more probable than a coincidental match to an unrelated Caucasian person 73.5 quadrillion times more probable than a coincidental match to an unrelated Hispanic person

Is anyone else in both items of evidence? There is no indication that any person, other than Leland Davis, contributed their DNA to both items of evidence.

Verdict Leland Davis was convicted of third degree murder and weapons charges in the 2008 McKeesport slaying of Tamir Thomas. His sentence was 23 years.

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