murder in mckeesport october 25, 2008 tamir thomas
TRANSCRIPT
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Murder in McKeesportOctober 25, 2008 Tamir Thomas
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Biological evidence
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DNA analysis
PowerPlex® 16 STR
Partial DNA profilesobtained for boththe gun and the cap
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Human review results
Black 420Caucasian 500Hispanic 470
Black 5.7 quadrillionCaucasian 9.3 quadrillionHispanic 1.8 quadrillion
Match to Leland Davis
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Prosecutor question
What is the truematch information of the evidenceto the suspect?
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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
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TrueAllele reportGenotype probability distributions
Evidence genotype Suspect 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.
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TrueAllele DNA match
Black 18.6 billionCaucasian 12.1 billionHispanic 3.37 billion
Black 89 quadrillionCaucasian 420 quadrillionHispanic 73.5 quadrillion
LR match to Leland Davis
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Trial preparation
• case report• direct examination• PowerPoint slides• background reading• other questions
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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.
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TrueAllele precedentCommonwealth of Pennsylvania v. Kevin James Foley
Superior Court, 2012
No pretrial admissibility hearing
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Computer Interpretation of Quantitative DNA Evidence
Commonwealth v Leland DavisCommonwealth v Leland DavisAugust, 2012August, 2012
Pittsburgh, PennsylvaniaPittsburgh, Pennsylvania
Mark W Perlin, PhD, MD, PhDMark W Perlin, PhD, MD, PhDCybergenetics, Pittsburgh, PACybergenetics, Pittsburgh, PA
Cybergenetics © 2003-2012Cybergenetics © 2003-2012
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DNA genotype
8, 91 2 3 4 5 6 7 8
ACGT
1 2 3 4 5 6 7 8
A genetic locus has two DNA sentences,one from each parent.
9
locus
Many alleles allow formany many allele pairs. A person's genotype is relatively unique.
motherallele
fatherallele
repeated word
An allele is the numberof repeated words.
A genotype at a locusis a pair of alleles.
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DNA evidence interpretationEvidence
itemEvidence
data
Lab Infer
10 11 12
Evidence genotype
Known genotype
10, 12 @ 50%11, 12 @ 30%12, 12 @ 20%
10, 12
Compare
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Computers can use all the dataQuantitative peak heights at locus vWA
peak size
peakheight
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People may use less of the dataOver threshold, peaks are labeled as allele events
Threshold
Under threshold, alleles vanish
All-or-none allele peaks,each given equal status
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How the computer thinksConsider every possible genotype solution
Explain thepeak pattern
Betterexplanationhas ahigher likelihood
One person's allele pair
Another Another person's person's allele pairallele pair
A third personA third person’’s s allele pairallele pair
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Evidence genotypeObjective genotype determined solely from
the DNA data. Never sees a suspect.
2%
91%
3% 1%1%1%
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DNA match information
Probability(evidence match)
Probability(coincidental match)
How much more does the suspect match the evidencethan a random person?
8x
11%
91%
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Match information at 15 loci
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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
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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
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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.
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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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