Separating DNA Mixtures by Computer to Identify and Convict a Serial Rapist Mark W Perlin, PhD, MD, PhD Cybergenetics, Pittsburgh, PA Garett Sugimoto,

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Presentation transcript:

Separating DNA Mixtures by Computer to Identify and Convict a Serial Rapist Mark W Perlin, PhD, MD, PhD Cybergenetics, Pittsburgh, PA Garett Sugimoto, MS Kern Regional Crime Laboratory, Bakersfield, CA Cybergenetics © American Academy of Forensic Sciences February, 2016 Las Vegas, NV

July & August 2013

Crime scene #1 Purse strap Phone cord Phone

Crime scene #2 Pants Shirt stain

Crime scene #3 Bathtub handle Zip tie in road

911 Call Girl: Hi. 911: Hello, 911. Girl: Ah yes, there’s this guy. He has my Mom naked. 911: OK. Girl: And he had me blind-folded. 911: OK. How old are you? Girl: Nine. 911: Nine. Where’s your Mom at now? Girl: What? You’re not my Dad. You’re not my Dad. Get off. (Scream)

DNA database hit Billy Ray Johnson Major contributor to DNA of zip tie in road

Biological evidence Kern Regional Crime Laboratory thirty seven swabs and cuttings garments blankets apartment surfaces skin body cavities telephones zip ties Early October 2013 low-level DNA mixtures containing 3 or 4 people

TrueAllele ® Casework ViewStation User Client Database Server Interpret/Match Expansion Visual User Interface VUIer™ Software Parallel Processing Computers

Quantitative mixture data KRCL tested biological items, generated STR data zip tie in road locus D13S317 Receive evidence data

DNA largely inconclusive Threshold Over threshold, peaks are labeled as allele events All-or-none allele peaks, each given equal status Under threshold, alleles vanish

Separate genotypes 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

Objective genotype determined solely from the DNA data. Never sees a comparison reference. Evidence genotype 68% 13% 19%

DNA match information Prob(evidence match) Prob(coincidental match) How much more does the suspect match the evidence than a random person? 10x 68% 7%

Match information at 15 loci

Is the suspect in the evidence? A match between the zip tie and Billy Ray Johnson is: 211 quintillion times more probable than a coincidental match to an unrelated Black person 100,000,000,000,000,000, zeros after the one 10x10x10x10x10x10x10x10x10x10x 10x10x10x10x10x10x10x10x10x10 20 powers of ten

Crime scene #1 matches Late October 2013 Receive reference data BRJ 2 5 5

Crime scene #2 matches BRJ 6 7 9

Crime scene #3 matches BRJ 3 20

Informative DNA mixtures 8 mixture items vs. 5 victims + 1 suspect DNA match statistics corroborate victim statements BRJ

December 2013: Grand Jury Cybergenetics TrueAllele Report Arrest

March 2015: Trial CaseItemDescription C1147X Billy Ray Johnson 13-CL-01721C0431XPurse strap1.63 C0432XPhone cord4.53 C0984XPhone CL-01927C0801SStain on pants6.24 C0802SStain from pants6.73 C0806SBack of shirt CL-02349C0929XBathtub handle2.74 C0937XZip tie20.32

Validation studies

Match error rates False positive rate in Specificity Table 8

Crime lab runs TrueAllele

Concordant Kern lab report Independent TrueAllele processing

April 2015

May 2015

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