Evaluating forensic DNA evidence Forensic Bioinformatics ( Dan E. Krane Biological Sciences, Wright State University,
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1 Forensic Bioinformatics (www.bioforensics.com) firstname.lastname@example.org Evaluating forensic DNA evidence: essential elements of a competent defense reviewDan E. KraneBiological Sciences, Wright State University, Dayton OH 45435Forensic Bioinformatics (
2 Three generations of DNA testing RFLPAUTORADAllele = BANDDQ-alphaTEST STRIPAllele = BLUE DOTAutomated STRELECTROPHEROGRAMAllele = PEAK
3 DNA content of biological samples: Type of sampleAmount of DNABlood30,000 ng/mLstain 1 cm in area2200 ngstain 1 mm in area22 ngSemen250,000 ng/mLPostcoital vaginal swab0 - 3,000 ngHairpluckedng/hairshedng/hairSaliva5,000 ng/mLUrineng/mL
4 Basic terminology: Genetics DNA Polymorphism (“many forms”)Regions of DNA which differ from person to personLocus (plural = loci)Site or location on a chromosomeAlleleDifferent variants which can exist at a locusDNA ProfileThe combination of alleles for an individual
5 Basic terminology: Technology Amplification or PCR (Polymerase Chain Reaction)A technique for ‘replicating’ DNA in the laboratory (‘molecular Xeroxing’)Region to be amplified defined by PRIMERSCan be ‘color coded’ElectrophoresisA technique for separating molecules according to their size
6 STR Short tandem repeat Describes a type of DNA polymorphism in which: a DNA sequence repeatsover and over againand has a short (usually 4 base pair) repeat unitA length polymorphism -- alleles differ in their length3 repeats: AATG AATG AATG4 repeats: AATG AATG AATG AATG5 repeats: AATG AATG AATG AATG AATG6 repeats: AATG AATG AATG AATG AATG AATG
7 Reading an electropherogram Peaks correspond to alleles BLUEGREENYELLOWREDD3vWAFGAD8D21D18D5D13D7AmelogeninXX = femaleXY = male75100139150160200245300 bpsRed = ROX size standard
9 Crime Scene Samples & Reference Samples Extract and purify DNADifferential extraction in sex assault cases separates out DNA from sperm cells
10 Extract and Purify DNAAdd primers and other reagents
11 PCR AmplificationDNA regions flanked by primers are amplifiedGroups of amplified STR products are labeled with different colored dyes (blue, green, yellow)
12 The ABI 310 Genetic Analyzer: SIZE, COLOR & AMOUNT
13 ABI 310 Genetic Analyzer: Capillary Electrophoresis Amplified STR DNA injected onto columnElectric current appliedDNA pulled towards the positive electrodeDNA separated out by size:Large STRs travel slowerSmall STRs travel fasterDetectorWindowColor of STR detected and recorded as it passes the detector
15 Statistical estimates: the product rule 0.222x0.222x2= 0.1
16 Statistical estimates: the product rule 1 in 101 in 22,200x1 in 1111 in 20= 0.11 in 1001 in 141 in 811 in 113,400x1 in 1161 in 171 in 161 in 31,552x1 in 79,531,528,960,000,0001 in 80 quadrillion
17 What more is there to say after you have said: “The chance of a coincidental match is one in 80 quadrillion?”
18 What more is there to say after you have said: “The chance of a coincidental match is one in 80 quadrillion?”• Two samples really do have the same sourceSamples match coincidentallyAn error has occurred
19 Sources of ambiguity in DNA testing results • Mixtures: deconvolution and relativesDegradation, inhibitionBackground noise• Stutter (n+4)Pull-upSpikes and blobs
24 How many contributors to a mixture? How many contributors to a mixture if analysts can discard a locus?Maximum # of alleles observed in a 3 person mixture# of occurrencesPercent of cases20.00331042,498,1395.53529,938,77766.32612,702,67028.148,1511,526,55032,078,97611,526,2190.023.3871.0725.53There are 45,139,896 possibledifferent 3-way mixtures of the 648individuals in the MN BCI database.There are 45,139,896 possible different 3-way mixtures of the 648 individuals in the MN BCI database.
25 Sources of ambiguity in DNA testing results • Mixtures: deconvolution and relativesDegradation, inhibitionBackground noise• Stutter (n+4)Pull-upSpikes and blobs
28 Sources of ambiguity in DNA testing results • Mixtures: deconvolution and relativesDegradation, inhibitionBackground noise• Stutter (n+4)Pull-upSpikes and blobs
29 Degradation SMALL LARGE When biological samples are exposed to adverse environmental conditions, they can become degradedWarm, moist, sunlight, timeDegradation breaks the DNA at randomLarger amplified regions are affected firstClassic ‘ski-slope’ electropherogramDegradation is unusual.
30 Degradation The Leskie Inquest Undegraded samples can have “ski-slopes” too.How negative does a slope have to be to an indication of degradation?Experience, training and expertise.Positive controls should not be degraded.
31 Degradation The Leskie Inquest DNA profiles in a rape and a murder investigation match.Everyone agrees that the murder samples are degraded.If the rape sample is degraded, it could have contaminated the murder samples.Is the rape sample degraded?
38 Spikes and blobs Blob: Peak Area / Peak Height > 10 + Spike: Peak Area / Peak Height < 4.5 -
39 ResourcesBooks‘Forensic DNA Typing’ by John M. Butler (Academic Press)InternetApplied Biosystems Website: (see human identity and forensics)Forensic Bioinformatics Website:STR base: (very useful)ScientistsLarry Mueller (UC Irvine)Simon Ford (Lexigen, Inc. San Francisco, CA)William C. Thompson (UC Irvine)William Shields (SUNY, Syracuse, NY)Marc Taylor (Technical Associates, Ventura, CA)Keith Inman (Forensic Analytical, Haywood, CA)Testing laboratoriesTechnical Associates (Ventura, CA)Indiana State Police (Indianapolis, IN)Other resourcesForensic Bioinformatics (Dayton, OH)