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Validating TrueAllele® genotyping on ten contributor DNA mixtures

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Presentation on theme: "Validating TrueAllele® genotyping on ten contributor DNA mixtures"— Presentation transcript:

1 Validating TrueAllele® genotyping on ten contributor DNA mixtures
1David W. Bauer, PhD 2Nasir Butt, PhD 1Mark W. Perlin, PhD, MD, PhD  1Cybergenetics Pittsburgh, PA  2Cuyahoga County Forensic Laboratory Cleveland, OH Cybergenetics ©

2 DNA evidence DATA Cold cases Corroborate answers Create new questions

3 What is an STR? Cell locus DNA Mother allele Father allele ACGT 1 2 3
4 5 6 7 8 9 Father allele 1 2 3 4 5 6 7 8 9 10 11 12

4 STR data Task: to infer genotypes from peaks
Data eliminates possibilities peak size before after 9 Single source … peak height 12 … 9, 9 9, 9 9, 10 9, 10 9, 11 9, 11 RFU 9, 12 9, 12 10, 10 10, 10 10, 11 10, 11 10, 12 11 10, 12 8 … … Size (bp)

5 STR mixture data more contributors more complexity more uncertainty
20 2 person mixture No single answer 18 19 18, 18 18, 19 18, 20 RFU Genotypes separated 19, 19 19, 20 17 20, 20 Size (bp)

6 Too complex? – Yes and No 3 person mixture RFU Size (bp) Reported
conclusions: “Mixture of at least 3 individuals…” “Number of contributors cannot reasonably be assumed…” “…therefore, no further conclusions can be drawn.” 3 person mixture 11 13 9 12 RFU 10 8 Size (bp)

7 How TrueAllele works - Computer tries thousands of possibilities
- Compares with data to calculate probability - Good explanations higher probability 3 person mixture 11 13 9 12 major minor RFU middle 10 8 Size (bp)

8 How TrueAllele works - Handles uncertainty with probability
Prob(evidence match) Probability Genotype for the major contributor 9,11 10,11 9,12 10,12 11,12 9,13 10,13 11,13 12,13 Allele pair

9 Degree of match Match statistics expressed as the numbers of zeros
Likelihood ratio (LR) = Prob(evidence match) Prob(coincidental match) Probability 9,11 10,11 9,12 10,12 11,12 9,13 10,13 11,13 12,13 Allele pair

10 Degree of match Match statistics expressed as the numbers of zeros
Likelihood ratio (LR) = Prob(evidence match) Prob(coincidental match) Probability 7x 14% 2% 9,11 10,11 9,12 10,12 11,12 9,13 10,13 11,13 12,13 Allele pair log(LR) = log(7) = 0.85 log(trillion) = 12 15 loci

11 minors difficult to ‘see’
Validation samples Laboratory created mixtures ( person) Don’t know real contributor number in casework 20 10 person mixture minors difficult to ‘see’ 19 18 23 RFU 21 17 15 22 16 25 Size (bp)

12 Sensitivity: simplicity from complexity
Match statistics to actual contributors 2 person Average match strength decreases 3 person 5 person Max log(LR) nearly constant 6 person -6 6 12 18 24 30 log(LR)

13 Sensitivity: More or less
log(LR) depends on contributor weight Match statistics for all samples 30 24 18 stronger match log(LR) 12 6 more DNA 20 40 60 80 100 -6 contributor weight (%)

14 Sensitivity: More or less
less DNA more contributors 30 weaker match 24 2-3 person 18 log(LR) 12 4 person 6 6 person 20 40 60 80 100 -6 contributor weight (%)

15 Specificity Sensitivity is important, but not sufficient 2 person
counts -42 -36 -30 -24 -18 -12 -6 log(LR) compare Each evidence genotype 10,000 random profiles Match statistic distribution for non-contributors

16 non-contributors excluded
Specificity Sensitivity is important, but not sufficient 2 person counts non-contributors excluded 3 person counts 4 person More contributors counts lower specificity 5 - 6 person counts -42 -36 -30 -24 -18 -12 -6 log(LR)

17 What limits specificity
Specificity not lost to complexity minors less specific counts 0 - 5 % counts % Information depends on DNA amount % counts 5 observed Majors more specific counts ≥ 50% -42 -36 -30 -24 -18 -12 -6 log(LR)

18 Reproducibility - Repeatable results using a random search algorithm?
- Compare independnet results for each individual contribs within group std dev 2 0.08 3 0.71 4 2.20 5 1.65 6 1.77 log(LR2) Software uses a random search algorithm log(LR1) more complexity more variation

19 Reproducibility - Software uses a random search algorithm
- How repeatable are the results? contribs within group std dev 2 0.08 3 0.71 4 2.20 5 1.65 6 1.77 log(LR2) log(LR1) more complexity more variation less DNA

20 How many contributors? Don’t know actual contributor number in casework Does assumed contributor number matter? 20 10 person mixture Actual: 10 19 18 23 RFU compare 21 17 Observed: 5 15 22 16 25 Size (bp)

21 Never too many contributors
Matches to true contributors counts -6 6 12 18 24 30 log(LR) Number of assumed contributors does not change results min mean std dev max N = False Exclusions Actual (2-10) -2.1 7.8 8.0 29.3 78 9 Observed (2-6) -9.1 7.9 8.9 29.4

22 Identification comes from information
Observed Actual counts -42 -36 -30 -24 -18 -12 -6 log(LR) Counts Observed Actual log(LR) 6,900 13,043 1 1,866 3,334 2 327 509 3 39 42 4 12 9 5 ‘Extra’ genotypes: - don’t affect chance of false inclusion 1 million comparisons - low inclusionary & exclusionary power Weak matches to low-information genotypes

23 Information grows Peeling matched references increases information
log(LR) DATA 1 4 1 3 2 1 2 assume known 2 2 2 2 2 3 3 match 2 1 1 2 3 3 4 15 3 3 4 4 6 8 weight (%) 13 4 2 3 3 6 16 4 4 5 6 12 5 5 5 22 6 7 13 7

24 Information grows Peeling matched-references increases information
log(LR) DATA 1 4 1 3 2 1 2 assume known 2 2 2 2 2 3 3 match 2 1 1 2 3 3 4 15 3 3 4 4 6 8 weight (%) more information 13 4 2 3 3 6 16 4 4 5 6 Higher match to minors 12 5 5 5 22 6 7 ‘Unseen’ minors not lost 13 7

25 Information includes AND excludes
Peeling matched-references increases information -7 -3 minor genotypes Before peeling After peeling counts -30 -24 -18 -12 -6 log(LR) Genotypes for minors more exclusionary

26 Complexity in casework
more probative weight more contributors more complexity Crime: Quintuple homicide in Sydney Australia Father, mother, 2 sons, aunt Suspect: Brother in-law Evidence: Blood stain in suspect’s garage

27 Complexity in casework
Blood mixture of 5 victims (blood-related) victim weight LR Son 40% 50 quadrillion Son 30% 2.2 billion Father 20% 230 thousand Aunt 7% 29 thousand Mother 3% 290

28 { Conclusions Tested TrueAllele on ten-contributor mixtures
• Sensitive Included true contributors DNA amount • Specific not Excluded non-contributors contributor number • Reproducible Concordance between runs • Objective Data dictate results, not user assumptions

29 More information • Courses • Newsletters • Newsroom • Presentations
• Courses • Newsletters • Newsroom • Presentations • Publications • Webinars TrueAllele YouTube channel:

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