DNA MIXTURES: CHALLENGES WITH INTERPRETATION
Issues with mixture interpretation
PCASTSimple v. Complex mixtures Issues with Complex mixturesHow has forensic community dealt with
complex mixturesPresent/Future
PCAST
PCAST contrasts simple and complex mixtures
Simple* Complex*
Sometimes sex cases are complex and touch items simple, so…
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Figuring out genotypes, not just alleles
A B
Mixture Deconvolution
A BC D
Complex Mixtures 9
Does A go with B? With C? With D?
Or is it a homozygote?
A B C D
DBAFC
E
Number of Contributors
Stochastic Effects
Low Template DNA
Drop-Out
Drop-In
Stutter
Source: John Butler, Advanced Topics in Forensic DNA Typing: Methodology (pg. 325)
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DROP IN DROP OUT
Stutter
John Butler, Forensic DNA Typing: Interpretation Elsevier (2015) p. 75
Degradation
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More Complex Modelling
Less binary modeling (CPI), more semi-continuous and continuous programs
Modelling Software ValidationHow stochastic effects/artifacts are modelled
Different drop-out ratesDifferent drop-in ratesStutter
Use of peak heights# of contributorsRelatednessDegradationHow low is too low
How program codedHow software is maintained
Lab environments are differentDifferent validation –quality?
Differences in Prob Gen Programs
Probabilistic Genotyping
Assumptions program is making in generating its likelihood ratio in your case
(what are the hypotheses?; # of contributors? Related/unrelated?)
Who else fits into the mixture? Would the judge/you/juror #5 have a LR>1?
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OCME interpretation
is it a mixture? meet concordance policy? (dupe rule) estimate the number of contributors what is the mixture ratio
If 1:1 can’t deconvolute (FST here we come) Rules for assigning alleles (heterozygote vs. homozygote) to a
deduced profile
https://nycocme.qualtraxcloud.com/ShowDocument.aspx?ID=1148
suitable for comparison?Look at how many peaks, peaks below threshold, quality of data
Inconclusive: too many alleles