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