DNA Mixture Interpretation

Summary

Interpreting a DNA mixture is necessary when two or more individuals (or contributors) are present in forensic evidence.  The resulting data (an electropherogram or "EPG") is reviewed by an analyst, and often, a weight of evidence (in the form of a likelihood ratio or "LR") is assigned.  In the DNA Mixture Interpretation project, we look into the different aspects that impact the variability of DNA interpretation methods. 

Description

An example of a three-person mixture - from Research Grade Test Material 10235. The STR profile is of a three-person mixture (with contributor ratios of 60:30:10, female:male:male)

Credit: Peter M Vallone

Interlaboratory Studies

Mixture interlaboratory studies provide a means to assess a laboratory's ability to process and interpret DNA mixtures. Initial ‘Mixed Stain’ studies (published in 2001) provided physical samples for DNA profiling with the first commercial STR typing tests.  Later, mixture interpretation studies conducted in 2005 and 2013 focused on the methods and variability in interpretation findings by providing electronic data (e.g., electropherogram) representing various case scenarios.

Reference Materials
NIST offers forensic DNA-based Standard Reference Materials (SRMs) and Research Grade Test Materials (RGTMs) that support the forensic DNA typing community.  A 2-person female:male (ratio 3:1) mixture is present in SRM 2391d DNA-based Profiling Standard.  In RGTM 10235, we have provided three DNA mixtures: a 2-person female:male (ratio 90:10), a 3-person female:male:male (ratio 20:20:60), and a 3-person (female:male:male (ratio 10:30:60).  These samples can be included in validation studies to assess DNA typing performance as well as performance check downstream interpretation and software tools. and

Sequencing Resources
The Next-Generation Sequencing Committee of the Federal Bureau of Investigation (FBI) Scientific Working Group on DNA Analysis Methods (SWGDAM) queried bioinformatic and statistical interpretation method developers to determine what data types would be useful in advancing this method/technology.  Based on this feedback, a set of mixture samples was conceived, which were composed of three-, four-, and five-person mixtures and run with commercially available STR sequencing assays.  The data is publicly available on the NIST Data Repository Page.

Mixture Interpretation
DNA mixture interpretation involves multiple steps that may impact the result's variability. Projects include setting analytical thresholds, estimating the number of contributors (NOC), the precision of replicate software analyses using Markov chain Monte Carlo algorithms, methods to assess variability between software models, and tools to support the validation of mixtures.

Further resources on DNA Mixture Interpretation are hosted on STRBase.