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Kern County resolves the DNA mixture crisis

Perlin, M.W. and Miller, K.W.P. Kern County resolves the DNA mixture crisis. Forensic Magazine, 11(4):8-12, 2014.



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Abstract: DNA mixtures containing DNA from two or more people comprise most biological evidence samples seen in United States crime labs. Mixtures can greatly complicate data interpretation since DNA analysts must account for data patterns that include many peaks of varying height. There are a vast number of weighted genotype combinations, with multiple ways to explain the data. This complexity makes it hard for analysts to readily differentiate probable from improbable genotypes.

The difficulty in manually interpreting complex DNA mixtures has consequences. Hundreds of thousands of evidentiary items have been collected and processed into DNA identification data that could implicate or exonerate—but these items have not been reported conclusively. A failure to fully use data from evidence is a failure of science to promote justice. This ongoing mixture crisis permits needless victimization by those whom DNA should have identified.

The forensic community is discussing genotype probability modeling as a way to interpret DNA mixtures. The Kern Regional Crime Laboratory (KRCL) was an early adopter of probabilistic genotyping for mixture interpretation. KRCL's adoption of these computer methods enables Kern County to use complex mixture evidence in routine casework, and easily report their match results.