How it works: Solving DNA mixtures with a visual calculator

Perlin, M.W. How it works: Solving DNA mixtures with a visual calculator. Forensic Magazine, 5(1):32, 2008.

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Abstract

DNA mixtures occur when two or more individuals contribute their DNA to a biological specimen. Sexual assaults produce mixtures, as do many homicides. Mixtures can be highly probative, establishing physical contact of a perpetrator with the victim. However, mixture data can be challenging to interpret.

DNA data is produced by PCR amplifying a sample at short tandem repeat (STR) genetic loci, and separating the fragments to observe different allele lengths. At a locus, an individual inherits two (perhaps identical) alleles, forming a genotype. The quantitative STR data has an easily interpretable genotype pattern comprising one or two tall peaks, surrounded by smaller PCR artifact and background peaks.

DNA mixtures add together the individual quantitative STR patterns to form a more complex pattern. The interpretation task is to identify the perpetrator's genotype possibilities at a locus that account for the data. (The population rarity of these possibilities gives a numerical match association that can be presented in court.) This is done by reconstructing data patterns for every possible genotype combination and mixture weighting (e.g., 70% victim plus 30% perpetrator), and comparing these hypothesized quantitative patterns with the original STR data peaks. Those genotype possibilities that best fit the data have the highest probability, and are most likely to account for the perpetrator's genotype.