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Scientific Combination of DNA Evidence: A Handgun Mixture in Eight Parts
M.W. Perlin and M. Greenhalgh, "Scientific combination of DNA evidence: A handgun mixture in eight parts", Twentieth International Symposium on the Forensic Sciences of the Australian and New Zealand Forensic Science Society, Sydney, Australia, 8-Sep-2010.
In a recent criminal case, a handgun was swabbed for touch DNA in four separate locations. Low-template DNA was recovered from each swab, and individually STR amplified in duplicate. A two person DNA mixture was seen in each of the resulting eight PCR products. Computer interpretation of the DNA mixtures with TrueAllele® probabilistic modeling yielded a definite (no uncertainty) 80%-85% major contributor genotype at two of the four STR profiles. This definite genotype matched the suspect with a likelihood ratio (LR) of over a quadrillion to one.
However, the minor contributor genotypes were not resolved in any of the four DNA templates, having respective mixture weights of 15%, 20%, 35% and 40%. Probabilistic modeling, though, permits construction of a joint likelihood function that treats each PCR experiment independently. Statistically combining the eight STR data profiles in this way, the computer can search for those genotypes that best fit all the evidence data. In this case, TrueAllele found a definite minor contributor genotype with a unique allele pair value at every locus. We conclude that scientific combination of DNA evidence may be helpful in interpreting low-template mixture DNA evidence.