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Calculating the Weight of Evidence in Low-Template Forensic DNA Casework


  • Supported in part by a Ruth Kirschstein National Research Service Award from the National Human Genome Research Institute (F32HG005308) and by the Miller Research Institute at UC Berkeley.

Additional information and reprint requests:

Kirk E. Lohmueller, Ph.D.

Department of Integrative Biology

University of California

1005 Valley Life Sciences, Building 3140

Berkeley, CA 94720-3140



Interpreting and assessing the weight of low-template DNA evidence presents a formidable challenge in forensic casework. This report describes a case in which a similar mixed DNA profile was obtained from four different bloodstains. The defense proposed that the low-level minor profile came from an alternate suspect, the defendant's mistress. The strength of the evidence was assessed using a probabilistic approach that employed likelihood ratios incorporating the probability of allelic drop-out. Logistic regression was used to model the probability of drop-out using empirical validation data from the government laboratory. The DNA profile obtained from the bloodstain described in this report is at least 47 billion times more likely if, in addition to the victim, the alternate suspect was the minor contributor, than if another unrelated individual was the minor contributor. This case illustrates the utility of the probabilistic approach for interpreting complex low-template DNA profiles.