Removal of a PCR Inhibitor and Resolution of DNA STR Types in Mixed Human-Canine Stains from a Five Year Old Case

The analysis of biological trace evidence from a reopened investigation into a 1991 murder from Vernon, B.C. revealed mixed human and dog bloodstains on blue jean pants that contained a PCR inhibitory substance. The presence of the inhibitory substance was detected by the inhibition caused from adding a small aliquot of the test DNA extract into a PCR reaction designed to produce a known standard product. The removal of the PCR inhibitory substance was accomplished by treating the extracted DNA with Thiopropyl Sepharose 6B beads. DNA profiles from two human contributors and a canine were obtained using species specific polymorphic STR markers. The two human DNA profiles obtained from blue jean pants were resolved, one matched the suspect and the other matched the victim. The DNA profile from the canine component matched that obtained from the known sample of the victim's dog who was also slain during the assault. This evidence along with other DNA typing evidence was critical in obtaining a resolution of the case.