Thursday, April 7, 2011

The Denise Johnson Case

In May 1992 the body of a woman later identified as Denise Johnson was found outdoors in the brush near some paloverde trees in Maricopa County, Arizona. Her clothing was scattered about the area, and she had been bound with cloth and braided wire. She was from nearby Phoenix and appeared to have been murdered and left at the location recently. A pager recovered at the scene led the police to a suspect, a man named Mark Bogan. The investigation developed circumstantial, but not definitive, evidence against him. One of the paloverde trees at the scene appeared to have been damaged, possibly by a vehicle. A search of the suspect’s pickup, pursuant to a warrant, revealed seed pods from a paloverde tree in the truck bed.

The suspect admitted that he had picked up Johnson, who had been hitchhiking, and had sexual relations with her in the pickup. But he said he had made her get out of the truck after they had argued. He denied being at the crime scene, and he denied killing her.

Police obtained the assistance of a plant molecular genetics specialist, Dr. Timothy Helentjaris of the University of Arizona, who could compare the DNA profile of the seed pods recovered from the suspect’s pickup with those of the trees in the vicinity of the crime scene. The geneticist conducted blind tests on a number of paloverde trees, and the tests showed that each exhibited a different profile. The seed pods from the pickup truck showed identical profiles (indicating that they fell from the same tree), and their profile matched that of one particular tree at the scene. This evidence went a long way toward convincing the trial jury that the suspect’s pickup was indeed at the crime scene, a fact that he had denied.

The plant genetics expert in this case was a university professor. He used a DNA profiling technique called RAPD (randomly amplified polymorphic DNA) that is not regularly used in forensic labs but is common in research. It is a good technique for looking at genetic variation in organisms whose total genetic makeup, or genomes, have not been mapped or sequenced very thoroughly (as was the case with the paloverde trees). The court allowed the evidence because the expert did a good job of running his tests blind and of establishing that there was much detectable variation in the trees.

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