DNA evidence in the trial of Amanda Knox and Raffaele Sollecito, Perugia, Italy
The kitchen knife:
The prosecution claims that DNA testing shows the presence of Amanda’s DNA on the handle and Meredith’s DNA on the blade. The following information pertains to the knife:
• The knife was selected from among several knives in the kitchen drawer of Raffaele’s apartment. It was the only knife collected from the kitchen, although it had no visible stains or notable characteristics.
• Testimony has been given in court that this knife could not have made two of the three slash wounds to the victim’s neck, but that a smaller knife could have made all three wounds. Furthermore, this knife did not match a bloody knife imprint left on the bed.
• An extremely sensitive chemical test for the presumptive presence of blood, tetramethyl benzidine (TMB, a chemical capable of detecting at least a 1:10,000 dilution of blood), was negative for both the handle and blade.
• A swabbing of the handle revealed the presence of Amanda’s DNA. This is not unexpected since she had used the kitchen knives to prepare food at Raffaele’s apartment.
• A swabbing of the center portion of the flat edge of the blade was taken for further analysis. This sample tested negative for blood with TMB.
An extremely low level, partial DNA profile was developed for the blade swabbing using the Identifiler kit. The alleles detected were consistent with the DNA of the victim. The highest peak in the electropherogram was approximately 100 relative fluorescence units (rfu), while 21 of the 29 peaks that were detected and labeled as alleles fell between 20 and 50 rfu.
This DNA does not originate from blood. A highly sensitive chemical test for blood was negative, and it is unlikely that all chemically detectable traces of blood could be removed while retaining sufficient cells to produce a DNA profile consistent with the victim.
Numerous samples were collected from the crime scene that were tested and shown to contain high quantities of the victim’s DNA. There exists the real possibility that the low level, partial profile attributed to the knife blade is a result of unintended transfer in the laboratory during sample handling. Numerous examples of this have been documented by other laboratories.
Electronic (.fsa) files that would allow independent analysis of the data have not been disclosed.
Neither the extraction nor amplification of the low template DNA from the kitchen knife blade was duplicated. The test can not be reproduced as the swab and DNA extract were consumed during testing.
Conclusion about the kitchen knife:
No credible scientific evidence has been presented to associate this kitchen knife with the murder of Meredith Kercher.
Elizabeth A. Johnson, Ph.D., Forensic Biology/DNA expert
Greg Hampikian, Ph.D., Professor and Director of the Idaho Innocence Project, Department of Biology Boise State University
Dan Krane, chief executive officer and chairman of the board of directors, Forensic Bioinformatics
Jason Gilder, systems engineer, Forensic Bioinformatics
Joy Halverson, DVM, director, Zoogen Services
Laurence D. Mueller, professor of ecology and evolutionary biology, University of California, Irvine
Marc Taylor, president, Technical Associates
Rick Staub, Ph.D., director of laboratory operations, Orchid Cellmark, Dallas, Texas
Simon Ford, Ph.D., Lexigen Science and Law Consultants