Part VI in a series on the Knox/Sollecito case
Let’s reexamine the problems with the electropherogram of DNA allegedly arising from the knife first, then interpret the profile in light of these problems.
There are three basic problems with the profile of DNA culled from the knife. The first is the weakness of the signals, as discussed in the previous post on the knife (Amanda Knox and Raffaele Sollecito and the Murder of Meredith Kercher, Part I). The signals one typically observes are in the many hundreds or thousands of RFUs, yet 22 of 29 peaks in this electropherogram fall below 50 RFU. Dr. Tagliabracci, a defense expert witness, noted in a document on DNA and paternity testing that the lower limit of detection is not less than 50 RFU (http://www.istitutoaffarisociali.it/flex/AppData/Redational/Ejournal/Articoli/Files/D.7e1010a4daa67d625c63/indagini_genetico_forensi.pdf). In Darkness Descending, the first book on the Kercher murder, one of Italy’s top forensic scientists, General Luciano Garofano, noted that 100-150 RFU is usually considered to be the minimum. There is a suggestion in the forensic literature to set two thresholds that are related to the average noise level, as opposed to a fixed value of RFUs (Gilder et al., Journal of Forensic Science, 2007, 52(1), 97-101). Two of the coauthors are signers of the open letter on the forensics of this case (http://www.friendsofamanda.org/articles.html).
Thus there is no universal threshold, but the lowest I have been able to document is 50 RFU, with one suggestion that peaks to 40 RFU may be interpreted with caution. Moreover, there ought to be consistency within one lab. Were the other DNA samples in the lab analyzed with the same threshold or a different one? Indeed, an equally serious issue is that one should always set the threshold before doing the experiment. The machine that Dr. Stefanoni used had a preset limit of 50 RFU (http://perugia-shock.blogspot.com/2009/09/psychiatrist-and-coroner-for-amanda.html), and changing the limit after the experiment was done opens the door to bias, as discussed in the previous post on the knife (http://viewfromwilmington.blogspot.com/2010/01/amanda-knox-and-raffaele-sollecito-and.html).
The second problem with this DNA profile is the appearance of two extra peaks in locus D3S1358, both of which have a signal-to-noise ratio (S/N) of about 20*. They have fifteen and sixteen repeats, respectively, and neither of which is part of Meredith’s profile, which has fourteen and eighteen repeats at this locus. There is one peak in locus D7S820 with a S/N ratio of only 15, which is interpreted to be part of Meredith’s profile. Why should this latter peak be treated as part of Meredith’s profile and the two other peaks ignored?
The third problem with this profile is that eight loci, D7S820, D16S539, D19S433, vWA, TPOX, D18S51, D5S818, FGA, have pairs of peaks in which the smaller peak is less than 70% of the height of the larger one. This value is typically 70-100% in a single source sample, and there are two peaks because there are usually two different alleles, one from each parent (Butler, Forensic DNA Typing, 2nd ed., pp. 155-156). Ordinarily, peaks below 70% that are not stutters (a type of artifact) are thought to indicate that the DNA arises from more than one person (is a mixture). In this case, a reasonable alternate explanation is that the peak heights are different because there are very few copies of the DNA template, perhaps 10-20. The special issues with low copy number DNA will be the subject of a future post. The problems noted above all support taking a conservative interpretation of the profile, as the signers of the open letter did. They refer to it as a “partial profile.” I lean toward referring it as a possible partial profile.
If the profile is Meredith’s, the question is how did it arise. Dr. Elizabeth Johnson said, “if someone had a knife covered in blood and they tried to
clean it very well, they would remove their ability to detect the DNA before they removed the ability to detect the chemical traces of blood.” The segment can be viewed at http://abcnews.go.com/GMA/AmandaKnox...cutorswitched-motives/story?id=9215634. Therefore, the lack of blood makes it impossible for there to be DNA on the knife, and the DNA that was observed has to arise from contamination from mechanisms similar to those documented in the previous post, “Forensic DNA Contamination,” such as the Mixer murder. General Garofano said, “they say it was cleaned with bleach. If an object is cleaned with bleach, there is no DNA left. And if any were left there would be the same amount of DNA belonging to Amanda Knox as to Meredith Kercher. Next to nothing…So the fact that there is a lot of Amanda Knox's DNA and a little of Meredith's doesn't sound logical to me.”
Dr. Johnson’s words on the ABC segment are more adamant than in the open letter. One might instead argue that it is only improbable that some DNA would remain on the knife. If so, then one would be left with choosing between two events, contamination or DNA somehow escaping removal, and deciding which seems more likely.
Dr. Donald Riley wrote (http://www.scientific.org/tutorials/articles/riley/riley.html):
(1) A partial profile essentially proves that one is operating outside of well-characterized and recommended limits.
(2) Contaminating DNA usually presents as a partial profile, although not always. For this reason, the risk that the result is a contaminant is greater than for samples that present as full profiles.
(3) A partial profile is at risk of being incomplete and misleading. The partial nature of it proves that DNA molecules have been missed. There is no way of firmly determining what the complete profile would have been, except by seeking other samples that may present a full profile.
Recall that we appear to be dealing with a partial profile in this instance.
The Italian forensics team might have made a stronger case for the DNA profile originating from blood on the knife, not contamination, had they done several things differently. General Garofano said**, “Did they open the knife to see if blood had dripped between the metal part of the handle and the plastic? No? Pity. That would have been a sure place to find blood if there was blood." Checking the knife more thoroughly for blood is the first thing that the forensics team should have done.
Second, the forensics team failed to perform a control experiment that would have shed light on whether or not the profile arose from contamination. Recently, I was fortunate enough to discuss this case with a graduate of our department who is now a forensic scientist in a municipal police department. When I told her about the knife, she asked whether or not they had checked other knives from the drawer for DNA (a good example of thinking like a scientist). If other knives had shown DNA from Meredith (or the other flatmates, for instance), then contamination certainly occurred.
Third, the prosecution has not released the fsa files, an almost unheard of occurrence in DNA forensics. These files allow independent forensic scientists to analyze the data. The open letter of 19 November 2009 noted their absence. Examination of these files might shed light on possible contamination and also give information on how signals were accepted or rejected as artifacts. To sum up, the lack of blood, the failure to look for DNA on other knives, and the lack of independent review greatly reduce the value of this DNA profile.
The authors of the open letter wrote, “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… No credible scientific evidence has been presented to associate this kitchen knife with the murder of Meredith Kercher.” That statement is more than sufficient to create reasonable doubt that this knife is a murder weapon, in addition to the many other problems associated with this knife (http://perugia-shock.blogspot.com/2009/09/neutral-expert-dismisses-murder-weapon.html). Sometimes a kitchen knife is just a kitchen knife.
*Update 11 February 2010. The words "signal-to-noise ratio" should be changed to "peak height" in this paragraph. All of the numbers are in RFUs. I am sorry about this error; however, their interpretation is not changed by this correction. **Update 17 February 2010. Corrected the spelling of General Garofano's name.