knife DNA profile
To all,
We are all agreed that the number of short terminal repeats in the peaks from the knife blade profile are mostly the same as that from Meredith Kercher’s reference profile. However, there are four basic problems with the profile of DNA culled from the knife blade, all of which make it poorer in quality to a normal profile. These problems were either addressed explicitly or implied in the open letter concerning the DNA evidence, co-signed by nine DNA forensic specialists (
http://www.friendsofamanda.org/articles.html). They refer to it as “an extremely low level, partial profile” with alleles that were “consistent with the DNA of the victim.”
The first problem is the weakness of the signal intensities. The signals one typically observes are in the many hundreds or thousands of relative fluorescent units (RFUs). I am aware of thresholds of 50 to 150 RFUs, below which one ignores the peaks (there is one paper in the literature that advocates setting thresholds on the basis of the mean level of noise, not a set value of RFUs). Yet, 21 of 29 peaks in the knife profile are between 20 and 50 RFUs in intensity.
A second problem is that one should always set the threshold before doing the experiment. The machine-set limit was 50 RFUs (from what I can gather), yet the smallest peak in the electropherogram is 15 RFUs. Changing the limit after the experiment was done opens the door to bias. The textbook An Introduction to Forensic DNA analysis, 2nd ed. (Rudin, N. and Inman, K., CRC Press 2002, p. 121) states (emphasis added), “It is important to have some
predetermined limit to distinguish what is signal and what is noise.”
The third problem with this DNA profile is the appearance of two peaks in locus D3S1358, both of which have an intensity of about 20 RFUs, and neither of which is part of Meredith’s profile. This may be a case of allele drop-in. In the D7S820 locus there is a peak with an intensity of 15 RFUs; therefore, it is difficult to see why the D7 peak should be included as part of Meredith’s profile but the two peaks in D3 ignored.
The fourth 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 (Butler, Forensic DNA Typing, pp. 155-156). In other words, the lack of equality of peak height is often taken to mean that one is dealing with a mixture. However, the more plausible explanation in this case is that the DNA template is producing stochastic effects because the copy number is so low. IMHO, the problems noted above all support using conservative language in discussing the profile, as the signers of the open letter did.
If the profile arose from Meredith’s DNA, the more important question is how did it arise. The knife was tested for blood with tetramethylbenzidine, and it came up negative. In an interview with ABC, Dr. Elizabeth Johnson, one of the cosigners of the open letter, indicated that cleaning the knife would remove DNA from the knife more quickly than cleaning would remove blood. In other words the lack of blood makes the possible presence of DNA on the knife at the time of collection to be unlikely, and the DNA that was observed must have arisen from contamination. 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.” However, one might argue that some DNA from non-blood tissue was on the knife. If one could support this supposition, then one would be left with choosing between two events, contamination or non-blood DNA somehow escaping removal during putative cleaning of the knife.
There are other problems with the knife that fall outside of the scope of this thread.
Chris
