In 2009, the journalist Baribie Nadeau mentioned something in an article which many people will have wondered about when they read it. She said that Vincenzo Pascali, a scientific consultant for the prosecution in the early days of the process, had “hinted” that a sample taken from the bra-clasp of the murder victim Meredith Kercher had contained the DNA of not only Raffaele Sollecito but also his alleged accomplice in the crime, Amanda Knox.
It may be thought unlikely that there was much in this. That’s not to impugn Nadeau’s journalism – if a DNA expert hints at something like that then it is worth reporting. At the same time, though, you would think that, if Knox’s DNA really were present on the clasp, the prosecution would have mentioned it by now.
Nevertheless, given the information that is now available about DNA evidence in the case, I thought it would be worth looking into this suggestion.
The first thing to note – and it’s an extremely important thing to note – is that the DNA of Kercher and Sollecito found in the sample from the clasp accounts for either all or very nearly all (depending on which expert you are listening to) of the alleles observed. Since there is so little left to play with, trying to make a further DNA match out of it would seem at bit like looking at a nine and a six and wondering whose phone number in particular they might form part of. Completely impossible, even if you believe that further DNA might be present. Impossible, that is, using any method that is generally considered scientifically reliable.
What you could do, though, is lower your standards and begin to look at peaks that would ordinarily be ignored on the grounds that they could well be nothing more than noise. This makes the peaks that you are including less reliable (how much less reliable depends on how radical you are in including smaller peaks), but it might be argued that there is a legitimate trade-off. If you can get a very good match, then maybe it can be argued that a questionable peak here or there is acceptable. After all, how likely is it that the noise has simply been kind enough to appear in the exact places you would need it to in order to make you incomplete match complete?
So, you can see on the image that I’ve added green dots to the DNA profile taken from the bra-clasp, indicating the places where Knox’s alleles would be expected to be located. And it turns out that it is possible to see a fairly good degree of compatibility with the sample. But I had to go all the way and include even the tiniest bumps on the printout. Because some of these are so small that no numerical information about them is included, I also had to judge a few of the locations by eye (the blue numbers on the image indicate where I did this). This raises the additional problem that my own human error may have some effect on the reliability of what I have done.
But how strong is the actual match? Well, first you have to explain away two things. At loci D8S1179 and D19S433 (the first and tenth groups of peaks on the image), there are small peaks which the DNA review says are above the threshold where we can be confident they are noise. But they are only above the threshold by a hair’s breadth. So maybe we can consider the possibility that they really are noise. Then at locus D7S820 (the third group of peaks in the image), there’s location where you would expect to see an allele relating to Knox, but there is nothing there. Again, maybe this is not so much of a problem. We’re presumably dealing with an extremely faint DNA trace. Maybe it’s simply the case that just one peak out of 29 is so faint as to not be there at all.
What’s starting to emerge, though, is the problem with not having any real standard to go by in terms of which peaks you consider to be real and which you don’t. The judgements you have to make in order to determine whether there is compatibility or not start to become fairly subjective, at least in this case. At each locus point, the ways in which compatibility could arguably be ascertained are numerous, so things are different from how they were in considering the matches to Sollecito and Kercher. Whereas their profiles are quite clear and match 100% to unquestionably genuine alleles on the graph, it seems like you can pretty much see Knox’s DNA profile in the printout according to how much you want to.
A commenter suggested adding dots for a sample known not to be connected to the case. I thought this was a good idea, and whose DNA profile could be more suitable for the task than that of Casey Anthony, recently acquitted of murder in the US? So I’ve added her alleles using pink dots.
I think this goes to further illustrate the problem. Although there are a greater number of problematic spots on the graph that you need to ignore if you want to make Anthony’s DNA fit, some things go the other way. For example, 69% of Knox’s allele’s pair up with peaks on the graph that are above 50 RFU in height (one measure of a potentially genuine allele). It may sound from that like Knox’s DNA is beating the odds. But Anthony’s DNA profile, on the other hand, matches to those >50 RFU peaks 75% of the time. For me, it seems hard to see how you could justify being confident that Knox’s profile is there without also thinking that there is a least a decent chance that Anthony’s could also be there.
Then again, it’s always possible that I have it backwards and that, in reality, the bra-clasp printout offers a new chance for that half of America outraged by Anthony’s aquittal to get her back in court.