In the mind of the public, there is something almost magical about the ability of a DNA match to absolutely positively link someone to a crime scene. So it's not difficult to understand why even just a very few traces of his DNA on one single fragment of clothing have been enough to convince so many that Sollecito participated in this horrible attack. And since he and Amanda Knox were so closely linked, the assumption has been that she too was involved, despite the fact that no trace of her DNA was found anywhere in the room where Meredith was assaulted.
There is a problem with the DNA evidence in both cases, however. Thanks to more sensitive methods of retrieving and analyzing DNA, developed only in recent years, it is no longer possible to assume that the presence of anyone's DNA on a particular object means that this person had ever been in direct contact with that object. This problem is summarized in a very interesting report, published on the Internet in 2007, the same year Kercher was murdered, of a Study of DNA Transfer, by Marc Taylor. According to Taylor,
The increasing sensitivity of DNA tests has affected the nature of criminal investigations and has created a new class of DNA evidence. Analysts talk of detecting "trace DNA," such as the minute quantities of DNA transferred through skin contact. DNA typing is currently being applied, with varying degrees of success, to samples such as doorbells pressed in home invasion cases, eyeglasses found at a crime scene, handles of knives and other weapons, soda straws, and even single fingerprints. These developments will bring more DNA evidence to court in a wider variety of cases and may well open new lines of defense. A key issue will be the potential for inadvertent transfer of small amounts of DNA from one item to another, a process that could easily incriminate an innocent person. (My emphasis.) . . .
Primary transfer occurs when DNA [is] transferred from a person to an item. Secondary transfer is when the DNA deposited on one item is transferred to a second item. Tertiary transfer is when the DNA on the second item is, in turn, transferred to a third.While primary and secondary transfer were well understood at the time, the possibility of tertiary transfer was not. In fact, when I posted regarding this possibility at a forum ostensibly devoted to justice for Meredith Kercher (but actually devoted to the demonization of Knox and Sollecito), I was challenged on that score by someone claiming that tertiary transfer had been tested and determined to be impossible. As documented by Taylor's report, that is not the case. Here is his description of the experiment he and a colleague carried out to test precisely this possibility (the references are to the case that prompted the experiment, commissioned by the defendant, Dr. Dirk Greineder):
Forensic scientists Marc Taylor and Elizabeth Johnson, of Technical Associates (an independent laboratory in Ventura, California) simulated the sequence of events posited by the defense theory: a man wiped his face with a towel, then a woman wiped her face with the towel, then gloves and a knife like those used in the murder were rubbed against the woman's face. DNA tests on the gloves and knife revealed a mixture of DNA from the man and woman-exactly what was found in the Greineder case. Taylor was allowed to present his findings to the jury. Although the jury ultimately convicted Greineder (there was other incriminating evidence besides the DNA), the case is a good example of how the amazing sensitivity of contemporary DNA profiling methods facilitate a plausible explanation for what might at first seem to be a damning DNA test result.In the Kercher case, the defense argued with good reason that the presence of Sollecito's DNA on the bra clasps was due to contamination, as the item in question had been left lying in the room for over 40 days before being tested. The prosecution countered that, while contamination might explain the presence of a policeman's DNA, it cannot explain the presence of Sollecito's, as nothing containing his DNA was thought to have been present in the building during that time. As I see it, the most likely explanation is that Sollecito's DNA, which by the way, could not be associated with any cell and must therefore be regarded as fragmentary trace evidence, was the result of perfectly innocent and easily understandable tertiary transfer.
Knox and Kercher used the same bathroom, which would have made it easy for the DNA of one to be found associated with the other, via secondary transfer. For instance, as in the above experiment, they could have at some point shared the same towel. Sollecito's DNA would of course have been all over Knox, since they were known to be intimate. Thus, some of the DNA transferred to Kercher from Knox may well have been Sollecito's. It's not difficult, in view of the experiment described by Taylor, to see how his DNA could therefore have been transferred first to Knox by direct transfer, then from Knox to Kercher, by secondary transfer, and finally, from Kercher to the clasps on her bra strap, by tertiary transfer.
I find it significant that Sollecito's DNA was found on an item that Kercher regularly touched, since the most likely means of transfer would be via the hands. If the transfer of Sollecito's DNA to the bra clasps had been the result of his touching them directly, then we would expect to find not only his DNA but his skin cells, yet no cells at all were found, just loose DNA. In view of this, the possibility of tertiary transfer seems far more likely than that of direct transfer, meaning that this "evidence" cannot be used to place Sollecito at the scene of the attack even if we discount the possibility of contamination.
Of course, as far as the general public is concerned, the fine distinctions outlined above mean very little. Once it's been established in people's minds, via simplifications disseminated by the media, but also still accepted in certain quarters, by law enforcement personnel, that someone's DNA has been found at the scene of a crime, then for a great many the issue is settled, and that person must be guilty. And by the same token, though with the opposite effect, interestingly enough, the same naive attitude has led many in the public, the media and law enforcement, to assume that JonBenet Ramsey could have been murdered only by some unknown intruder, who will some day be identified when a DNA match is found.
The fact that the traces of DNA found on JonBenet's clothing and in her blood can easily be explained as the result of secondary and/or tertiary transfer, from some innocent source to JonBenet's hands (perhaps when she touched something someone else had touched, or even petted a dog or cat) and from there to her longjohns and panties, is not at all obvious and can all too easily be overlooked. In view of all the many reasons why no intruder could have been present in the Ramsey home that night, coupled with the lack of any match over so many years to any of the millions of DNA samples in the FBI's Codis files, indirect transfer seems by far the most likely explanation.
The problem with DNA is that the methods used to detect it have become so sensitive as to, in many cases, be counter-productive. The danger that some innocent person will be implicated in a crime has become all too real.