I think random DNA transfer from a specific individual when that individuals DNA constitutes a minuscule proportion of the DNA lying around is rare. Perhaps rare enough that it is not a likely explanation as to how Sollecito's DNA came to be on the bra clasp.
This is a point that I have discussed before and it seems that others may have disagreed with it. I am not sure why. If samples were collected all over Kercher's room how often would a piece of Sollecito DNA be detectable? My guess is almost never. I continue to think that contamination from random DNA lying around in Kercher's room or the apartment that produced a false positive for Sollecito on the bra clasp is unlikely. There was vastly more of other people's DNA in Kercher's room and that apartment.
I was hoping to find something in English, but here's something from the
RTIGF that may be helpful, go to page 272 of that PDF. This is the list of the unknown profiles found in the cottage, eight males and three females. Note that two of the males and two of the females are from downstairs/outside (bloody tissues that didn't have anything to do with the murder were found outside the cottage--the area had a drug element) or are Filomena and Laura. That still leaves six unidentified males and a female that investigators found in the girls' flat. Outside maybe Filomena's boyfriend and perhaps Laura's, no other male had spent significantly more time in the cottage than Raffaele, especially the week previous to the murder.
Those other males are likely postal police, Filomena's boyfriend and his buddy who were at the discovery, the remainder of the boys downstairs (not covered by the two found in their flat that I excluded above) as well as perhaps
polizia scientifica and
polizia di stato at the scene. At any rate there isn't six males who'd have significantly more DNA at the scene than Raffaele, who had one 'hit' in the initial
polizia scientifica sweep. The assumption that Raffaele's DNA was 'miniscule' in proportion to all the others is mistaken, they picked up
full profiles (just like Raffaele from the cigarette butt) of several men who would have little to no more reason to have DNA at the upstairs flat of the cottage than Raffaele.
Raffaele's DNA was there, just like theirs was, and it was picked up in the normal course of doing the investigation. That was centered on the murder room (where they found nothing of Raffaele or Amanda or these people) and places where they thought traces might be found in relation to the murder, but they didn't swab
everywhere and Raffaele's DNA (and theirs) was
also without doubt (not a phrase I often use!) in places they didn't swab or swab well.
Especially trace DNA that they wouldn't see if they didn't go looking for it.
As for it getting in the murder room, that's the horror of coming back six weeks later having gone in and out of the room all those times without changing booties/gloves and letting it all blow around a little. By the time they were done with their work taking hundreds of samples where they were (generally) careful enough with the one they were
doing but not so much with the rest of the area when they were done, that crime scene was definitely contamination prone. The idea that they could come back six weeks later and that site have any integrity left to it (i.e. that things--
especially trace elements--were necessarily where they were when the murder was committed) is laughable. Anything of Raffaele's only had to be transferred a few feet from where he tried to knock down that door, and the bra clasp definitely traveled as well, and each of the people who handled it at the collection had (obviously) just been outside the murder room and by the way they acted on camera when they discovered it there's little reason to think they were especially careful beforehand when they weren't being seen and hadn't found something that might be of interest.
That doesn't mean I think the positive test for Sollecito's DNA was the result of his contact with the bra during the crime. I think there are two more likely explanations:
1. A gross contamination event occurred. That is a bit of Sollecito's DNA came to be on the sample as the result of contamination in the lab or during the collection process with Sollectio DNA.
That's what I'm talking about here, something from outside the murder room where Raffaele's DNA definitely was (and in greater quantities than just the cigarette butt) getting passed on to the bra clasp either during the forensic investigation where they went inside and outside that room with the bra clasp on the floor somewhere, or the bra clasp might have skittered to the doorway and then been kicked back in, (perhaps while removing the body or mattress) or during the six weeks after the
polizia scientifica were done and that bra clasp was in a pile in a room with a broken lock in a cottage with a widow out, and then finally during the collection was handled freely by
foresnic technicians clowns in bunny suits who'd just been outside that room themselves.
They collected a number of things the second trip, they only had to make one 'mistake' on one item, and they may have made duplicate ones with the bra clasp, meaning those other males on that electropherogram might well match one of the other unknown males from outside the room as well, it would take the EDFs to be able to properly analyze that. Also not all the electropherograms from the samples that were taken were ever released, there's data missing there as well. Keep in mind this is LT DNA, going below the RFU threshold
only on select items, and
not allowing anyone to look at the other samples the same way. There are no doubt other people's profiles that could be found in other items if anyone else was allowed to zoom in and analyze those peaks, Stefanoni wants to only show you what helps her case, and suppress that which doesn't--like with negative blood tests...
2. The DNA test results were flawed because of misinterpretation of the test results.
I think that's unlikely but that gets complicated, they ran a y-haplotype and found Raffaele's. Incidentally, while my guess is that Raffaele's contribution to the sample was significantly larger than the others (mostly based on the results of the y-haplotype test) there's nowhere you can draw a line (on the autosomal) and include Raffaele and exclude all the others, like you can (easily) with Meredith and Raffaele's peaks. You could not 'call' those alleles and come up with a match for Raffaele, you'd have to have Raffaele's profile and then go looking for it, which is one of the dangers with analyzing mixed LT samples to begin with. With all those alleles I could take your profile and mine and it's (roughly) even odds I could 'find' one of ours there, but the other reason to be wary of such 'evidence' is: if it has all those alleles it must have really gotten around somehow!
I also think that other innocent explanations exist for Sollecito's DNA on the bra clasp including such things as the fact that Kercher's clothes would have had the opportunity to come into contact with Sollecito's DNA since they were washed in the same house that Knox's clothes were and she had had significant contact with Sollecito.
These actually amount to the same thing as the above if you think on it, somehow material transferring from something Raffaele touched to the bra clasp.
