"So want to read the link re: DNA evidence from New Scientist, but can't get link to work."
ah - this might explain why Dittany has ignored it every time I have posted it.
"The case against them largely hinges on two pieces of DNA evidence. One is from a knife recovered from the kitchen drawer at Sollecito's apartment. The other comes from a portion of a clasp that was cut away from the bra Kercher was wearing. The forensic specialists question both.
In respect of the knife, the letter says contamination from other DNA present in the lab that did the analysis cannot be ruled out. The initial method used was standard: the DNA was amplified, then analysed using electrophoresis. This generates a graph consisting of a series of peaks, whose heights represent how much of certain DNA snippets are present. Taken together the peaks create a DNA "fingerprint" unique to an individual.
The lab says that DNA taken from the knife's blade produced a series of peaks that matched Kercher's DNA, while DNA from the handle produced peaks that matched Knox's.
To minimise the risk that some peaks arise from contamination, most US labs only count peaks falling above a height threshold of 150 relative fluorescence units (RFUs) and all dismiss those below 50. The trouble with the DNA found on the knife is that "most of the peaks are below 50", says Greg Hampikian of Boise State University in Idaho, who signed the letter and reviewed the DNA evidence.
Contamination fear
When this happens, samples can be rerun, but this doesn't appear to have been done in the Knox and Sollecito case. This means contamination cannot be ruled out, the open letter claims. The same lab may also have been running DNA profiles from other evidence in the case at the same time, it says, and tiny amounts of this could have contaminated the knife samples.
What's more, a sensitive chemical test for blood on the knife was negative, and it is unlikely that all chemically detectable traces of blood could be removed from the knife while retaining sufficient cells to produce a DNA profile. "No credible scientific evidence has been presented to associate this kitchen knife with the murder of Meredith Kercher," the letter concludes.
Evidence from the clasp is equally inconclusive, according to the letter. What looks like a mixture of different people's DNA was found on it, and Sollecito could not be excluded. However, because Sollecito had visited the women's home several times before the murder, his DNA could have made its way onto the clasp "through several innocent means", the letter says.
Neither Sollecito's nor Knox's DNA was found on the remainder of the bra, other items of Kercher's clothing, objects collected from Kercher's room, or in samples from her body ? although Guede's DNA was found everywhere, the letter points out."