The Bra Clasp

From The Murder of Meredith Kercher
Revision as of 18:05, 4 May 2017 by Ergon (Talk | contribs)

(diff) ← Older revision | Latest revision (diff) | Newer revision → (diff)
Jump to: navigation, search
A bra clasp containing Meredith and Raffaele's DNA was recovered from Meredith's room during the second collection exercise by the Scientific Police, on December 18, 2007. The clasp is from the bra Meredith was wearing when she was murdered. The bra had been removed some time after her death by cutting the back strap.[1]

The bra clasp was tested by a team of scientists from the Scientific Police, headed by Patrizia Stefanoni. DNA was found on the clasp and two separate kinds of test confirmed that Raffaele Sollecito's DNA was present.

The DNA evidence of the bra clasp presents a formidable problem for the defense and there have been various attempts by them to discredit it.


Is the DNA on the Bra Clasp Sollecito's DNA?


Yes. There is absolutely no doubt that it is Sollecito's DNA.[2]

Human DNA exists within 22 pairs of chromosomes that are not sex-related (known as autosomal chromosomes) and an additional pair of sex chromosomes, X and Y. Two different types of DNA test are in common use: one relies on the autosomal DNA and the other is based on testing the sex chromosomes. Both types of DNA tests were run on the sample from the bra clasp: a test for autosomal DNA and a test looking specifically for y-chromosomes, which is the male sex chromosome. On the autosomal DNA test Stefanoni found the DNA on the bra matched Sollecito on 16 locus-points.[3] That is an exceptionally strong match. In the United Kingdom only ten locus-points were used (till 2014). The CODIS system in the United States maintained a database of only 13 markers (till 2017) and having 10 was considered a match for most purposes.[4]

A y-chromosome test was also performed. Since women don't have y-chromosome there is no reassortment of the y-chromosome during fertilization. The y-chromosome only changes through chance mutations during spermatogenesis. This causes complications for identification, since close male relatives all share the same Y chromosome. The bra clasp DNA was also a match for Sollecito's y-chromosome.[5] This confirms that the DNA belongs to a male Sollecito likely not more than two or three degrees of separation from Raffaele. The benefit of the y-chromosome test is that it removes Meredith's DNA from the interpretation and so we have only Sollecito's DNA. The autosomal profile is a perfect match to Sollecito but because the quantity of Meredith DNA is so much greater, it can create false peaks (known as "stutters") in the DNA profile, which are close to the height of Sollecito's profile. Having the additional y-chromosome test gives us a level of certainty through redundancy.

At the appeal, the two court-appointed DNA experts, Stefano Conti and Carla Vecchiotti, reviewed the testing of the bra clasp. Vecchiotti conceded in court that Sollecito's y-chromosome was on the clasp, despite trying to avoid making that claim in her written report. Even after Vecchiotti stated it was Sollecito's y-chromosome, Judge Hellmann nevertheless concluded that there is no way to determine if DNA on the bra clasp was a match to Sollecito. Conti and Vecchiotti's criticisms of the autosomal DNA are, to put it plainly, wrong but, even if we ignore the autosomal DNA completely, the y-chromosome match that Vecchiotti concedes is present, limits the possible matches to Raffaele Sollecito, his father, and maybe his uncles.

With respect to the autosomal DNA, Conti and Vecchiotti seem to be contesting four loci of the fifteen matches. Dr. Tagliabracci, a DNA defense expert who testified during the original trial, contested the same four loci plus an additional one.[6] With the autosomal DNA being a 16 loci match, the probability that the DNA belongs to someone other than Sollecito is one in a trillion or two. If instead of debating the issue with the defense experts we just grant them all the contested locus-points, the probability that it is someone other than Sollecito is still one in over ten billion. This still makes it Sollecito's DNA profile, without even considering that the y-chromosome test really limited the possible contributors to just Sollecito and his close male relatives.

Having defense experts argue over issues that will never disprove the presence of Sollecito's DNA on the bra clasp is silly. Tagliabracci would need to bring into question considerably more before he would even come close to being able to argue that we can't be certain that Sollecito's DNA is on the bra clasp. Tagliabracci doesn't even try because the argument is not there to be made. There is no way to deny that the DNA profile on the bra clasp is the DNA profile of Raffaele Sollecito.

Problems with Collection and Risks of Contamination

Crime scene photograph of the collection of the bra clasp, December 18, 2007
The bra clasp was collected on December 18, 2007. Meredith's body was discovered November 2nd and the initial evidence collection at the cottage happened between the night of the 2nd and November 7th.[7] As such the bra clasp was collected 43 days later and this is often cited as grounds for why it should not be considered reliable. Despite its oversight not being a high point in the investigation, DNA evidence is routinely used in American cases months later and in cold cases even decades later: the delay in collection is not in itself a sufficient reason to reject the clasp from consideration.

While supporters of Knox are quick to point out the 43-day delay they are at a loss to explain why that matters: Two items with Guede profiles, the purse and the sweatshirt, were also collected on this second pass. DNA does not spontaneously appear and the cottage was a sealed crime scene. As long as the cottage remained closed and more specifically as long as Raffaele Sollecito did not enter Meredith's room the bra clasp could remain there uncollected for any length of time and Sollecito's DNA profile would never magically appear on it. While the passage of enough time might lead to a degradation of DNA no amount of time will ever spontaneously create DNA.

The second issue with the collection of the bra clasp is that the clasp moved 1.5 meters from the time it was first photographed to where it was eventually collected. Much like the delay in collection this again sounds bad but is mostly irrelevant. The bra clasp never left Meredith's room and there is no innocent reason why Sollecito's DNA would be elsewhere in the bedroom to contaminate the bra clasp, nor was his DNA found anywhere else, apart from on cigarette butt in the kitchen. Without a vehicle and with no Raffaele DNA to act as source, contamination is impossible.

Conti and Vecchiotti's Criticism of the Bra Clasp

Conti and Vecchiotti had two major criticisms of the bra clasp. The first was an abstract argument for unreliability based on a theoretical risk that was extrapolated from lapses in collection protocol that Conti and Vecchiotti had no evidence happened, but would like us to imagine the possibility that they did. That argument is as bad as it sounds, which is what led observers of the trial including the police and the lawyer representing the victim's family to wonder aloud that the independent experts were actually colluding with the defense.

The second criticism that Conti and Vecchiotti advance is that the forensic expert who analyzed the bra clasp for the police was incorrect to treat some peaks as stutters. This argument was based on claims that are not accepted by the scientific community but which can not be dismissed completely. Accepting Conti and Vecchiotti's criticism in this respect would have no impact on the fact that Sollecito's DNA is on the bra clasp but it would mean that a possible third faint male profile is also present. That is very likely but also useless information.

Anything is Possible — An Argument for Contamination

Conti and Vecchiotti created a DVD from the video footage of the evidence collection and cataloged a series of minor lapses in proper protocol. For example, at one point you see a technician without a hairnet. While that is improper the only risk is that the technician will contaminate the evidence with his own DNA. There is no reason why Sollecito's DNA would be in the hair of a technician from the forensic police. Another example that Conti and Vecchiotti point out is that when the forensic police run out of paper bags they use plastic bags that have a higher risk of destroying DNA. Again while that might be true, destroying DNA leads to the loss of evidence, not the spontaneous creation of suspect DNA. At one point Conti and Vecchiotti are critical of the frequency that the team changes their gloves. According to Conti and Vecchiotti, a forensic technician needs to change gloves every time they touch anything. While latex glove manufacturers might support Conti and Vecchiotti's position we are unable to find any criminal evidence collection manual that shares it. The instruction in the manuals is that technicians are to use discretion when deciding when it is appropriate to change gloves.

If the goal here is to determine if there is any reason to doubt the reliability of Sollecito's DNA on the bra clasp, Conti and Vecchiotti fail. None of the lapses they document are possible explanations for Sollecito's DNA on the bra clasp. Conti and Vecchiotti's position is made even less defensible when they are asked to explain how contamination might have happened. No one is asking Conti and Vecchiotti to tell the court definitively how contamination happened but since they are raising contamination as a reason to reject the bra clasp they are required to give some explanation of how that would come about. To this question Conti answered only that "anything is possible."

Why Contamination is not Possible

There are three main reasons why contamination is not possible. The first is that tertiary transfer has never been seen in the laboratory and second even if tertiary transfer were possible you'd still lack a source of Sollecito DNA. The final reason is that the bra clasp had an abundant quantity of Sollecito's DNA and as such even if the first two reasons did not apply the quantity alone would be sufficient to rule out contamination as a possibility.

Tertiary Transfer

Any claim that Sollecito's DNA got was the result of contamination would require tertiary transfer. What this means is that Sollecito would first need to transfer his DNA to some surface. This would be primary transfer of touch DNA. It happens although it is uncommon during the normal handling of objects[8] and even difficult when excessive force is applied.[9] A technician would then have to come into contact with the deposited Sollecito DNA and that contact would need to lead to DNA transfer. Most studies of secondary transfer look at person to person to person/object transfer where secondary transfer is possible but unlikely.[10] We should give Sollecito the benefit of the doubt and assume that person to object to person transfer is possible.

The problem for Sollecito is that he needs the DNA to be transferred one more time from the technician to the bra clasp and that is where he runs into problems -- tertiary transfer doesn't happen.[11] As technology advances and we gain the ability to obtain DNA profiles from ever decreasing quantities of genetic material tertiary transfer might become commonly detected but this testing happened in 2007 and the quantity of Sollecito DNA was sufficient to not make it a LCN sample.[12]

Lack of Sollecito Source DNA

For transfer to happen you need a source of Sollecito's DNA for the technician to touch. The only sample of Sollecito's DNA found in the cottage was a mixed sample with Knox on a cigarette butt from an ashtray in the kitchen. The bra clasp never left Meredith's room and none of Raffaele's DNA should have been in Meredith's room. That would be sufficient but in this situation we have the extra reassurance that the DNA was located nowhere in the entire cottage.

Two additional points with respect to a lack of a source for Sollecito DNA. The first is the claim that since Raffaele has been to the cottage his DNA would be in the dust. There is no DNA in dust. To be more accurate while there is DNA in dust the quantity of DNA and the fact that the number of contributors is so high makes it impossible to get a DNA profile from dust. A method for detecting human DNA in dust was only discovered a year after the testing of the bra clasp. As it currently stands we still can't obtain a DNA profile from dust by any method. It is impossible to attribute the profile that Stefanoni obtained to contamination from Raffaele's DNA being in dust.

The second rebuttal to the lack of a Sollecito DNA source being found is that the forensic police did not test every square inch of the cottage. As such it is possible that Sollecito's DNA was present but just never detected. While that is possible it is rather unlikely—the Scientific Police collected and tested over 160 samples from the cottage.[13] The decision would be between accepting that Raffaele's DNA was present, that it was not detected, that it managed to be transferred by a method which has never been successfully done in studies of DNA transfer, and that the DNA was transferred only to the bra clasp of the victim versus the DNA was there because Raffaele participated in the murder and cut off Meredith's.

The Quantity of Raffaele's DNA Excludes Contamination

Raffaele's DNA was not discovered anywhere other than the bra clasp and the mixed sample on the cigarette butt but addressing the valid claim that Sollecito DNA might have been present but just not on anything that was tested we should explore that possibility. The first thing that we know is that if Raffaele's DNA was present it would have been touch DNA. Touch DNA and LCN DNA are often confused by non-experts and while they are connected the connection is one of correlation rather than classification. Touch DNA is DNA transferred through contact with skin while LCN DNA is any DNA where the quantity of genetic material is so minute that the technician has to use additional amplification to get a profile. The confusion in equating the two rests in the fact that since the quantity of DNA transferred by touch DNA is so low that touch DNA transfers are often also LCN samples.

A second fact of DNA transfer is that the transfer has to always be smaller than the source. This is simple logic -- If you have 200 picograms of DNA and it transfers then the quantity of DNA must be less than 200 picograms since no transfer is perfect. This causes a problem since any argument for contamination is based on the belief that this case involves contamination in a previously undocumented tertiary transfer. That means the quantity of DNA was transferred at least twice after Sollecito's primary touch transfer. That is incompatible with the sample that was actually found on the bra clasp. The sample on the bra clasp is not even LCN DNA. In fact it is at the upper limits of what you'd expect for touch transfer. The quantity is easier to understand when you consider it is a metal hook that is an excellent candidate for transfer but the quantity makes it almost certainly primary transfer. Even if we ignore the fact that tertiary transfer has never been successfully documented the quantity would preclude this sample being the result of touch DNA transferred multiple times.

Are There Any Additional DNA Profiles on the Bra Clasp?

The best answer is unlikely but it doesn't matter. Raffaele Sollecito's DNA is definitely present and there is no way to deny that. The controversy over the existence of a possible additional profile stems from a claim that Stefanoni declared some peaks as meaningless data called stutters. Conti and Vecchiotti claim that the International Society for Forensic Genetics (ISFG) recommendations are strict rules that must be followed. Their position is that peaks should never be rejected as stutters if they are above 50 RFU in height and also over 15% of the height of the next known allele to their right. Conti and Vecchiotti's position is not generally accepted by DNA analysts including defense DNA expert Tagliabracci. Stefanoni had already explained that the ISFG guidelines do not set out a rigid formula but instead set out parameters to be used when interpreting peaks.

Conti and Vecchiotti unlike Tagliabracci are not attempting to claim that Raffaele's DNA is not present. They concede that Raffaele's DNA is on the bra clasp but they wish to make the claim that someone else's DNA is also present. Conti and Vecchiotti contend that in at least four loci there are additional peaks that suggest a faint third DNA profile. Even if we accept that the peaks are genuine rather than stutters there isn't enough information to use it to definitively identify someone. Meredith had a boyfriend so the proposition that another male profile might have been on the bra would not be hard to accept. More importantly if someone chooses to accept that the peaks are stutters and thus meaningless noise or if someone decides the peaks are genuine, Raffaele Sollecito's DNA is still undeniably present in a much greater quantity. For the purpose of determining if Raffaele came in contact with the bra, the peaks being discussed have no relevance.

Conclusions of the Nencini appeal court

All the evidence and arguments about the bra clasp were reviewed by Judge Nencini at Knox and Sollecto's 2011 appeal in Florence. Nencini concludes:

"It is thus possible to assert that the genetic investigations performed by the Scientific Police on the hook of the clasp of the bra worn by Meredith Kercher on the evening she was killed yielded a piece of evidence of indisputable significance. Both by the quantity of DNA analyzed and by the fact of having performed the analysis at 17 loci with unambiguous results, not to mention the fact that the results of the analysis were confirmed by the attribution of the Y haplotype to the defendant, it is possible to say that it has been judicially ascertained that Raffaele Sollecito’s DNA was present on the exhibit; an exhibit that was therefore handled by the defendant on the night of the murder."[14]


  1. Who Returned To Move Meredith?
  2. Dr. Stefanoni's Technical Assessment of Biologicals page 126
  3. Dr. Stefanoni's Technical Assessment of Biologicals page 126
  4. Combined DNA Index System
  5. Dr. Stefanoni's Technical Assessment of Biologicals page 126
  6. Adriano Tagliabracci's Testimony
  7. Giacinto Profazio's Testimony
  8. Phipps M and Petricevic S. The tendency of individuals to transfer DNA to handled items. Forensic Sci. Int. 168 (2007) 162-168.
  9. Rutty GN. An investigation into the transference and survivability of human DNA following simulated manual strangulation with consideration of the problem of third party contamination. Int. J. Legal Med. (2002) 116: 170-173.
  10. Phipps M, Petricevic S. The tendency of individuals to transfer DNA to handled items. Forensic Sci. Int. 168 (2007) 162-168
  11. The only support for tertiary transfer is from a Massachusetts trial. Dr. Greineder stood accused of murdering his wife and his DNA was found on gloves and a knife used in the crime. Greineder wished to explain the DNA as tertiary transfer so he hired a non-accredited private laboratory to test his theory. The non-accredited private laboratory testified that tertiary transfer was possible but the paper for that study was never published in any peer-reviewed journal. Attempts to replicate the results by respected laboratories failed.
  12. The British Crown Prosecution Service guide to Low Copy Number DNA testing in the Criminal Justice System says that non-LCN DNA tests involve 50 - 100 cells or more
  13. The full list of samples was presented to the Massei Court by Dr Stefanoni. A copy of her presentation, Dr. Stefanoni's Technical Assessment of Biologicals, is in the public domain. It lists over 160 samples that were taken from the cottage. Of these, only three matched Sollecito's DNA: Rep 145A was the mixed Knox/Sollecito DNA on a cigarette butt in an ashtray in the kitchen/living room, and Reps. 165A and 165B were the two samples taken from the bra clasp.
  14. Nencini Sentencing Report, p.250