Part 31 in the Knox/Sollecito case
The subject of DNA contamination has been a frequent topic of this blog. DNA contamination is again near the forefront of the Kercher murder and the second trial of Amanda Knox and Sollecito. The court-appointed, independent experts, Drs. Conti and Vecchiotti, have issued a report that raises the strong possibility of contamination. The odds that a forensic DNA sample are contaminated are hard to determine yet very important. Let us first examine a specific statement made with respect to environmental contamination and this case, then look at the question more generally. A subsequent entry will examine the collection practices of the Rome lab and Conti’s and Vecchiotti’s evaluation of their work in this case.
Professor Giuseppe Novelli, a researcher into medical genetics and forensic DNA profiling, said, «Il contaminante va dimostrato, dove nasce e dove è. Il gancetto contaminato dalla polvere? Più probabile che cada un meteorite e butti giù questo tribunale» "The contaminant needs to be demonstrated, where it comes from and where it is. The clasp contaminated by dust? It's more likely that a meteorite comes down and knocks down this courthouse." (translation by komponisto)
Let us assume that Dr. Novelli is specifically referring to household dust. The potential for dust being an issue in forensic DNA testing first came to light with the publication of a paper from Bonnie Brown and coworkers: Toothman et al., “Characterization of human DNA in environmental samples,” Forensic Science International 178 (2008) 7–15. These workers sampled dust from offices, research laboratories, and classrooms.
Figure 3 in this paper is an electropherogram of dust from a classroom, and it shows peaks of 1000 to 3500 RFU corresponding to 3-6 alleles in some loci associated with shorter pieces of DNA. As the length of the DNA fragments increases (moving from left to right on the elecropherogram), the height of the peaks decreased. The authors point out that this is consistent with DNA template that is partially degraded. The combination of a sample’s being a mixture and its yielding only partial profiles makes it very difficult to identify individual profiles.
The authors wrote, “Results of this study have implications regarding the processing of forensic samples. First, the presence of genotypeable human DNA in dust illustrates a significant
potential contamination source in forensic investigations. Twenty-five of 36 samples contained sufficient input human DNA for STR analysis using the AmpFlSTR® Profiler PlusTM assay (~1.0 ng), and 36% (including low-input samples) produced alleles at multiple loci. These results demonstrate that even though anti-contamination measures may be in place at a crime scene and the laboratory, trace DNA derived from dust in the vicinity of other evidence is capable of producing signals higher than background noise in STR analyses.”
Trace DNA and the environment
Secondary transfer is the movement of DNA from a donor to an intermediate object then to another object from which it is collected. Some forensic scientists only classify it as contamination when it occurs subsequently to the object’s being taken into custody; however, other workers prefer to treat innocent secondary transfer as equivalent to contamination on the basis that neither is relevant to the investigation. Both secondary transfer prior to an object’s being collected and contamination need to be understood more thoroughly for trace DNA to be used routinely in DNA forensics. Van Oorschot and colleagues wrote, “Greater effort needs to be made by police/crime investigators to investigate how a DNA sample arrived at the location where it was found, as well as by scientists to better understand the impact of activities on the relative amounts of DNA from particular sources at a crime scene… Some preliminary contributions to our knowledge of transfer in relation to residential burglary and street robbery have recently been made [67].
In their review of trace DNA in forensics Van Oorschot and colleagues suggested six remedies to address the problem of contamination:
“1. perform more studies similar to those of Raymond et al. [67], Cook and Dixon [202], Dowlman et al. [203] and Toothman et al. [201] in order to learn more about the occurrence and persistence of DNA on particular surfaces in different environmental conditions” Understanding how prevalent and persistent background DNA is in the environment is far from being completely understood. Dr. Novelli declined to comment for this report, but his comparison with a meteorite is premature, at best.
The relationship between DNA sample size and the ease of transfer
In their 2010 review article Van Oorschot and colleagues wrote, “Contaminant DNA may appear as either the major or minor sample within a mixture or, alternatively, may overwhelm the target DNA completely.” One explanation for this somewhat counterintuitive statement is that contamination in the laboratory may introduce DNA from previous PCR amplifications by a number of possible routes. However, the two main DNA profiles of interest (Meredith’s on the knife blade and Raffaele’s putative profile on the bra clasp) both involve relatively small amounts of DNA; therefore, we will focus on small samples.
The discussion of low copy number (LCN) testing from the Crown Prosecution Service noted, “This increased sensitivity means ultra-clean laboratories are needed for the testing to minimise contamination of the sample by DNA from any other source.” The New Zealand Institute of Environmental Science and Research has spent $1 million building anticontamination areas for low copy number (LCN) DNA forensics. The New Zealand Herald wrote, “The bogey is contamination. The very sensitivity of the technique which enables it to extract a DNA profile from the tiniest sample also makes it extremely vulnerable to contamination. Stringent measures are needed to minimise that risk… We live in a ‘soup’ of DNA, explains ESR forensic programme manager Keith Bedford. ‘If I were to shed dandruff, massive amounts of dna could fall ... hair could carry DNA. The way I am speaking at the moment, we could probably detect DNA on this pad in front of me.’”
Sara Gino testified for the defense in the trial of the first instance, and some of what she had to say is pertinent to this issue. From the Massei report (p. 258, English translation): “She reaffirmed that [the risk of] contamination exists, and emphasised that in minimal quantities of DNA there is not necessarily a greater risk of contamination but it was easier to notice the effects of the contamination and be misled (‘...It's not that the risk of contamination is greater; but it is easier to see the contamination...’ page 92).” In response to a question on this subject, Professor Dan Krane responded, “There is absolutely no question but that contamination is a much greater problem in LCN cases than conventional DNA testing. The reasons that it is a greater problem are both because it is easier to detect contaminants ([Sara] Gino's point) and because it is easier to transfer (and to transfer without knowing) smaller amounts of DNA than larger amounts of DNA.”
Some examples of DNA contamination
Farah Jama was a young man accused of rape on the basis of his DNA seemingly being found on the alleged victim. Mr. Jama is black and at 21 was too young to have entered the club at which the incident occurred, which catered to people over 28. Moreover, the alleged victim did not recall seeing a black man at the club that night. Yet, as Milanda Rout wrote, “But the judge and the jury did not buy his alibi, despite supporting evidence from his father, brother and friend. Instead, they believed the forensic scientist who testified there was a one in 800 billion chance that the DNA belonged to someone other than the accused man.” After Mr. Jama spent more that eighteen months in prison, he was released because prosecutors said that they could not rule out contamination. The contamination event may have occurred during two forensic medical examinations, one of the victim and the other of Mr. Jama on an unrelated matter that occurred one day earlier.
Perhaps the most thoroughly studied case of contamination is that which occurred in the Jaidyn Leskie case. This blog has covered the Leskie case on two previous occasions. The DNA of a woman who probably never left her village was found on the clothing of the submerged body of a toddler. She was a mentally challenged woman who may have been raped, which is why her DNA was being examined. Because the woman was such an exceedingly unlikely suspect, the only reasonable explanation was contamination. Contamination has been documented on several occasions when evidence items from unrelated cases are examined within a few days in the same lab.
Russell John Gesah was charged with the rape and murder of a mother and child. Kathleen Skeen wrote, “A Victorian Police Forensic Services Centre review found clothing with Mr Gesah's DNA from an unrelated offence had been examined on the same day and same surface as clothing from the Tapp case." The Gesah case, and the murders of Jane Mixer and Jane Durrua (see below) are all examples of DNA cold hits. The Gesah case prompted the State of Victoria to reexamine thousands of cases (see below).
Gregory Turner might have been convicted of murder on the basis of DNA evidence. However, a forensic worker contaminated a key piece of evidence with his and her DNA. She also acknowledged contamination in two other cases on which she had worked. An interesting aspect of the Turner case is that the DNA from the victim came from her fingernails, and Mr. Turner’s DNA came from his wedding ring. These facts suggest that the presence of liquids is not necessary to bring about cross-contamination, in contrast to the implications of Patrizia Stefanoni’s testimony in the present case.
The murder of Jane Mixer was initially attributed to a serial killer. When Gary Leiterman’s DNA was found on the decades-old evidence, he was convicted. However, the presence of the DNA matching then four-year old John Ruelas on the same item of evidence (despite Ruelas’s living in another city) strongly points to this being another example of contamination. This illustrates another important principle. One does not always know the precise moment that contamination occurred, but one can infer contamination when the direct deposit of DNA is shown to be highly unlikely.
A seemingly solved cold case that turned out to be contamination involved the 1968 murder of Jane Durrua. Jerry Lee Bellamy’s DNA was found when the evidence was tested in 1999. Evidence against Mr. Bellamy in an unrelated case was tested on the same day as items from the Durrua case. The actual evidence of contamination was not conclusive, but despite this, charges against Mr. Bellamy were dropped. Alleged serial killer Robert Zarinski was later arrested, but he died before he could be tried.
The difficulties in quantifying the frequency of DNA contamination
Not all labs document contamination events. Some labs argue that contamination that is identified with the use of negative control experiments does not count as contamination. Negative controls will spot wholesale contamination events but will not necessarily catch sporadic contamination. These facts make it difficult to quantify how frequently contamination occurs. However, it does not seem to be an especially rare event. Professor Thompson is a lawyer who specializes in probability theory as it relates to DNA profiling. Maura Dolan reported that he is among the leading authorities on laboratory errors in the United States. In response to a request from the Los Angeles Times to review the records from some California forensics labs, Thompson said, “’on a regular basis, laboratory personnel make mistakes that could lead to false identifications’ of suspects.” He also indicated that what has emerged in recent years is just “the tip of the iceberg.”
In 2008 Professor Thompson wrote an article, “The Potential for Error in Forensic DNA Testing (and How That Complicates the Use of DNA Databases for Criminal Identification"). “Doubt was also cast on a number of convictions in Queensland when a forensic scientist who had previously worked for a state forensic laboratory publicly expressed concerns about the reliability of the lab’s work. He told The Australian newspaper that it was not uncommon for the lab to mix up DNA samples from different cases.[62] For example, he said that analysts’ own DNA, from blood samples used as analytical controls, often was mixed up with (or found its way into) casework samples, creating false matches: “[Q]uite often my (colleague) would walk down the aisle and say, ‘I’ve just committed another rape on the Gold Coast.’”[62] The analyst said that while many such errors were caught, sample limitations made it impossible to resample or retest in some questionable cases.” These remarks underscore the notion that DNA contaminations are not a rare event.
[62. A. McDonald, “DNA evidence claim clouds Australian convictions,” The Australian, July 8, 2006.]
In response to the Russell John Gesah contamination incident (see above), the Victorian police reexamined their cases involving DNA forensics. During the period from 1988 to 2008 the Victorian police service handled 7000 cases involving DNA. According to Peter Gregory and coauthors, “In 2003, Mr Scheffer told an inquest on Moe toddler Jaidyn Leskie that since late 1999, 39 cases had been identified as requiring "diagnostic and corrective action", with most involving contamination.
Finally, testimony reported by Annabelle McDonald (in The Australian) implied that mixing up samples is a not uncommon event. Although mislabeling of samples (if that is what mixing up means) is not itself contamination, it has the potential to lead to the same erroneous judicial result. A mislabeling in Nevada was uncovered during an independent review of the Lazaro Sotolusson case. In addition Dwayne Jackson was also the victim of a similar mistake at the Las Vegas forensics lab.
Conclusions
There is not yet enough information on environmental contamination to make conclusive statements about how common environmental contamination is; however, DNA is deposited routinely in all sorts of ways that are unrelated to criminal activity. The authors of a recent study believe that environmental dust can give rise to extra alleles in evidence samples. The frequency of contamination is difficult to quantify, but it is not an especially rare occurrence. The chances of contamination are greater for DNA in the low template range than they are for larger samples. Historical examples of contamination suggest that it is more likely to occur when items of evidence are processed closely in time. Contrary to the implication of Dr. Novelli's remarks, it is rarely the case that the exact mechanism of contamination is proven.
Showing posts with label LCN. Show all posts
Showing posts with label LCN. Show all posts
Sunday, September 4, 2011
Monday, September 27, 2010
An overview of some DNA evidence in the murder of Meredith Kercher
Part 23 in the Knox/Sollecito case
Executive summary
The most severe problems with the bra clasp are that three other partial profiles are present, that it was moved by unknown means before it was collected, that it was handled way too much during its collection, and that it might be within LCN range. Massei’s reasoning with respect to the disputed loci is fundamentally flawed. The most severe problems with Meredith’s profile on the knife are lack of blood, the low peak heights, the alleles that dropped in and dropped out, and the fact that an inferior version of low copy number (LCN) DNA analysis was used. Amanda’s DNA found with Meredith’s blood is not inculpatory, because DNA in one’s home is rarely out of the ordinary, and there are collection problems with these samples. Finally, the electronic data files were not released, and this has hampered the ability of the defense to challenge the evidence properly.
General defense arguments
Besides the argument that a profile is insufficient to identify a person (see below), an argument that the defense can use is that the DNA is present due to innocent activity. For example, Amanda’s DNA on the handle of the knife may be due to her using it to cook. In addition, the defense can argue three things with respect to how a defendant’s DNA came to be on an item of evidence: secondary/tertiary transfer, contamination, and evidence tampering.
The legal standard for a DNA profile and low copy number DNA
One argument that the defense can use is to say that a given DNA profile fails to clear the legal bar for identifying someone. One might use the analogy to a partial fingerprint. An example here is the necessity of testing low copy number (LCN) DNA twice, whereas the knife was only tested once. The bra clasp falls into a gray area in terms the amount of Raffaele’s DNA present; the defense is arguing that it, too, falls into the LCN range. Meredith’s profile on the knife also shows other evidence of being in the LCN range, such as peak height imbalance. Moreover, the majority of the peaks are below 50 RFU in peak height, most labs have set a threshold of at least 50 RFU as the minimum height for a peak to be counted. Meredith’s DNA profile should not have been accepted by the court as evidence, and Raffaele’s lawyers may have success with their argument.
Contamination
If we only allowed the contamination argument in cases where the defense could demonstrate the exact mechanism of when and how it happened, we would exclude known cases where it did happen. The prosecution must perform negative controls under the same conditions as the evidence and disclose the results of these control experiments to the defense. If DNA shows up in the negative controls, my understanding is that all evidentiary samples processed at the same time must be performed over. When no rational explanation for the presence of DNA on an item can be offered, one is forced to assume that the DNA arrived via contamination. Some cases of DNA contamination are the Jaidyn Leskie murder, the Jane Mixer murder, the Farah Jama rape case, the Gregory Turner case, and the profile N case in New Zealand.
The lack of frequent glove changes and the handling of the clasp by many forensic technicians are problematic for the prosecution. Moreover, Dr. Stefanoni’s testimony as presented in the Massei report (pp. 202-203 in the English translation at Perugia Murder FIle) on this subject is open to serious challenge. Her argument is that they did not change gloves with respect to a certain item of evidence and this piece of evidence did not show contamination. Therefore, contamination is difficult, requiring either liquids or vigorous rubbing. This view seems to be at odds with the consensus of the field, as well as the facts of the Gregory Turner case, which involved transfer of DNA from fingernails to a wedding ring.
Confirmation bias
Not doing what amounts to substrate controls on the mixed Knox/Kercher DNA samples might have been due to confirmation bias. Not obtaining DNA reference samples from Laura and Filomena are behaviors consistent with confirmation bias. Using a lower peak height threshold for the knife than any other piece of evidence contradicts the words of an introductory textbook on DNA forensics as well as general scientific principles. However, there is an additional reason to suspect that some form of investigator bias was at work with respect to the knife profile. Sample 164 was blood from the wall of a bedroom, but it was not tested because of a “negative preliminary (quantification) result.” One surmises that there was not enough DNA to continue the forensic analysis. Why should testing have been stopped for item 164, which had blood, when it was continued for the knife, which had no blood?
Expert testimony
The prosecution’s witnesses and the defense’s witnesses do not have equal scientific standing. Dr. Stefanoni has not published any articles on DNA forensics of which I am aware. On the other hand the nine signers of the open letter (Dr. Johnson, Dr. Hampikian, and the seven co-signers) publish regularly. That is not to say that publication record or academic prestige is everything; there is also variation in the abilities of expert witnesses to convince a jury that they are correct. In this matter the defense may have not fared as well.
The bra clasp
The following discussion assumes that a full DNA profile corresponding to Raffaele Sollecito’s is present, but that does not answer the question of how or when it got there. One problem with the bra clasp as evidence against Raffaele is that his DNA is not found on the bra itself, a point that his lawyers raised in the Micheli preliminary hearing. A more serious problem is that partial profiles of three other people have also been reported.
An answer to the question of how Raffaele’s DNA was deposited on the clasp is that it might have been deposited the same way as three DNA profiles from unknown individuals. To put it another way, if we acknowledge that DNA from three unknown individuals came to be on the clasp innocently, then what makes Raffaele’s DNA different? It is very unlikely that four people handed the bra clasp as part of a sexual assault and murder. Primary transfer before the assault seems equally unlikely; most people fold their own laundry, and someone else folding it would only account for one profile. So the unknown DNA had to arrive either from secondary transfer or from contamination.
One possibility involving secondary transfer involves the towels. The towels that Rudy probably took from the bathroom might have had DNA from anyone who washed his or her hands in the bathroom and used the towels to dry off, including Raffaele, who had cooked there. If the towel were placed over the clasp and stepped on, it could transfer DNA to the clasp. This might also explain the deformation of the clasp. Another possible route of secondary transfer is that whoever moved the clasp before it was collected deposited Raffaele’s DNA (possibly originating from the door).
There is a good deal of misunderstanding involving DNA contamination and the clasp. It is often said that the only item that tested positive for Raffaele’s DNA was a cigarette butt, and so how could contamination occur? There are several problems with this argument. First, one should not equate the DNA that the investigators found with the total amount of DNA Raffaele left at the cottage. The investigators seemed to be focused on blood, as well as Meredith’s body. They were not taking a random sampling of the cottage. Second, there is no reason to exclude contamination from the cigarette butt in the lab, although if they were tested far apart in time, contamination is less likely. Third, Raffaele’s reference sample is a potentially serious source of contamination. In PCR-based DNA forensics, the DNA is amplified very roughly a millionfold in amount. That is why good labs separate the pre-PCR from the post PCR-samples.
The amount of Raffaele’s DNA on the clasp is borderline LCN. If it is judged to be below the LCN cutoff, it would ordinarily have to be tested twice and only those alleles that showed up in both runs should be counted. It is sometimes said that the amount of Raffaele’s DNA was so large as to mean that it could only arise from vigorous rubbing (primary transfer). The fact that the amount of DNA is actually low would seem contradict such an argument. However, it is the premise that is wrong; the DNA profile itself can rarely give an indication of when and how it was deposited. One cannot rule out primary transfer when the amount of DNA is low any more than one can rule out secondary transfer when the amount of DNA.
Raffaele’s appeal with respect to the bra clasp
The discussion above presumes that a good, complete profile was found. However, Dr. Tagliabracci disputed that the profile matched Raffaele’s for at least six of the loci. If Sollecito’s profile were strong and if the bra clasp DNA were not a mixture, there might be fewer opportunities for disagreement between Dr. Tagliabracci and Dr. Stefanoni. Of the six disputed loci from the bra clasp DNA profile, Massei wrote (pp. 296-297 of the Perugia Murder File English translation):
“Consequently, there are apparently a considerable number of loci that are not the subject of dispute, a number which seems to be greater than the number of disputed loci and greater than the number of six loci with reference to which Professor Tagliabracci had previously declared, before the current systems were available‚ it was enough ... we made hypotheses even with six loci‛ (page [319] 103). The circumstance now exposed allows, it was held, the following consideration: if, despite the subjective contribution of the geneticist, the interpretative disagreement regarding the non-compatibility of Raffaele Sollecito’s profile with the loci that had contributed to forming trace 165B involved those loci indicated by Professor Tagliabracci during the course of the hearing and at pages 20 and 21 of the previously mentioned memorandum conclusions, it must be held that, for the greatest number of loci at least, the peaks were so clear and the interpretation so sound that they could not be contested. Consequently, the overall result should be considered fully reliable, even disregarding the repetition of the analysis. It should however be noted that Dr. Stefanoni, during the hearing at which she testified, had offered suitable explanations and answers which this Court considers acceptable.”
Raffaele’s appeal document correctly notes that Massei’s argument about the numbers of disputed and undisputed loci is contrary to the principles of forensic genetics. Let us assume that the data are clear enough to avoid ambiguity and consider the following analogy. Suppose that a winning lottery number is 12497635834, and I have a lottery ticket that is 12497235834. And suppose I claim that since my ticket has 10 out of the 11 numbers identical, I am a winner. That argument makes as much sense as Massei’s does.
But what of Dr. Tagliabracci’s statement that six loci used to be enough to form hypotheses? Suppose that initially a complete profile consisted of six loci. If a person matched all six loci, he or she would not be excluded as the DNA donor to that sample. However, if that person matched at only 5 loci and failed to match at the sixth locus, then he or she would be excluded. Now suppose an improved test with 10 loci became available. Then a person who matched all ten loci would not be excluded, and the number of other people who could also match would be much smaller than in the case with 6 loci. However, a person who matched at 9 loci but failed to match the tenth locus would still be excluded, even though 9 is greater than 6.
Massei must believe that at all six disputed loci, the DNA is Raffaele’s, or at the very least that the results in all six loci are indeterminate (if the latter were true, it would indicate that Raffaele’s DNA constituted a partial profile, not a complete one). Massei does not provide a clear reason for rejecting Dr. Tagliabracci’s assessment in favor of Dr. Stefanoni’s. It is difficult to see why a sentencing report the fails to provide reasons is any better than no sentencing report at all.
The knife profile
The peak heights on the DNA profile culled from the kitchen knife are all below 100 relative fluorescence units (RFU), and most are below 40 RFU. This is below any threshold of which I am aware. What was the harm in using a lower peak threshold? One can argue that it obliges the forensic scientist to use the same threshold for all the samples on the basis of consistency. It is a dollars-to-donuts bet that some evidence of contamination could be found at this atypically low peak threshold among the hundreds of samples run.
There is no detectable blood on the knife. The open letter asserts that if a bloody knife were cleaned, one would remove detectable traces of DNA before detectable traces of blood. If one claims that the DNA arose from other tissue, then I would ask how it is possible to remove blood cells and not other cells. The cleaning problem only grows more severe if one claims that the knife were cleaned with bleach and that traces of bleach were found. Even trace amounts of bleach are known to destroy DNA for forensic profiling.
The profile shows evidence of alleles dropping in and dropping out. In other words there is one allele where Meredith’s profile is weak or absent, and there is one locus with two peaks that are not part of Meredith’s profile. The peaks within each locus are often very uneven (as much as roughly threefold) in peak height, yet they should be approximately the same height in a good profile. These problems are to be expected when DNA is in the low copy number (LCN) range. When DNA falls into such a low range of amounts, forensic scientists generally test it at least twice and accept only those peaks that appear in both runs.
One can argue that LCN profiling should ordinarily be accepted by a court. However, Dr. Stefanoni used an inferior version of LCN DNA profiling, one that has never appeared in the scientific literature. LCN profiling is typically done in specialized buildings, away from the laboratory doing regular profiling. These precautions are necessary because LCN profiling is more prone to contamination than ordinary PCR-based profiling. These precautions were not followed with respect to the knife, and it was only tested once.
Another problem with the knife is that the second officer to have possession of the knife was at Meredith’s cottage just before receiving it. This raises the odds of contamination outside of the lab. Meredith’s profile probably arose through contamination in the laboratory, but contamination during the time it was taken into custody is also a possibility.
The mixed DNA samples
A number of samples that appeared to be blood had both Meredith’s and Amanda’s DNA. Three of the mixed DNA samples were probably blood and three may or may not have been blood. This would be very weak evidence under most conditions. Amanda’s DNA is expected to be in many locations in her own home. The fact that samples were not taken close to the blood (essentially substrate controls) means that one cannot rule out an innocent explanation for their existence. To argue that these samples are inculpatory, one is almost forced to assume that Amanda’s DNA is from her blood. White blood cells contain DNA; therefore, Amanda’s DNA might have arisen from her blood. Yet without characterizing or quantifying the amount of biological material that gave rise to her DNA, there is no reason to believe that the samples must be from blood. No such tests were done.
However, the prosecution has two additional problems with its case. First, Dr. Stefanoni did not change gloves when collecting multiple samples (see above). Therefore, she might have mixed samples herself. Second, at least one of these samples had a third profile in it, from an unknown individual. If this person’s DNA arose from innocent means, there is no reason to exclude the possibility that Amanda’s did also.
The lack of DNA
The lack of Raffaele’s DNA or Amanda’s DNA on Meredith’s body, when Raffaele is thought to have restrained her and Amanda to have throttled her calls into question this part of the prosecution’s narrative. The number of actual instances where DNA was used in strangulation cases where the DNA originated from the victim’s neck, as opposed to the victim’s fingernails, is small. However, some instances of alleged domestic violence cases have used swabbing of bruised or reddened areas on the alleged victim as evidence.
Conclusions
The way that the bra clasp was handled and the lack of a clear chain of custody cast doubt on this piece of evidence. The lack of blood on the knife calls into grave question whether the DNA got there before or after the police took it into evidence, as argued in the Johnson/Hampikian open letter. Secondary transfer is a likely means for Raffaele’s DNA being in the bra clasp, and contamination, either in the lab or during collection, is a likely means for Meredith’s DNA being found on the knife. The mixed DNA samples are virtually meaningless. The single most troubling aspect of the DNA evidence is the lack of full disclosure of the electronic data files and other documentation relating to the DNA forensics. The prosecution is acting as if it had something to hide.
Executive summary
The most severe problems with the bra clasp are that three other partial profiles are present, that it was moved by unknown means before it was collected, that it was handled way too much during its collection, and that it might be within LCN range. Massei’s reasoning with respect to the disputed loci is fundamentally flawed. The most severe problems with Meredith’s profile on the knife are lack of blood, the low peak heights, the alleles that dropped in and dropped out, and the fact that an inferior version of low copy number (LCN) DNA analysis was used. Amanda’s DNA found with Meredith’s blood is not inculpatory, because DNA in one’s home is rarely out of the ordinary, and there are collection problems with these samples. Finally, the electronic data files were not released, and this has hampered the ability of the defense to challenge the evidence properly.
General defense arguments
Besides the argument that a profile is insufficient to identify a person (see below), an argument that the defense can use is that the DNA is present due to innocent activity. For example, Amanda’s DNA on the handle of the knife may be due to her using it to cook. In addition, the defense can argue three things with respect to how a defendant’s DNA came to be on an item of evidence: secondary/tertiary transfer, contamination, and evidence tampering.
The legal standard for a DNA profile and low copy number DNA
One argument that the defense can use is to say that a given DNA profile fails to clear the legal bar for identifying someone. One might use the analogy to a partial fingerprint. An example here is the necessity of testing low copy number (LCN) DNA twice, whereas the knife was only tested once. The bra clasp falls into a gray area in terms the amount of Raffaele’s DNA present; the defense is arguing that it, too, falls into the LCN range. Meredith’s profile on the knife also shows other evidence of being in the LCN range, such as peak height imbalance. Moreover, the majority of the peaks are below 50 RFU in peak height, most labs have set a threshold of at least 50 RFU as the minimum height for a peak to be counted. Meredith’s DNA profile should not have been accepted by the court as evidence, and Raffaele’s lawyers may have success with their argument.
Contamination
If we only allowed the contamination argument in cases where the defense could demonstrate the exact mechanism of when and how it happened, we would exclude known cases where it did happen. The prosecution must perform negative controls under the same conditions as the evidence and disclose the results of these control experiments to the defense. If DNA shows up in the negative controls, my understanding is that all evidentiary samples processed at the same time must be performed over. When no rational explanation for the presence of DNA on an item can be offered, one is forced to assume that the DNA arrived via contamination. Some cases of DNA contamination are the Jaidyn Leskie murder, the Jane Mixer murder, the Farah Jama rape case, the Gregory Turner case, and the profile N case in New Zealand.
The lack of frequent glove changes and the handling of the clasp by many forensic technicians are problematic for the prosecution. Moreover, Dr. Stefanoni’s testimony as presented in the Massei report (pp. 202-203 in the English translation at Perugia Murder FIle) on this subject is open to serious challenge. Her argument is that they did not change gloves with respect to a certain item of evidence and this piece of evidence did not show contamination. Therefore, contamination is difficult, requiring either liquids or vigorous rubbing. This view seems to be at odds with the consensus of the field, as well as the facts of the Gregory Turner case, which involved transfer of DNA from fingernails to a wedding ring.
Confirmation bias
Not doing what amounts to substrate controls on the mixed Knox/Kercher DNA samples might have been due to confirmation bias. Not obtaining DNA reference samples from Laura and Filomena are behaviors consistent with confirmation bias. Using a lower peak height threshold for the knife than any other piece of evidence contradicts the words of an introductory textbook on DNA forensics as well as general scientific principles. However, there is an additional reason to suspect that some form of investigator bias was at work with respect to the knife profile. Sample 164 was blood from the wall of a bedroom, but it was not tested because of a “negative preliminary (quantification) result.” One surmises that there was not enough DNA to continue the forensic analysis. Why should testing have been stopped for item 164, which had blood, when it was continued for the knife, which had no blood?
Expert testimony
The prosecution’s witnesses and the defense’s witnesses do not have equal scientific standing. Dr. Stefanoni has not published any articles on DNA forensics of which I am aware. On the other hand the nine signers of the open letter (Dr. Johnson, Dr. Hampikian, and the seven co-signers) publish regularly. That is not to say that publication record or academic prestige is everything; there is also variation in the abilities of expert witnesses to convince a jury that they are correct. In this matter the defense may have not fared as well.
The bra clasp
The following discussion assumes that a full DNA profile corresponding to Raffaele Sollecito’s is present, but that does not answer the question of how or when it got there. One problem with the bra clasp as evidence against Raffaele is that his DNA is not found on the bra itself, a point that his lawyers raised in the Micheli preliminary hearing. A more serious problem is that partial profiles of three other people have also been reported.
An answer to the question of how Raffaele’s DNA was deposited on the clasp is that it might have been deposited the same way as three DNA profiles from unknown individuals. To put it another way, if we acknowledge that DNA from three unknown individuals came to be on the clasp innocently, then what makes Raffaele’s DNA different? It is very unlikely that four people handed the bra clasp as part of a sexual assault and murder. Primary transfer before the assault seems equally unlikely; most people fold their own laundry, and someone else folding it would only account for one profile. So the unknown DNA had to arrive either from secondary transfer or from contamination.
One possibility involving secondary transfer involves the towels. The towels that Rudy probably took from the bathroom might have had DNA from anyone who washed his or her hands in the bathroom and used the towels to dry off, including Raffaele, who had cooked there. If the towel were placed over the clasp and stepped on, it could transfer DNA to the clasp. This might also explain the deformation of the clasp. Another possible route of secondary transfer is that whoever moved the clasp before it was collected deposited Raffaele’s DNA (possibly originating from the door).
There is a good deal of misunderstanding involving DNA contamination and the clasp. It is often said that the only item that tested positive for Raffaele’s DNA was a cigarette butt, and so how could contamination occur? There are several problems with this argument. First, one should not equate the DNA that the investigators found with the total amount of DNA Raffaele left at the cottage. The investigators seemed to be focused on blood, as well as Meredith’s body. They were not taking a random sampling of the cottage. Second, there is no reason to exclude contamination from the cigarette butt in the lab, although if they were tested far apart in time, contamination is less likely. Third, Raffaele’s reference sample is a potentially serious source of contamination. In PCR-based DNA forensics, the DNA is amplified very roughly a millionfold in amount. That is why good labs separate the pre-PCR from the post PCR-samples.
The amount of Raffaele’s DNA on the clasp is borderline LCN. If it is judged to be below the LCN cutoff, it would ordinarily have to be tested twice and only those alleles that showed up in both runs should be counted. It is sometimes said that the amount of Raffaele’s DNA was so large as to mean that it could only arise from vigorous rubbing (primary transfer). The fact that the amount of DNA is actually low would seem contradict such an argument. However, it is the premise that is wrong; the DNA profile itself can rarely give an indication of when and how it was deposited. One cannot rule out primary transfer when the amount of DNA is low any more than one can rule out secondary transfer when the amount of DNA.
Raffaele’s appeal with respect to the bra clasp
The discussion above presumes that a good, complete profile was found. However, Dr. Tagliabracci disputed that the profile matched Raffaele’s for at least six of the loci. If Sollecito’s profile were strong and if the bra clasp DNA were not a mixture, there might be fewer opportunities for disagreement between Dr. Tagliabracci and Dr. Stefanoni. Of the six disputed loci from the bra clasp DNA profile, Massei wrote (pp. 296-297 of the Perugia Murder File English translation):
“Consequently, there are apparently a considerable number of loci that are not the subject of dispute, a number which seems to be greater than the number of disputed loci and greater than the number of six loci with reference to which Professor Tagliabracci had previously declared, before the current systems were available‚ it was enough ... we made hypotheses even with six loci‛ (page [319] 103). The circumstance now exposed allows, it was held, the following consideration: if, despite the subjective contribution of the geneticist, the interpretative disagreement regarding the non-compatibility of Raffaele Sollecito’s profile with the loci that had contributed to forming trace 165B involved those loci indicated by Professor Tagliabracci during the course of the hearing and at pages 20 and 21 of the previously mentioned memorandum conclusions, it must be held that, for the greatest number of loci at least, the peaks were so clear and the interpretation so sound that they could not be contested. Consequently, the overall result should be considered fully reliable, even disregarding the repetition of the analysis. It should however be noted that Dr. Stefanoni, during the hearing at which she testified, had offered suitable explanations and answers which this Court considers acceptable.”
Raffaele’s appeal document correctly notes that Massei’s argument about the numbers of disputed and undisputed loci is contrary to the principles of forensic genetics. Let us assume that the data are clear enough to avoid ambiguity and consider the following analogy. Suppose that a winning lottery number is 12497635834, and I have a lottery ticket that is 12497235834. And suppose I claim that since my ticket has 10 out of the 11 numbers identical, I am a winner. That argument makes as much sense as Massei’s does.
But what of Dr. Tagliabracci’s statement that six loci used to be enough to form hypotheses? Suppose that initially a complete profile consisted of six loci. If a person matched all six loci, he or she would not be excluded as the DNA donor to that sample. However, if that person matched at only 5 loci and failed to match at the sixth locus, then he or she would be excluded. Now suppose an improved test with 10 loci became available. Then a person who matched all ten loci would not be excluded, and the number of other people who could also match would be much smaller than in the case with 6 loci. However, a person who matched at 9 loci but failed to match the tenth locus would still be excluded, even though 9 is greater than 6.
Massei must believe that at all six disputed loci, the DNA is Raffaele’s, or at the very least that the results in all six loci are indeterminate (if the latter were true, it would indicate that Raffaele’s DNA constituted a partial profile, not a complete one). Massei does not provide a clear reason for rejecting Dr. Tagliabracci’s assessment in favor of Dr. Stefanoni’s. It is difficult to see why a sentencing report the fails to provide reasons is any better than no sentencing report at all.
The knife profile
The peak heights on the DNA profile culled from the kitchen knife are all below 100 relative fluorescence units (RFU), and most are below 40 RFU. This is below any threshold of which I am aware. What was the harm in using a lower peak threshold? One can argue that it obliges the forensic scientist to use the same threshold for all the samples on the basis of consistency. It is a dollars-to-donuts bet that some evidence of contamination could be found at this atypically low peak threshold among the hundreds of samples run.
There is no detectable blood on the knife. The open letter asserts that if a bloody knife were cleaned, one would remove detectable traces of DNA before detectable traces of blood. If one claims that the DNA arose from other tissue, then I would ask how it is possible to remove blood cells and not other cells. The cleaning problem only grows more severe if one claims that the knife were cleaned with bleach and that traces of bleach were found. Even trace amounts of bleach are known to destroy DNA for forensic profiling.
The profile shows evidence of alleles dropping in and dropping out. In other words there is one allele where Meredith’s profile is weak or absent, and there is one locus with two peaks that are not part of Meredith’s profile. The peaks within each locus are often very uneven (as much as roughly threefold) in peak height, yet they should be approximately the same height in a good profile. These problems are to be expected when DNA is in the low copy number (LCN) range. When DNA falls into such a low range of amounts, forensic scientists generally test it at least twice and accept only those peaks that appear in both runs.
One can argue that LCN profiling should ordinarily be accepted by a court. However, Dr. Stefanoni used an inferior version of LCN DNA profiling, one that has never appeared in the scientific literature. LCN profiling is typically done in specialized buildings, away from the laboratory doing regular profiling. These precautions are necessary because LCN profiling is more prone to contamination than ordinary PCR-based profiling. These precautions were not followed with respect to the knife, and it was only tested once.
Another problem with the knife is that the second officer to have possession of the knife was at Meredith’s cottage just before receiving it. This raises the odds of contamination outside of the lab. Meredith’s profile probably arose through contamination in the laboratory, but contamination during the time it was taken into custody is also a possibility.
The mixed DNA samples
A number of samples that appeared to be blood had both Meredith’s and Amanda’s DNA. Three of the mixed DNA samples were probably blood and three may or may not have been blood. This would be very weak evidence under most conditions. Amanda’s DNA is expected to be in many locations in her own home. The fact that samples were not taken close to the blood (essentially substrate controls) means that one cannot rule out an innocent explanation for their existence. To argue that these samples are inculpatory, one is almost forced to assume that Amanda’s DNA is from her blood. White blood cells contain DNA; therefore, Amanda’s DNA might have arisen from her blood. Yet without characterizing or quantifying the amount of biological material that gave rise to her DNA, there is no reason to believe that the samples must be from blood. No such tests were done.
However, the prosecution has two additional problems with its case. First, Dr. Stefanoni did not change gloves when collecting multiple samples (see above). Therefore, she might have mixed samples herself. Second, at least one of these samples had a third profile in it, from an unknown individual. If this person’s DNA arose from innocent means, there is no reason to exclude the possibility that Amanda’s did also.
The lack of DNA
The lack of Raffaele’s DNA or Amanda’s DNA on Meredith’s body, when Raffaele is thought to have restrained her and Amanda to have throttled her calls into question this part of the prosecution’s narrative. The number of actual instances where DNA was used in strangulation cases where the DNA originated from the victim’s neck, as opposed to the victim’s fingernails, is small. However, some instances of alleged domestic violence cases have used swabbing of bruised or reddened areas on the alleged victim as evidence.
Conclusions
The way that the bra clasp was handled and the lack of a clear chain of custody cast doubt on this piece of evidence. The lack of blood on the knife calls into grave question whether the DNA got there before or after the police took it into evidence, as argued in the Johnson/Hampikian open letter. Secondary transfer is a likely means for Raffaele’s DNA being in the bra clasp, and contamination, either in the lab or during collection, is a likely means for Meredith’s DNA being found on the knife. The mixed DNA samples are virtually meaningless. The single most troubling aspect of the DNA evidence is the lack of full disclosure of the electronic data files and other documentation relating to the DNA forensics. The prosecution is acting as if it had something to hide.
Tuesday, July 13, 2010
DNA transfer in Strangulation
Part XIX in the Knox/Sollecito case
Update 1, 22 August 2010
I communicated with an anonymous forensic nurse about their domestic violence program. If an alleged victim complained of an attempted strangulation, complained of a partner’s grabbing their arm, or showed bruising or redness, they have been swabbing that area for DNA, for two years. Positive results in at least one case helped to convict someone. This information confirms the reasonableness of swabbing Meredith’s body in appropriate places, although the bruise on the nape of Meredith’s neck may have been the result of her being thrown against the wall (as some have theorized). We do not know whether swabbing the bruised areas was done or not.
____________________________
PM Guiliano Mignini’s reconstruction of the murder of Meredith Kercher was shown to the jury in the form of an animated video. In his speculation Amanda Knox grabbed Meredith Kercher by the throat and slammed her against a wall. The video superimposed actual shots of Meredith’s bruises with Amanda’s animated hand to imply that Amanda’s action produced the bruises. Later, Rudy Guede and Raffaele Sollecito held Meredith’s arms back, and Amanda stabs her (Barbie Nadeau, Angel Face, p. 160). In forensics professor Carlo Torre’s reconstruction of the crime, the single assailant grabbed Meredith by the throat and stabbed her. The area under Meredith’s chin and her nape were bruised; the former bruising was the result of the knife and the latter when the assailant put Meredith down.
Although Mignini’s theory of the crime suggests a much longer amount of contact between the assailants and victim than does Torre’s, both imply contact. None of Amanda Knox’s DNA was found in Meredith’s bedroom or on her body. Raffaele Sollecito’s DNA was found on a bra clasp (although the defense contests this piece of evidence) but not the bra itself. Rudy Guede’s DNA was found in several places, including on the sleeve of Meredith’s sweatshirt. Let us examine some forensic DNA studies to see if they shed any light on this tragedy.
The study by Wiegand and Kleiber gives a case study in which DNA evidence was collected 48 hours after a strangulation. They wrote, “Strangulation marks were clearly visible on the neck of the victim. Epithelial cells could be removed from the neck of the victim using separate cotton swabs for the left and the right side of the neck. Only the swab from the right side could be typed and included the pattern of the suspect (Fig. 3), a result which corresponded to the autopsy findings (the right neck side showed a higher intensity of bleeding in the muscles than the left side indicating a more intensive pressure against the right side). Altogether clear results could be obtained using four STRs (TH01, VWA, FGA, CD4) demonstrating the high utility and sensitivity of the method described.” These authors also conducted simulated strangulations, and they reported a success rate of better than 70%.
The simulated strangulation study in 2002 by Rutty used both SGMplus and LCN amplification. The simulated strangulation experiment was done with periods between the force and the sampling were 1, 5, 10, 15, 30, and 60 min, 2, 3, 4, 5, 6, 7, 8, 24, and 48 h and 3, 4, 5 and 10 days. When the author used SGMplus, he observed a full profile of the offender 7 out of 29 times, and always in the presence of the victim’s profile. When the author used LCN all 17 experiments yielded offender profiles, with the majority being partial profiles. Dr. Rutty wrote, “Of the test neck swabs, 19 yielded positive amplification results using SGMplus, 12 showed a victim-only profile and 7 a victim and offender profile with a full offender profile detectable up to 6 h after contact. When LCN was used (17 tests) all showed the offender to be present for all time periods i.e. up to 10 days. In the majority of cases it was a partial offender profile with the majority of the amplification result being a full victim profile.”
Dr. Rutty stated that “When considering the apparent time periods of DNA survival, passive transfer of the offender’s DNA onto the victim’s neck could also explain the presence of offender DNA several days after contact.” This study did not break down the results by time periods between simulated strangulation and DNA collection; therefore, it is difficult to draw firm conclusions based solely on the data presented.
A 2008 study by Graham and Rutty reexamined the question of DNA transfer to and from strangulation victims with an emphasis on innocent DNA transfers that might deposit DNA on the neck of a victim. In 24% of samples collected showed nonself DNA on the simulated victim from third party sources. The authors believe that such DNA transfers might confuse an investigation.
Finally, it might be helpful to return to the subject of primary and secondary DNA deposition. The study by Lowe and coworkers in 2002 looked at DNA transfer from a good DNA shedder to a poor DNA shedder to an object. By definition transfer was secondary from the good shedder, and transfer was primary from the poor shedder. When mixtures were observed, secondary transfer from the good shedder provided the major component, not primary transfer from the poor shedder. This study illustrates the dictum that one generally cannot infer the mechanism of how the DNA was deposited from the DNA itself. Along with the study by Graham and Rutty, this work suggests caution in the interpretation of nonself DNA on the body of the victim of strangulation. A similar caution should also be applied in the interpretation of DNA on the bra clasp.
In addition to the case study discussed by Wiegand and Kleiber, there are a small number of news reports of strangulations that mention DNA. One is from greater Detroit, and another is from Chicago. However, these articles do not specify the place on the body that was tested for DNA. In addition to the possibility that DNA was collected from the neck, it is possible that the victim’s fingernails contained the perpetrator’s skin cells or vice versa.
There are no reports that ILE found anyone’s DNA on Meredith’s bare wrists or her neck, and it is unclear whether or not the forensic police swabbed for DNA in these areas. It is also unclear whether some areas on the neck would have been free enough of Meredith’s blood to allow swabbing for the assailant’s DNA. Nevertheless it is difficult to see why the forensic police should not have swabbed the nape of Meredith’s neck or her wrists. If they did and found nothing, it would be strongly exculpatory, although it might fall short of proof of innocence due to uncertainties over collecting enough DNA or collecting it quickly enough after the murder. If the forensic police failed to swab these areas, it would suggest that they did not do as thorough a job as one would wish.
Bibliography
P. Wiegand and M. Kleiber, “DNA typing of epithelial cells after strangulation,” International Journal of Legal Medicine (1997) 110 :181–183. abstract
G. N. Rutty, “An investigation into the transference and survivability
of human DNA following simulated manual strangulation
with consideration of the problem of third party contamination,” International Journal of Legal Medicine (2002) 116 :170–173.
abstract
A. Lowe, C. Murray, J. Whitaker, G. Tully, P. Gill, “The propensity of individuals to deposit DNA and secondary transfer of low level DNA from individuals to inert surfaces.” Forensic Science International (2002) 129(1):25-34. abstract
E. A. M. Graham and G.N. Rutty, “Investigation into ‘normal’ background DNA on adult necks: implications for DNA profiling of manual strangulation victims.” Journal of Forensic Science, (2008) 53(5):1074-82. abstract
Update 1, 22 August 2010
I communicated with an anonymous forensic nurse about their domestic violence program. If an alleged victim complained of an attempted strangulation, complained of a partner’s grabbing their arm, or showed bruising or redness, they have been swabbing that area for DNA, for two years. Positive results in at least one case helped to convict someone. This information confirms the reasonableness of swabbing Meredith’s body in appropriate places, although the bruise on the nape of Meredith’s neck may have been the result of her being thrown against the wall (as some have theorized). We do not know whether swabbing the bruised areas was done or not.
____________________________
PM Guiliano Mignini’s reconstruction of the murder of Meredith Kercher was shown to the jury in the form of an animated video. In his speculation Amanda Knox grabbed Meredith Kercher by the throat and slammed her against a wall. The video superimposed actual shots of Meredith’s bruises with Amanda’s animated hand to imply that Amanda’s action produced the bruises. Later, Rudy Guede and Raffaele Sollecito held Meredith’s arms back, and Amanda stabs her (Barbie Nadeau, Angel Face, p. 160). In forensics professor Carlo Torre’s reconstruction of the crime, the single assailant grabbed Meredith by the throat and stabbed her. The area under Meredith’s chin and her nape were bruised; the former bruising was the result of the knife and the latter when the assailant put Meredith down.
Although Mignini’s theory of the crime suggests a much longer amount of contact between the assailants and victim than does Torre’s, both imply contact. None of Amanda Knox’s DNA was found in Meredith’s bedroom or on her body. Raffaele Sollecito’s DNA was found on a bra clasp (although the defense contests this piece of evidence) but not the bra itself. Rudy Guede’s DNA was found in several places, including on the sleeve of Meredith’s sweatshirt. Let us examine some forensic DNA studies to see if they shed any light on this tragedy.
The study by Wiegand and Kleiber gives a case study in which DNA evidence was collected 48 hours after a strangulation. They wrote, “Strangulation marks were clearly visible on the neck of the victim. Epithelial cells could be removed from the neck of the victim using separate cotton swabs for the left and the right side of the neck. Only the swab from the right side could be typed and included the pattern of the suspect (Fig. 3), a result which corresponded to the autopsy findings (the right neck side showed a higher intensity of bleeding in the muscles than the left side indicating a more intensive pressure against the right side). Altogether clear results could be obtained using four STRs (TH01, VWA, FGA, CD4) demonstrating the high utility and sensitivity of the method described.” These authors also conducted simulated strangulations, and they reported a success rate of better than 70%.
The simulated strangulation study in 2002 by Rutty used both SGMplus and LCN amplification. The simulated strangulation experiment was done with periods between the force and the sampling were 1, 5, 10, 15, 30, and 60 min, 2, 3, 4, 5, 6, 7, 8, 24, and 48 h and 3, 4, 5 and 10 days. When the author used SGMplus, he observed a full profile of the offender 7 out of 29 times, and always in the presence of the victim’s profile. When the author used LCN all 17 experiments yielded offender profiles, with the majority being partial profiles. Dr. Rutty wrote, “Of the test neck swabs, 19 yielded positive amplification results using SGMplus, 12 showed a victim-only profile and 7 a victim and offender profile with a full offender profile detectable up to 6 h after contact. When LCN was used (17 tests) all showed the offender to be present for all time periods i.e. up to 10 days. In the majority of cases it was a partial offender profile with the majority of the amplification result being a full victim profile.”
Dr. Rutty stated that “When considering the apparent time periods of DNA survival, passive transfer of the offender’s DNA onto the victim’s neck could also explain the presence of offender DNA several days after contact.” This study did not break down the results by time periods between simulated strangulation and DNA collection; therefore, it is difficult to draw firm conclusions based solely on the data presented.
A 2008 study by Graham and Rutty reexamined the question of DNA transfer to and from strangulation victims with an emphasis on innocent DNA transfers that might deposit DNA on the neck of a victim. In 24% of samples collected showed nonself DNA on the simulated victim from third party sources. The authors believe that such DNA transfers might confuse an investigation.
Finally, it might be helpful to return to the subject of primary and secondary DNA deposition. The study by Lowe and coworkers in 2002 looked at DNA transfer from a good DNA shedder to a poor DNA shedder to an object. By definition transfer was secondary from the good shedder, and transfer was primary from the poor shedder. When mixtures were observed, secondary transfer from the good shedder provided the major component, not primary transfer from the poor shedder. This study illustrates the dictum that one generally cannot infer the mechanism of how the DNA was deposited from the DNA itself. Along with the study by Graham and Rutty, this work suggests caution in the interpretation of nonself DNA on the body of the victim of strangulation. A similar caution should also be applied in the interpretation of DNA on the bra clasp.
In addition to the case study discussed by Wiegand and Kleiber, there are a small number of news reports of strangulations that mention DNA. One is from greater Detroit, and another is from Chicago. However, these articles do not specify the place on the body that was tested for DNA. In addition to the possibility that DNA was collected from the neck, it is possible that the victim’s fingernails contained the perpetrator’s skin cells or vice versa.
There are no reports that ILE found anyone’s DNA on Meredith’s bare wrists or her neck, and it is unclear whether or not the forensic police swabbed for DNA in these areas. It is also unclear whether some areas on the neck would have been free enough of Meredith’s blood to allow swabbing for the assailant’s DNA. Nevertheless it is difficult to see why the forensic police should not have swabbed the nape of Meredith’s neck or her wrists. If they did and found nothing, it would be strongly exculpatory, although it might fall short of proof of innocence due to uncertainties over collecting enough DNA or collecting it quickly enough after the murder. If the forensic police failed to swab these areas, it would suggest that they did not do as thorough a job as one would wish.
Bibliography
P. Wiegand and M. Kleiber, “DNA typing of epithelial cells after strangulation,” International Journal of Legal Medicine (1997) 110 :181–183. abstract
G. N. Rutty, “An investigation into the transference and survivability
of human DNA following simulated manual strangulation
with consideration of the problem of third party contamination,” International Journal of Legal Medicine (2002) 116 :170–173.
abstract
A. Lowe, C. Murray, J. Whitaker, G. Tully, P. Gill, “The propensity of individuals to deposit DNA and secondary transfer of low level DNA from individuals to inert surfaces.” Forensic Science International (2002) 129(1):25-34. abstract
E. A. M. Graham and G.N. Rutty, “Investigation into ‘normal’ background DNA on adult necks: implications for DNA profiling of manual strangulation victims.” Journal of Forensic Science, (2008) 53(5):1074-82. abstract
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