This post returns to the subject of forensic DNA contamination. It will define primary and secondary transfer of DNA and explain how they are related to contamination. It also treats clerical errors and the need for independent review of the forensic DNA data. It reconsiders the lessons of the Leskie case. These concepts will be applied to some of the DNA forensics in the murder of Meredith Kercher, especially as it relates to the evidence against Amanda Knox and Raffaele Sollecito.
People sometimes confuse DNA contamination with secondary transfer of DNA. Let us define these terms and give a few examples. In Forensic DNA Typing, p. 152, John Butler wrote, “Contamination implies the accidental transfer of DNA.” Implicit in his definition is that the transfer happened after the evidence was collected. In An Introduction to Forensic DNA Analysis, p. 14, Norah Rudin and Keith Inman “define contamination as the inadvertent addition of an individual’s physiological material or DNA during or after collection of the sample as evidence…A contaminated sample is one in which the material was deposited during collection, preservation, handling, or analysis.”
Primary transfer of DNA
According to forensic scientist Suzanna Ryan, “Primary transfer is described as the transfer of DNA from an individual to an item.” Ryan summarized some of the factors that contribute to how much DNA is transferred: “Through studies by Ladd, et al and others, many in the forensic community believe that some individuals just naturally shed more cells when handling an item (‘good’ shedders) than others (‘poor’ shedders). Other factors including the substrate being handled (rougher items collect more DNA), the time since the individual last washed their hands, how nervous the person is (nervousness can lead to increased sweating), and how often they touch their mouth, eyes, hair, face or other body parts (thus gathering DNA on their hands) play a heavy role in whether a DNA profile will be obtained through simply touching an item.” One presumes that rougher items cause more cells to transfer than smoother items, but Ms. Ryan did not give specific examples.
Secondary transfer of DNA
Suzanna Ryan wrote, “In secondary transfer, there is no direct contact between a person and an object. It is instead transferred through an intermediary. This could be another person or another object. An example—expanding upon the same knife example as above—is as follows: Person A shakes Person B's hand. Person B touches the handle of a knife. Secondary transfer theory would allow that Person A's DNA could be transferred first to Person B's hand and second to the knife handle. This would mean that even though Person A never actually touched the knife handle, his DNA could be present on it.”
Secondary transfer at the scene and the lab
Suzanna Ryan continued, “At the laboratory, the analysts are careful to wear gloves when handling evidence items. Great care is generally taken to clean scissors, tweezers, and other utensils between testing items. But what about items that are not cleaned as regularly? For example, is it possible to transfer DNA from an item of evidence to a ruler, and then when the next item of evidence is examined and photographed, could the DNA transfer again from the ruler to this item? A 2006 study by Poy and van Oorschot showed an example of secondary transfer when a mixed DNA profile was found on a swab taken from an examination magnifying lamp. This profile was searched in the lab's database and a match was found with a case that had been worked on the bench-top with the magnifying lamp. It was determined that DNA was transferred from the item being examined to the analyst's gloves and then onto the top of the magnifying lamp.”
This incident should be considered both secondary transfer of DNA (the item transferred DNA to the glove, which transferred it to the lamp) and DNA contamination because it happened in the analysis lab. This incident also illustrates how easy accidental DNA transfer can be in a lab. DNA contamination occurs when an analyst transferred his or her own DNA onto an object, but this would be classed as primary transfer.
Sometimes DNA contamination occurs in ways that do not involve the item of evidence. DNA profiles have been observed because an item of labware (plastic tubes, cotton buds, etc.) was contaminated with DNA from a person who helped to manufacture the item. The chemical reagents and equipment needed to perform the polymerase chain reaction, the DNA amplification step, are occasionally contaminated. These are examples of contamination that are due neither to primary nor to secondary transfer.
Primary and secondary transfer when several people handle an object
In the journal Nature, 387, p. 767 (1997), R. van Oorschot and M. Jones wrote, "Objects handled by many individuals all produced profiles with multiple alleles of varying intensity. To determine the effect of multiple handlers, we exchanged polypropylene tubes between individuals (2 or 3, 10 min each) with different genotypes. Although the material left by the last holder was usually present on the tube, that of previous holders was also retrieved to varying extents. The strongest profile obtained was not always that of the person who last held the object, but was dependent on the individual. We regularly observed profiles of previous holders of a tube from swabs of hands involved in these exchanges, showing that in some cases material from which DNA can be retrieved is transferred from object to hand (secondary transfer).” This article was the first one of which I am aware to document that DNA profiles could be recovered from fingerprints. This may also be the first one to use the term “secondary transfer.”
Primary and secondary transfer of DNA to the bra clasp
Some have argued that the amount of Raffaele Sollecito’s DNA on the clasp can only have been deposited through primary (direct transfer), not contamination or secondary transfer. This argument is false on several grounds. It ignores that that the amount of DNA in primary transfer depends upon several factors (see above). It ignores that contamination can produce strong profiles that analysts have mistaken for genuine ones (see below). It ignores the fact that Mr. Sollecito’s DNA profile on the clasp was relatively weak in intensity, roughly 200 RFUs. Perhaps most fundamentally, this argument runs counter to a principle of DNA forensics. As part of his analysis of the Leskie case (see below), forensic scientist Dan Krane wrote, “In fact, it is quite uncommon for DNA tests themselves to say anything about the circumstances (or even the time frame) associated with the transfer.” Jason Gilder wrote in an email to me, “One of the standard axioms of DNA typing is, the presence of a DNA profile says nothing about the time frame or the circumstances under which DNA was transferred to that item. Contamination falls into is one such instance of an issue that cannot be identified by the electropherogram alone (unless you are dealing with a control sample or other known profile).”
On the other hand, an anonymous commenter on a discussion board offered this hypothetical scenario for secondary transfer of Mr. Sollecito’s DNA: “For example, let's say that DNA from Meredith, Amanda and Raffaele was on the hand towels Rudy took from the bathroom (highly probable, since they'd all used the bathroom in the days before the murder). Drying your hands on a hand towel is exactly the kind of rubbing action that would shed DNA. If Rudy had trodden on one of the towels, then trodden on the bra clasp (one of the hooks was deformed, of course) that could have transferred DNA from the towels to the hook, with no direct contact from Raffaele necessary. In turn, Raffaele's DNA was more easily identified because they could use the more sensitive tests for the Y chromosome, whereas the other DNA would have been mixed female and thus tricky to correctly identify.” This is a speculative scenario, but it is not the only kind of secondary transfer that one could imagine. Sollecito’s DNA was likely on the door to Meredith’s bedroom, for example, because he tried to break the door down.
Clerical errors in DNA forensics
Previously we have explored why the electronic files are important. However, simply having an independent expert review the case file can catch some types of errors even without the electronic files. DNA forensic consultant Norah Rudin encountered a case where two reference samples were switched, due to a clerical error. Lazaro Sotolusson was held for a year and would have been tried for rape involving juveniles, facing multiple life sentences. In a preliminary hearing, one of the female victims identified Sotolusson as her attacker, possibly a case of a mistaken eyewitness. Dr. Rudin examined handwritten notes and computer files (not the sort of files one uses to create a DNA electropherogram, such as a .fsa file) and concluded that the lab had switched the two samples. The lab retested samples from Mr. Sotolusson and Joseph Coppola and confirmed the error. Rudin stated to a reporter, “If I hadn't looked at the original data, I likely wouldn't have found the mistake. That's one of the reasons that I always look at the original data… I believe every case needs to be reviewed by an independent analyst.” (emphasis added)
Errors of a similar kind have occurred elsewhere. Professor William Thompson wrote, “I recently reviewed the corrective action file for an accredited California laboratory operated by the District Attorney’s Office of Kern County (Bakersfield). Although this is a relatively small laboratory that processes a low volume of samples (probably fewer than 1,000 per year), during an 18-month period, it documented multiple instances in which (blank) control samples were positive for DNA, an instance in which a mother’s reference sample was contaminated with DNA from her child, several instances in which samples were accidentally switched or mislabeled, an instance in which an analyst’s DNA contaminated samples, an instance in which DNA extracted from two different samples was accidentally combined into the same tube, falsely creating a mixed sample, and an instance in which a suspect tested twice did not match himself (probably due to another sample-labeling error).” (emphasis added)
Whether or not an independent analysis would have caught possible clerical errors here is an open question. The Knox defense team asked for an independent analysis of all of the forensic data, but the judge denied the request. As long as human beings are part of the testing process, clerical errors are likely to happen at some frequency. This factor and the possibility of contamination ought to be considered when one ponders the astronomical odds that are sometimes used when describing a particular DNA result.
The Leskie case and its relationship to the knife and bra clasp as evidence
Jaidyn Leskie was a toddler who was murdered in Australia over a decade ago. A DNA profile belonging to a Ms. P. was found on two pieces of Jaidyn’s clothing, a bib and some track pants. Ms. P’s profile had varying intensities at different loci, suggesting sample degradation. However, her profile was quite strong in some loci, having peak intensities of over 1000 RFUs. Another example of contamination occurred in State of Illinois v. Sean Evans (2003). In this example a positive control was contaminated with a sample containing a DNA ladder, a series of DNA fragments of increasing length that might be used as a calibration standard. Note that the intensity of the fragments making up the ladder is 500-1500 RFU, similar to a typical evidence sample. The unusual nature of the ladder DNA helps to show that contamination occurred, but this is seldom true for other kinds of contamination.
Ms P’s DNA profile was ultimately attributed to contamination partially on the basis that Ms. P lived hundreds of miles away from the crime and never left her village. Yet she could have lived next door to the Leskie family, and contamination would still be a likely explanation for finding her DNA on the toddler’s clothing. Jaidyn’s body was found submerged in water for months. These conditions are antithetical for the preservation of DNA evidence. Significantly, none of Jaidyn’s DNA was recovered from his clothing. This should have been a reason to suspect contamination on its own.
The bloodless kitchen knife in the Knox/Sollecito case is similar to Jaidyn’s clothing in that no human DNA was likely to have been on the blade at the time of collection at any section that tested negative for blood; therefore, the DNA was most likely deposited in the lab. The Leskie case was one in which the electronic data files were particularly helpful. The Knox/Sollecito case would also benefit by independent examination of these files. For example, one could use the .fsa files to examine other samples along the knife blade and handle using the same threshold value for all samples. One might find profiles belonging to other individuals, and this would strengthen the argument that DNA on the knife arose from contamination or secondary transfer.
Could contamination have occurred in the Leskie case? The evidence with Ms P’s DNA was examined within a few days the evidence from the Leskie case, and Dr. Krane noted “even if evidence samples from the two cases did not come into direct physical contact, secondary transfer (by way of common contact with a surface or implement) of biological material from the condom or articles of Ms. P’s clothing in the rape investigation to the bib and track pants could have occurred.” The lab in question had argued that contamination was unlikely on at least two grounds, one of which was the lack of DNA from Ms P showing up in other profiles. Dr. Krane agreed that “no other samples including negative controls associated with both investigations display indications of Ms. P’s allele’s being present.” However, he rebutted the lab’s point by stating, “there is no good reason to expect that contamination would uniformly affect all samples if and when it does occur. In much the same way, Salmonella-tainted meat placed on one part of a countertop at the beginning of food preparation does not mean that all food subsequently prepared for the same (or even a later) meal will be similarly tainted – just those items that also come in contact with the same portion of the countertop can be reasonably expected to have a chance of being tainted.” Although the amplification of DNA through use of the polymerase chain reaction (PCR) creates the possibility of contamination via the amplified DNA, Dr. Krane did not think it is as likely in this case as secondary transfer.
There is one more point with respect to the bra clasp that needs to be addressed. It is sometimes argued that Mr. Sollecito’s DNA could not have arrived on the clasp by contamination because his DNA was found on no other items except for a cigarette butt. This is essentially the reasoning of Judge Micheli, who oversaw Rudy Guede’s trial. From the Wikipedia article on the murder of Meredith Kercher, “The judge at the trial of Guede acknowledged that the DNA sample was considered small, but described the claim of contamination at the laboratory as making ‘no sense’, since there was no material from which such contamination could have come, and so ‘the risk would have been the loss of traces found there, not the risk of somehow discovering new traces.’” This argument is false for at least two reasons. It ignores the possibility of contamination from Mr. Sollecito’s DNA in the lab, and it ignores the empirical facts of the Leskie case, where Ms. P’s DNA did not show up in other items or controls.
One way to distinguish between secondary transfer and contamination is to say that the difference between primary and secondary transfer depends upon how the DNA was deposited on the evidence item, but contamination depends upon when the DNA was deposited: if the DNA were deposited before the item was collected as evidence, it would not be contamination. We have also seen that contaminating DNA does not evince a difference in intensity that would allow it to be distinguished from DNA arising from primary or secondary transfer before the item was collected as evidence. Clerical mix-ups are another source of error in DNA forensics, and catching them is one of many reasons to welcome independent reviews of the results. Raffaele Sollecito’s DNA could have arisen on the bra clasp from contamination or secondary transfer, as well as primary transfer. Meredith Kercher’s weak, dubious DNA profile on the knife is almost certain to have arisen from contamination.