Ed. Note: Chris Halkides has been kind enough to try to make us lawyers smarter by dumbing down science enough that we have a small chance of understanding how it’s being used to wrongfully convict and, in some cases, execute defendants. Chris graduated from the University of Wisconsin-Madison with a Ph.D. in biochemistry, and teaches biochemistry, organic chemistry, and forensic chemistry at the University of North Carolina, Wilmington.
Not all DNA evidence is of equal weight; the presence or absence of a body fluid for which forensic tests are commonly available (blood, semen, and saliva) is a critical factor. We will examine two kinds of misunderstandings which have appeared.
- With respect to DNA evidence Professor Peter Gill describes a framework (hierarchy) of propositions.
- The sub-source level refers to the strength of the DNA profile itself.
- The source level refers to an evaluation of strength of evidence if a DNA profile can be associated with a particular body fluid, such as semen or blood.
- The activity level associated the DNA profile with the crime itself, e.g., sexual assault.
- The highest level deals with the ultimate issue of guilt/innocence.
In the Amanda Knox/Raffaele Sollecito case, a forensic worker found a low template number DNA profile belonging to Meredith Kercher while testing the blade of a knife from Mr. Sollecito’s home. The handle had the DNA profile of Amanda Knox. The prosecution claimed that the DNA profile came from Meredith’s blood and that the position of the DNA on the handle indicated that it had been used for stabbing, not cutting.
There was no blood on the knife; therefore, this was a sub-source profile, for which a source was assumed. There is no evidence in the forensic literature on distinguishing whether a knife was used for stabbing or cutting. There is, however, evidence that profiles can be transferred from one part of an item to another after being packaged.
Daniel Holtzclaw is serving a 262-year sentence for a series of sexual assaults which the prosecution argued he committed while a police officer in Oklahoma. One of the victim’s profiles was found on a pair of his trousers. The prosecution argued that this profile arose from vaginal fluid, but lacked supporting evidence of the source of the DNA: it was a sub-source profile for which a source was assumed without evidence.
It is the responsibility of the forensic scientist to provide the court with an unbiased list of the alternatives by which a profile came to be on an item of evidence. It is undisputed that Mr. Holtzclaw interacted with the women in question. The hypothesis that Mr. Holtzclaw touched the victim, then touched his trousers is a reasonable scenario, in that secondary and tertiary transfers of DNA profiles are now thoroughly documented in the forensic literature. Peter Gill and Suzanna Ryan were two of the authors of a thorough report supporting Mr. Holtzclaw’s appeal, but the appellate court did not accept this report.
Although different body fluids have different concentrations of DNA, the amount of DNA in a forensic sample is not an unambiguous guide to its origin. Jane Taupin wrote (p. 34), “Currently DNA profiling can identify an individual from a sample of biological material but it does not reveal the body fluid or tissue source from which the profile originated.”
Yet even a positive result from a confirmatory test for a body fluid should not lead to an automatic, unconditional assumption that the DNA profile arose from the body fluid. To do so is to make the association fallacy. Peter Gill wrote, “However, the observation of a body fluid and the detection of a DNA profile are two separate tests. It cannot be implied with certainty that the body fluid and the DNA have the same source.” Different tests have different lower limits of detection, and substances which interfere with one test might not interfere with another.
In the Knox/Sollecito case, the Amanda Knox’s profile was found in her sink along with Meredith Kercher’s profile and blood. Discussions of the case often conflated mixed DNA with mixed blood. Yet the alternative, that Ms. Kercher’s blood was responsible for her profile, and that Amanda’s profile arose from ordinary ablutions, is equally consistent with the evidence.
In the Adam Scott case, testing of fluid from a rape victim yielded evidence of semen and two DNA profiles. One belonged to the victim’s boyfriend; the other belonged to Mr. Scott. The prosecution claimed that both profiles arose from semen. Mr. Scott claimed that he had never been to the city in which the victim lived, yet he was convicted entirely on the strength of the DNA evidence. It was eventually shown that the laboratory had accidentally re-used a plastic tray that contained DNA from Mr. Scott’s saliva.
It is relatively easy to understand the association fallacy in situations in which there are two DNA profiles. Yet a single profile might arise from a different individual from the victim or perpetrator of a crime. One reason for the lack of a victim’s or perpetrator’s profile is that little DNA was deposited, and another is that the DNA profile has decayed over time.
Non-perpetrator DNA might arise from post-crime deposition, or from a contamination within the laboratory. Professor Gill and a colleague used drops of blood and saliva at various dilutions, which were tested presumptively for blood or saliva over time, as well as being subjected to DNA profiling. The samples were handled by a different individual. The more dilute or degraded a stain, the higher the proportion of the handler’s DNA. Cold cases are prone to this sort of false association, among other problems.
Jane Mixer was murdered in 1969, and her body was found in a cemetery west of Ann Arbor. Samples from the crime scene were tested for DNA over thirty years later. Although the crime had been thought to be related to a series of murders taking place at that time, details of the crime suggested a different perpetrator. DNA donated from Gary Leiterman was found on pantyhose and a stocking.
When a sample of blood from Ms. Mixer’s hand was tested, DNA donated from John Ruelas was found. The prosecutor said that it was Ruelas’s blood. However, Mr. Ruelas was four years old and lived in Detroit. The dates at which the samples were processed at the Michigan State Crime Laboratory are such that a more reasonable explanation for Mr. Ruelas’s DNA is that cross-contamination occurred. The actual perpetrator might not have been either a possible serial killer or Mr. Leiterman.
For Further Reading
Peter Gill, Misleading DNA Evidence, Academic Press, 2014.
Jane Moira Taupin, Introduction to Forensic DNA Evidence For Criminal Justice Professionals, CRC Press, 2014.