Reasonable Doubt: Drawbacks with DNA dragnets, Part 2

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      Second of two columns

      DNA dragnets are an investigative tool used by the police to help identity suspects when a search of the DNA database fails to identify a possible match. Forensic science plays a crucial role by creating a DNA profile of the culprit based on DNA evidence found at the crime scene or on a victim. That DNA profile is then compared to DNA samples collected during the police investigation.

      Utilizing the dragnet, the police request voluntary DNA samples from a targeted group in the community, such as those who could have had contact with the victim. Anyone who declines then becomes a suspect.

      Last week’s discussion focused on the practical concerns of this investigative tool. The potential consequences for innocent people, whether or not they decline to provide a sample, include the risk of being stigmatized as a person who would commit the types of crimes being investigated. Such stigmatization, although unfounded, could result in significant social and financial consequences.

      Government monitoring of citizens a concern

      Privacy considerations are also a significant concern, whereby DNA samples may be retained in some sort of database lacking standards to protect such personal information. Further, when countries such as the United Kingdom are collecting DNA samples taken from anyone accused of a crime, not just convicted of one, it is not a stretch to imagine an attempt to justify the collection of DNA samples from every person at birth or the use of other forms of public monitoring, like traffic and security cameras, facial recognition, biometrics, and GPS tracking devices.

      Directly on point is the fact that city council in Williams Lake, B.C., recently voted unanimously in favour of a motion to implant GPS tracking devices in its most prolific criminal offenders. Although this initiative would likely not be approved in Canada any time soon, there is a developing trend toward increased government monitoring of its citizens.

      Scientific misgivings about errors

      With the practical concerns aside, there are also scientific concerns when using DNA dragnets. The risk is that an innocent person who voluntarily provides a DNA sample is falsely convicted because of an error. Such errors may include the fact that the theoretical probabilities of the certainty of a match, such as one in a billion, are overstated when taking into account the existence of identical twins and rare chimeras, humans with two sets of DNA.

      More significant concerns include limitations in DNA-profiling software requiring subjective input by forensic scientists, the possibility of contamination, and the risk of inaccurate DNA profiles derived from mixed DNA samples.

      The theoretical probability that a match is coincidental is extremely low. Just like fingerprints, each person’s DNA is unique. From those odds it can be argued that if a particular suspect’s DNA is a match to the DNA profile created by the evidence, then that suspect is the person who committed the crime.

      Human "chimeras" have two sets of DNA

      However, those probabilities are affected by the fact that 0.2 percent of the world’s population are identical twins, which decreases the odds. The existence of chimeras also affects the odds. A chimera, an individual who has two sets of DNA, is created by the fusing of two fertilized eggs shortly after conception, and may unfairly exclude a suspect.

      More significantly, however, are the other concerns. Limitations in software and less sensitive techniques require subjective input by forensic scientists whereby similar, but not identical, DNA patterns are deemed to be identical genetic matches after accounting for imperfections in processing. This may lead to inaccurate results.

      For example, three scientists at a New York state police lab have claimed they were targeted after they questioned long-standing DNA-testing practices. They allege those practices promoted DNA analysis focusing on one suspect, subjectively building a case around that person despite other data.

      Sample contamination a risk

      The potential for contamination is another risk. In one case, it was discovered that the cotton swabs used to collect DNA from crime scenes had been contaminated by one of the workers who helped manufacture them. This woman’s DNA was on the swab at the time it left the factory, even though the swabs had been sterilized. It wasn’t until the woman’s DNA on the swab matched the DNA of a male that the contamination was discovered.

      This secondary transfer of genetic material may also occur from nearby objects or left-over cells transferred from a pervious test. Anything that has had contact with a victim and is then brought into the lab is a potential source of contamination. A full DNA profile, as opposed to only segments, should be compared and should differ (except for identical twins) to ensure a person was not matched in error to their own DNA in another sample.

      Mixed DNA another challenge

      DNA profiles derived from mixed DNA samples present a particular challenge. Mixed DNA samples comprise the DNA of both the victim and the perpetrator, such as those involved in sexual assault or crimes involving a handgun handled by multiple individuals. In these cases, the potential for inaccurate DNA profiles is significant due to the mixed genetic materials, particularly when the two individuals are related.

      For example, in incest rape cases. It has been claimed that more advanced software has identified the fact that many previously calculated probabilities related to mixed DNA profiles have been consistently inaccurate in both directions—either by incriminating an innocent person or exculpating a guilty one.

      When keeping in mind all of these practical and scientific concerns of using DNA dragnets, those who are unwilling to voluntarily provide a DNA sample should not be so quickly labelled a suspect. Quite legitimate reasons exist for wanting to safeguard such private information. Furthermore, a cautious juror should look at all the evidence of a crime to determine whether other factors raise a reasonable doubt of guilt—and should not convict on genetic evidence alone.

      Sherry Baxter practises criminal and civil law on Vancouver Island, as well as provides legal research and litigation services. Reasonable Doubt appears on on Fridays. You can send your questions for the column to its writers at

      A word of caution: you should not act or rely on the information provided in this column. It is not legal advice. To ensure your interests are protected, retain or formally seek advice from a lawyer.