Asked & Answered: Is DNA evidence infallible?

By Jo Mathis

Legal News

A law professor at the University of California, Hastings recently wrote a letter to The New York Times challenging the infallibility of DNA.

Washtenaw County Trial Court Chief Judge Donald Shelton is the author of "Forensic Science in Court" and teaches forensic science in the criminology department at Eastern Michigan University during the spring semester as well as forensic science evidence at Cooley Law School in the summer.

MATHIS: Are we placing too much faith in DNA forensic technologies?

SHELTON: DNA is different from all of the prior forms of forensic science evidence that has been routinely admitted in the past. DNA is the molecular structure in all living things that contains genetic information. It is very durable and can be extracted from the smallest of remains many years after a crime. DNA is polymorphic in the genetic sense, meaning that, depending on the method of extraction, it is unique among humans and can identify the owner of a specimen with overwhelming accuracy.

DNA profile analysis arose out of pure medical research and was not developed for criminal prosecution purposes, as was fingerprint or ballistics or other similar analyses. Perhaps most importantly, the results of DNA comparisons are statistical rather than judgmental or opinion based. While other types of forensic science evidence relies upon the experience, and perhaps bias, of the examiner, the DNA analysis is biology rather than opinion.

DNA analysis is of course not infallible because it is conducted by humans and humans make mistakes. Courts must insure that the sample collection and analysis was conducted properly. But the underlying science of DNA analysis is superior to every other form of forensic science evidence we have ever utilized.

MATHIS: In a New York Times editorial, University of California Law Professor Osagie K. Obasogie insisted that when the government warehouses DNA samples on a large scale-as is the case in California-there is a chance of contamination, and innocent citizens might be arrested as a result. Is this warehousing done in Michigan?

SHELTON: DNA samples, like any other forensic evidence, are subject to being contaminated by human error. That error may occur at the crime scene when a piece of evidence is improperly collected, or at any time during its storage or transport, or by a fault laboratory procedure. That possibility is no different for DNA than for ballistics or fingerprints or blood samples or whatever. The "warehousing" comment is not a reflection of what really happens. DNA samples are routinely locally collected with cheek swabs ( a "Q-tip") from persons who are convicted of felonies in Michigan (some States collect samples from misdemeanants or even arrestees). The samples are analyzed and the results posted as part of a nation-wide database called CODIS (Combined DNA Index System). When a crime scene specimen is collected and analyzed, it is then electronically compared to the over 10 million samples in CODIS. If a "hit" occurs, then that person becomes a suspect but it is not that "hit" that ever comes into evidence. Before the case ever comes to trial, a new sample of the suspect's DNA is taken and that sample is compared to the actual crime scene evidence. In other words, the CODIS "warehouse" as he calls it is used as an investigative tool by police and not as admissible evidence.

MATHIS: Have you ever presided over a case in which you doubted the guilt of a defendant, regardless of the DNA evidence against him or her?

SHELTON: No. There was however a high profile case, People v. Leiterman, in which I presided where it was clear to me, and the jury, that one of the samples used for a DNA analysis had been contaminated at the laboratory, in spite of the laboratory's denials to the contrary.

There was however ample other evidence, including several uncontaminated DNA samples, which proved the defendant's guilt.

MATHIS: What are your thoughts on the way DNA evidence is interpreted and presented to juries?

SHELTON: As I said, DNA evidence is especially valuable because it presents probabilistic evidence rather than opinion evidence.

No good DNA expert will testify (as some fingerprint experts are wont to do) that there is a "match" and that the specimen could not have come from anyone else. Rather, DNA experts testify as to the statistical likelihood of those same alleles appearing in another human. It is not the CODIS database that those statistics are based upon. Rather, it is the huge genotype database of persons of similar race and gender to which the defendant's sample is compared.

MATHIS: Do jurors ever doubt DNA evidence?

SHELTON: Judges are the "gatekeepers" of forensic science evidence, meaning that the judge must be satisfied that the evidence is based in good science and that it was properly analyzed and evaluated.

I have no doubt as to the reliability of the science of DNA analysis. But as in every other form of forensic science, the procedures used by humans in the collection and analysis of DNA samples must be demonstrated to be proper before I will allow the jury to hear the evidence. We can never disregard the apparently distinct possibility of human error.

MATHIS: How are the laboratory standards enforced to reduce the chance of human error?

SHELTON: Ironically, the intense demand for DNA testing by police and prosecutors may inadvertently pose the most serious threat to its use in criminal trials. The overwhelming demand may be resulting in poor laboratory practices by inexperienced or overworked technicians to the degree that confidence in DNA test results may be affected. Suspected and documented cases of poor practices by various state and local crime laboratories have been widely reported, including locally the forced closure of the City of Detroit Crime Laboratory. The reality is that there are few nationally accepted standards for the performance and practices of forensic science laboratories. The Congressionally commissioned National Academies of Science Report documented that lack and recommended the establishment of such standards but no action by Congress has been forthcoming. In the end, trial judges are the primary guardians of the integrity of scientific evidence that is sought to be introduced to prove guilt. When it comes to DNA evidence, the gatekeeper role assigned to judges may find its most important function not in an evaluation of the reliability of the scientific theory but rather in the very traditional role of insuring that the evidence meets basic standards of authenticity, relevance, and reliability in the application of that scientific theory to the facts of the particular case.

Published: Thu, Aug 28, 2014


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