Can a Defense Lawyer Challenge DNA Evidence?
What is DNA? How does it Work?
For a Massachusetts criminal defense lawyer representing a client charged with murder or a sex offense, DNA, or Deoxyribonucleic Acid, may prove to be the most difficult evidence to effectively challenge at trial. Without a fundemental understanding of DNA and its shortcomings as evidence, especially with collecting DNA samples, a criminal defense lawyer may be unable to prevent an innocent man or woman with being unfairly convicted.
Cambridge Criminal Defense Lawyer Explains: DNA Evidence
DNA is perhaps best explained as the building block of a cell. A double-stranded molecule found in all cell nuclei, DNA contains the genetic material that makes us who we are (human, blonde, green-eyed, etc.). Each DNA molecule resembles a spiral ladder, often referred to as the “double helix”. Inside each of these “ladders”, exists four organic bases: adenine, cytosine, thymine, and guamine (A, C, T, and G). These organic bases form pairs to complete the “rungs” of this ladder. Every sequence of these pairs carries genetic code. Each segment of this code that governs characteristics of an organism is called a gene. A complete system of human DNA, or “genome,” contains close to four billion base pairs.
Throughout most of this genome, the sequences of base pairs is nearly identical in every human. This is what makes us human. However, in some parts of the genome, the DNA is “polymorphic,” where different sequences occur in different people. These different or distinctive configurations are called “alleles.”
The number of alleles in a human population depends on the “loci,” or specific locations in the genome. Humans, as beings born of two parents, have two copies of genetic material in each cell (one from the mother and one from the father). Because of this, at each locus examined by DNA tests, a person generally has two alleles. This pair is called a “genotype.” A set of genotypes possessed by a person at two or more loci is a “multi-locus group” or a “DNA profile,” meaning that this genotype occurs at multiple places in the sample.
Because DNA profiles are highly variable in a given population, it is highly unlikely that two people will have the same profile. For example, if there are three alleles, (g, h, i), then there are six possible combinations or genotypes (gg, gh, gi, hh, hi, ii). In a larger sample, of , say 100 alleles, there will be many more possibilities. Therefore, in a larger locus, there are many different pairs and the chances of two unrelated people containing the same pairs are few and far between.
Problems with DNA testing
While studies of DNA are an important part of scientific development, the concept of forensic DNA testing is potentially flawed. According to Black and Lee’s Expert Evidence: A Practitioner’s Guide to Law, Science, and the FJC Manual (1997), “…forensic DNA testing is not an independent area of academic science; it is a technical field in which procedures are designed, shaped and performed specifically to play a role in litigation” (p. 200). Therefore, courts and attorneys must then consider the environment in which DNA evidence is being produced. Those who have created the tests, due to financial concerns and reputations to consider in the field of DNA testing, are also the ones validating it.
Another issue to consider is that of human error. One such concern is found in the improper collection and handling of samples; cross-contamination being the largest concern. It has been found, as in the O. J. Simpson case, that technicians were handling blood samples of the alleged perpetrator, and then handling the evidence they were supposed to be typing. This is apparently a common problem, especially when DNA samples and evidence are tested and contained in the same room.
Another aspect of human error that can affect DNA testing is found in the failure to be objective and/or to use blind testing. According to Black and Lee, images gathered from DNA samples are occasionally unclear or foggy. When this occurs, the technician occasionally makes a judgement call as to the markings, therefore skewing the test results. Problems with not using blind testing, or using methods where the technicians have no knowledge of the ownership of the samples or even of the case, also cause problems. An examiner’s results can also be skewed when the examiner knows the expected outcome of the test (p. 249-250).
Kevin J. Mahoney is a Cambridge criminal defense lawyer and author of Relentless Criminal Cross-Examination.
These are just a few examples of problems with DNA testing. For more information, review the following material:
Black, Bert and Patricia Lee (Eds.), Expert Evidence: A Practitioner’s Guide to Law, Science, and The FJC Manual, West Group, St. Paul, MN, 1997.
Gonick, Larry and Michael Wheelis, The Cartoon Guide to Genetics, 1983.
Saks, Michael J. “Prevalence and Impact of Ethical Problems in Forensic Science.” 34 Journal of Forensic Science 772, 1989.
“Reference Guide on Forensic DNA Evidence.” FJC Reference Manual on Scientific Evidence.