DNA Typing Evidence in Forensic Casework

J. Thomas McClintock, Ph.D.

Fairfax Bar Journal
Newsletter of the Fairfax Bar Association
Vol. 26, No. 3, March, 1999

Each person has a unique set of fingerprints. As with a person's fingerprint no two individuals share the same genetic makeup. This genetic makeup, which is the hereditary blueprint imparted to us by our parents, is stored in the chemical deoxyribonucleic acid (DNA), the basic molecule of life. Examination of DNA from individuals other than identical twins, has shown that variations exist and that a specific DNA pattern or profile could be associated with an individual. These DNA profiles have revolutionized criminal investigations and have become powerful tools in the identification of individuals in criminal and paternity cases.

In the past, genetic identity or similarity between evidentiary and known samples were inferred by comparison of known blood groups (ABO blood typing) and other protein-based tests. However, these traditional tests provided only limited information regarding the inclusion or exclusion of suspected offenders. In contrast, DNA testing is highly specific and significantly more discriminating. Despite early controversies and challenges, the admissibility of DNA test results in the courtroom has become routine and an integral part of the criminal justice procedure.

The first widespread use of DNA tests involved RFLP (restriction fragment length polymorphism) analysis, a test designed to detect variations in the DNA from different individuals. In the RFLP method, DNA is isolated from a biological specimen (e.g., blood, semen, vaginal swabs) and cut by an enzyme into restriction fragments. The DNA fragments are separated by size into discrete bands in a gel (gel electrophoresis), transferred into a membrane, and identified using probes (known DNA sequences that are "tagged" with a chemical tracer). The resulting DNA profile is visualized by exposing the membrane to a piece of x-ray film which allows the scientist to determine which specific fragments the probe identified among the thousands in a sample of human DNA. A "match" is made when similar DNA profiles are observed between an evidentiary sample and those from a suspect's DNA. A determination is then made as to the probability that a person selected at random from a given population would match the evidence sample as well as the suspect.

If the evidentiary sample contains an insufficient quantity of DNA for RFLP testing or if the DNA is degraded, a PCR (polymerase chain reaction)-based test may be used to obtain a DNA profile. The PCR-based tests are generally less involved providing rapid results that can serve as an alternative or as a complement to RFLP testing. As in RFLP analysis, the process involves the isolation of DNA from a biological specimen (e.g., blood, semen, saliva, and fingernail clippings). Next, rather than cutting the DNA with an enzyme, the PCR amplification technique is used to produce millions of copies of a specific portion of a targeted DNA segment. The PCR amplification procedure can be likened to a molecular xeroxlng machine. The amplified PCR products are then identified either by the addition of known DNA probes or separated by gel electrophoresis followed by chemical staining. Such detection procedures eliminate the need for critically sensitive DNA probes, as in the RFLP procedure, thus reducing the analysis time from several weeks to twenty-four (24) to forty-eight (48) hours. The resulting DNA profiles are routinely interpreted by direct comparison to DNA standards. As in RFLP testing, probability calculations are determined based upon classical population genetic principles.

Once DNA typing results are obtained in an individual case, the prosecutor, as well as the defense, must develop an approach for the successful presentation of that evidence in the courtroom. One approach involves the use of expert testimony to present a thorough and basic understanding of the principles and methodologies used in DNA testing. The actual DNA test results are presented to the jury after the scientific techniques and principles have been discussed. A second approach is to present the DNA evidence in a nontechnical manner. This approach provides the jury information that focuses on the reliability of the testing method and the significance of the results obtained. This ensures that the results are not obscured and that this type of forensic evidence is not singled out for special scrutiny in the minds of the jury. Expert testimony is still essential; however, the nontechnical direct examination will focus on the experience and integrity of the expert, the goal of DNA testing, the widespread use and acceptance of DNA testing techniques, the test results, and that the evidence can be retested if claims are presented that the DNA test results are flawed. Such a simple presentation will provide the jury with a full appreciation of the DNA test results and allow them to conclude that the biological evidence contains a 'match" or an "exclusion."

Since either approach warrants the use of a DNA expert, qualifications of the expert witness is essential to establish jury confidence in the witness's testimony. Areas that should be discussed include, but are not limited to, formal education, teaching and research experience, authored publications, training and experience with DNA testing techniques, and previous court expert testimony. The DNA expert witness should provide candid testimony about the potential for error in the DNA analyses and the advantages and disadvantages of the DNA test(s) used.

Since careful preparation is essential to the successful presentation of DNA evidence in the courtroom, our role as a scientist and expert witness is to provide the jury, the judge, and legal counsel a basic understanding of the principles and methodologies used in DNA testing. Numerous resources are available that provide information on DNA typing techniques. Such resources include biology and molecular biology textbooks, scientific journals, and reporter's transcripts of expert witness testimony in prior proceedings. Seminars and workshops are also available that provide an overview of DNA testing principles as well as providing "hands-on" training in laboratory techniques. A pretrial conference with the DNA expert should also provide legal counsel with a thorough understanding of DNA testing as well as deciding a presentation strategy. Consequently, effective presentation of DNA evidence in the courtroom by the DNA expert witness or legal counsel will resolve uncertainties regarding the source or the identify of the donor of evidentiary samples and, when coupled to the probability calculations, will allow the jury to decide the innocence or guilt of a defendant.

DNA typing technology has revolutionized the analysis of crime scene evidence and has provided a method to discriminate between individuals. Despite early challenges, DNA test results have been presented in numerous courts throughout the United States resulting in many appellate rulings accepting this technology. More importantly is the fact that improved testing technologies are emerging that provide efficient and effective DNA evidence processing which promise to widen the use of DNA evidence as an investigative tool.