DNA & Forensic Analysis

The introduction of forensic DNA testing in criminal cases revolutionized the nation’s criminal justice system. DNA testing has become the leading technique in identifying and excluding suspects in criminal cases. It has also given many wrongfully convicted people the chance to prove their innocence, years after their wrongful conviction.

What is DNA?

DNA, or deoxyribonucleic acid, is the hereditary material that determines all the characteristics of a living organism. In forensic DNA testing, both nuclear DNA and mitochondrial DNA are used to identify or eliminate suspects.

Nuclear DNA is found in the nucleus of a cell. The nuclear DNA structure is a double helix, consisting of four chemical nitrogen bases (adenine (A), thymine (T), guanine (G) and cytosine (C)) that pair up with each other to form base pairs. This DNA structure resembles a ladder, and the base pairs form the “rungs” of the ladder. Two of these “ladders” come together to form an “X” shape, called a chromosome. The order and number of these bases (A, T, G, C) is what determines your genetic code and makes you unique.

Mitochondrial DNA (mtDNA) also consists of the same four chemical nitrogen bases.  Instead of an “X” shape, however, mitochondrial DNA forms a circular shape. Mitochondrial DNA is almost entirely passed from a mother to her child through the egg cell. Hence, an individual’s mitochondrial DNA profile will match all others in their maternal line. This makes the use of mitochondrial DNA in identifying suspects relatively useless, especially if suspects are maternally related.  However, it can be a powerful tool in excluding suspects or obtaining results from severely degraded biological samples.

When DNA testing began in the 1980s, scientists needed large quantities of biological material in order to create a profile that could be matched to a suspect. Now, however, scientists can use a process called “polymerase chain reaction” (or PCR) testing which is a form of DNA replication to test small amounts of DNA. With the PCR process, billions of copies can be made of a small DNA sample sufficient for a DNA test.

What is forensic science?

Forensic science is the application of sciences such as biology, chemistry, and physics to a criminal investigation and the law. Scientists practicing in the forensic science field are often called as expert witnesses in many trials. Forensic science is an extremely important tool on which jurors tend to rely heavily. Thus, it is extremely important to be sure forensic sciences are accurate in order to secure convictions of the guilty, to prevent wrongful convictions, and to overturn wrongful convictions that have already occurred.

The history of forensic science has evolved over time and some areas once thought to be sound forensic sciences have been called into question, some have even become completely unraveled, by advancements in methodology or research.  Examples include fire/arson investigation, bite mark evidence/forensic odontology, lead bullet analysis, shaken baby syndrome/abusive head trauma, pattern evidence analysis (i.e., tire treads, shoe prints, etc.), and microscopic hair comparison.  

In 2009, the National Academy of Sciences published a report entitled Strengthening Forensic Science in the United States: A Path Forward (the NAS Report) that noted serious problems with almost all disciplines of forensic science.   Notably, there was an absence of standardized terminology and reporting practices within disciplines, unknown rates of error, problems with forensic scientist bias, lack of quality control, etc.  Further research and development was needed to ensure the accuracy, reliability, and validity of many disciplines including handwriting comparison, firearms/bullets analysis, pattern analysis (tool marks, footwear impressions, tire impressions), forensic odontology/bitemark comparison, microscopic hair comparison, fiber evidence comparison and analysis, bloodstain pattern analysis, and fingerprint analysis (friction ridge analysis).

In 2016, the President’s Council of Advisors on Science and Technology (PCAST) released a report entitled, Forensic Science in Criminal Courts: Ensuring Scientific Validity of Feature-Comparison Methods.  The report concluded that any forensic science disciplines were still lacking in scientific validity and reliability.  These disciplines involved the “feature-comparison” methods mentioned above.

Many individuals have been wrongfully convicted on the basis of these improper or invalidated forensic “sciences.”   Such cases from the California Innocence Project include William Richards, JoAnn Parks, Kenneth Marsh, Alan Gimenez, and Suzanne Johnson.