DNA & Forensic Analysis


DNA is a tremendous tool now available to the criminal justice system, and its introduction has revolutionized the way courts have looked at criminal convictions – and how innocent people can prove their claims. DNA testing has given many wrongfully convicted people the chance to prove their innocence, years after their wrongful conviction.

What is DNA?

DNA is the hereditary material in each person. Every person’s DNA is unique to them; outside of identical twins, no two people have the same DNA makeup. DNA is found in every living cell. For the purposes of DNA testing, DNA is taken from two sources: the cell’s nucleus (nuclear DNA) and the cell’s mitochondria (mitochondrial DNA).

The DNA structure is a double helix, consisting of four chemical bases that pair up with each other to form base pairs. This DNA structure makes DNA look something like a ladder, and the base pairs form the “rungs” of the ladder. The DNA sequencing of the base pairs is what makes each individual person unique.

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 PCR and the DNA replication process, Innocence Projects are able to amplify and test the DNA. This has led to many successful exonerations in cases where a person has been wrongfully accused by the criminal justice system.

What is forensic science?

Essentially, forensic science is the application of scientific principles to the “art” of criminal investigation. Scientists are called as expert witnesses in many trials. Forensic science is an extremely important tool on which jurors tend to heavily rely. Thus, it is extremely important to be sure the science being used is accurate – and reliable – in order to help prevent wrongful convictions.

The history of forensic science has had many twists and turns. There are areas of science that have changed drastically over time as more research has been conducted. There are many areas of science, now referred to as junk science, that have been completely debunked in recent years. Examples of junk science can be seen in cases dealing with fire investigation, bite mark evidence, firearms analysis, lead bullet analysis, and shaken baby syndrome.

There are many cases where junk science was the driving force behind a wrongful convictions, such as William Richards, JoAnn Parks, Kenneth Marsh, and Suzanne Johnson. Innocence Projects continue to present these cases to courts in hopes of freeing the many victims of junk science.


Hair analysis has been used in crime scene investigations for a number of years. In 2012, the Washington Post published an article that discusses the possibility that flawed hair analysis by the FBI’s crime lab may have used in thousands of criminal cases. At the time of publication, the Post had identified two Washington, D.C. men who had been wrongfully convicted using flawed evidence.

Hair comparison analysis was widely used before the development of DNA testing. Microscopic hair analysis was thought to be a way to match up two pieces of hair. The theory, similar to firearms analysis, was that an individual’s hair contained distinguishable features that allowed for exclusions and matches of people. By comparing two hairs under a microscope, an expert would be able to tell if the source was the same.

The Old View

In 2000, FBI Trace Evidence Unit Chief Douglas Deedrick published an article (see here) that contained three possible conclusions regarding hair comparisons:

(1) “Consistent With” – the questioned hair exhibits the same microscopic characteristics as the hairs in the known hair sample and, accordingly, is consistent with originating from the source of the known hairs.

(2) “Exclusion” – the questioned hair is microscopically dissimilar to the hairs found in the known hair sample and, accordingly, cannot be associated to the source of the known hairs.

(3) “Inconclusive” – similarities and slight differences were observed between the questioned hair and hairs in the known hair sample. Accordingly, no conclusion could be reached as to whether the questioned hair originated from the same source as the known hairs.

These conclusions are similar to those used across all fields of forensic science. However, Deedrick included the following as a possible qualifying statement to add to a report:

“Hair comparisons are not a basis for absolute personal identification. It should be noted, however that because it is unusual to find hairs from two different individuals that exhibit the same microscopic characteristics, a microscopic association or match is the basis for a strong association.”

Experts were, for the most part, comparing two pictures of hairs and determining whether they looked sufficiently similar to the extent they may have come from the same source. As such, the subjective nature of the science, combined with the fact that there is no statistical approach to distinguishing characteristics between individuals, left plenty of room for error.

The New View

In 2009, the National Academy of Sciences published a report (Strengthening Forensic Science in the United States: A Path Forward [“NAS Report”]) that noted there is no scientifically accepted statistics about the frequency of particular characteristics of hair distributed within the population. The NAS Report went on to cite an FBI study in which 80 hair comparisons were thought to be “associated,” but using Mitochondrial DNA testing showed that 9 of the comparisons (12.5%) were actually from different sources.

The NAS Report concluded that, because of advances in DNA testing, microscopic hair analysis can be useful in determining which hairs to test, but should not be relied upon otherwise. Similarly, macroscopic hair comparisons can be helpful in determining whether different hairs have similar color, length, and size, but should not be used in identifying an individual suspect.

STR (root) v. Mitochondrial (shaft)

Today, scientists can use Short Tandem Repeat (STR) DNA testing so long as the hair sample contains a root. In crime scene investigations, it is often the case that hairs forcefully pulled from a person’s head contain a root. Typically, however, hairs are shed without a root. A hair without a root requires mitochondrial DNA (mtDNA) testing. MtDNA testing is limited due to the fact that all siblings from the same mother have identical mtDNA. Regardless, the use of DNA testing on hairs is far superior to the subjective guess work used by experts in microscopic hair comparisons.