Forensic Science HQ

Investigate the forensic science world

Forensic DNA Analysis

DNA analysis was first introduced to the U.S. courts in the early 1990’s. It is without a doubt the biggest advancement in Forensic Science since the discovery of fingerprints.

Forensic DNA STR analysis

Why is DNA analysis so important in criminal investigations?

  • A DNA profile can positively include or exclude a person as source of evidence.
  • DNA results can be obtained from a very small, old or degraded sample, and is found in almost all biological material.
  • With the exception of identical twins, no two people have the same DNA. It is unique.

What are the disadvantages of DNA analysis?

  • Cost. DNA equipment and trained personnel are not cheap, although advances in science and technology are making it more affordable every day.
  • Sensitivity. Because DNA is so prevalent and the techniques so sensitive, extreme care must be taken not to contaminate the evidence.
  • Complexity. While the interpretation of a single source of DNA is usually fairly straight forward, it can be very difficult to correctly interpret samples with multiple sources and concentrations. Forensic scientist must undergo a long, continual training program to stay current with the technology.
  • Time. Because of the complexity, DNA analysis usually takes several days at best. Also due to the demand, there is often a backlog of cases to be analyzed and a limited number of resources.


Forensic DNA Yield Gel

DNA Yield Gel

What types of tissue can be analyzed for DNA? There are many, but the most common are:

  • Blood
  • Saliva
  • Hair (with a root)
  • Bone
  • Teeth
  • Semen
  • Vaginal secretions

What types of evidence have been used for DNA analysis?

  • Bloodstains
  • Toothbrush
  • Beverage containers
  • Hats
  • Cheek swabs
  • Sexual assault stains
  • Gloves
  • Chewing gum
  • Cigarettes

Almost anything that comes in contact with the human body will contain traces of DNA and is a potential source of evidence.

Forensic Biology

Forensic Biology is the analysis of body fluids, stains and other bodily materials to help solve a crime. Typically, this involves the positive identification of blood, semen, or saliva and further genetic testing (DNA) to determine who the material may have originated from, typically the alleged victim, suspect or other involved party.

Some laboratory systems expect the Forensic Biologist to handle all evidence in their area from a given case from “cradle to grave”, i.e from initial detection to identification and DNA testing and prosecution. Some use an “assembly line” approach, where one scientist may do only evidence screening, which means they only look for the body fluids, another would do tests to determine what the fluids are, and yet another would do DNA analysis. This approach allows the scientist to focus on a single area of expertise, although the “cradle to grave” approach gives a better overall understanding of the case and what may or may not be important based on related factors.

Depending on the agency or company you work for, you therefore may be trained in a variety of biological techniques or possibly just a single specialty, like DNA analysis. Some agencies also expect their scientists to go to the crime scene and collect evidence Most, however have crime Scene Technicians dedicated to this task. The evidence is then transported to the laboratory for analysis.

A Forensic Biologist usually begins their analysis by examining a piece of evidence for the presence of hairs, fibers and stains. Any collected hairs or fibers of evidential value are most often transferred to a Forensic Microscopist for further analysis. However, it is usually the job of the Forensic Biologist to collect and preserve the hairs and fibers initially.

Many times an alternate light source, such as a laser is used to find stains on articles such as bed sheets or clothing. A portion of the stain is removed and tested for identification. The most common types of test are those for blood and semen, although at times it is necessary to test for other body fluids or tissues such as urine or saliva. A forensic scientist must know not only how to perform the appropriate test, but also how to interpret the results. Some tests can provide absolute answers, for instance, if a stain is in fact blood. Others can only provide a “likely” answer, like in the case of saliva, where there is no absolute conclusion. A positive analysis for this type of test means only that a substance is indicated (probable), not conclusively identified. Overstating conclusions is always a risk for the poorly trained or over confident Forensic Biologist.

Once a stain is identified, the scientist is often asked to determine who it could or could not have originated from. For example, if blood is found in a car, it may be beneficial to the case to find out if it originated from a people thought to be involved in the case, usually a victim and suspect. In this scenario, DNA analysis would be conducted on the blood in the “questioned stain” to develop a DNA profile. The questioned profile would then be compared to the known DNA profiles of the victim and suspect. If the profiles are the same, the are considered a “match” and the next step is to figure out what that match means. How many other people could be expected to have the same profile? Depending on amount of detail discovered in the DNA profile, this could range from a lot of people, to one in several billion, or essentially only a single person. With this type of strong match, DNA analysis if very similar to a fingerprint, and the analyst is able to testify in court that the blood originated from specific individual.