Physical Evidence Handbook

DNA Analysis Unit

The DNA Analysis Unit of the Crime Laboratory analyzes and compares body fluids and materials for the purpose of investigating or prosecuting crimes.

Forensic Comparisons
Forensic science is comparative in nature. The DNA Analysis Unit uses comparative characterization also. The purpose of the analyses performed in the DNA Analysis Unit is to determine if there is any physical evidence of a biological nature which might support or refute a possible connection between a victim of a crime and a suspect of a crime. Sometimes a connection is sought between the people, the scene, and/or an instrument of the crime.

The comparisons used in this Unit involve the characterization of genetic markers. In this context, a genetic marker is an inherited biochemical characteristic present in almost every cell of the body. An individual's genetic markers are determined by inheritance from their parents. Genetic marker types do not change throughout a person's lifetime. In addition, the genetic profile found in one body fluid or tissue will be identical to the genetic profile found in any other body fluid or tissue from the same individual. Identical twins are the only individuals that have the same genetic markers.

In order to be forensically useful a genetic marker must also meet certain other criteria. It must be present in the biological material of interest. It must be analyzable by reliable techniques. It must show different types in different people and we must know how rare or how common a particular marker is in the various population groups.

Some Questions Can Be Answered by Forensic DNA Analysis...
In forensic DNA analysis, all of the tests performed are designed to help answer two basic questions.

Question 1: What is the genetic profile of the person who left the biological evidence?

This question is answered by characterizing the evidence through the analysis of genetic markers.

Question 2: Could the biological material have come from a given individual connected with or suspected of being connected with the crime?

This question is answered by characterizing the genetic markers of the individual in question and then comparing them to the genetic markers found in the evidence. Another way to ask this question is: Could the individual in question have contributed the biological evidence?

Corollary to Question 1: How many individuals could have left the evidence?

This question is not answered by examination of the evidence. This question is answered by population statistics. Population statistics are generated by analyzing samples from a large number of people for the genetic marker in question. The statistics generated are used to make assumptions about the frequency of those genetic markers in the whole population. When the number of individuals who could contribute a certain type of biological evidence is so small, it becomes an identity, and, statistically, only that one person could contribute that evidence.

A Question that Cannot be Answered by Forensic DNA Analysis
What is the probability that this person is guilty?

The guilt question assumes that the evidence is connected to the crime. The same probabilities exist depending on whether or not the individual is actually guilty. Laboratory analysis alone cannot tell if the evidence is related to the crime or which of the possibilities is the correct one. That is the purpose of the trial and the function of the jury.

DNA Evidence
Most of the work in the DNA Unit is concerned with the examination of sexual assault evidence. The Unit examines evidence which is collected from victims at the hospital in the form of a Sexual Assault Evidence Collection Kit and evidence which is collected from the scene of the crime. The major purpose of these examinations is to seek evidence of sexual activity and then to determine the individuals that were involved in the sexual activity. The major type of evidence, which can be used to demonstrate sexual activity, is semen or seminal stains.

The second most common type of evidence examined in the DNA Unit is bloodstains. Most often, bloodstains are submitted from murder cases but they also may be associated with sexual assaults, aggravated assaults, burglaries, etc.

Once semen, blood, or any other biological evidence has been located, further testing is performed to characterize the evidence. Biological material can provide clues to the genetic makeup of the person who contributed the material. These clues come in the form of genetic markers as described previously. DNA is the genetic material, found in most cells in the body, which determines the genetic characteristics of each person. DNA can be characterized to provide a profile of DNA information for the evidence material. The DNA profile found in the evidence is compared to the DNA profile found in a standard sample from a person suspected of being the source of the evidence.

Three Possible Results
When a comparison is made there are three possible results. The first is exclusion. The second is inconclusive. The third is a match or inclusion.

  • An "exclusion" means that the evidence has a DNA profile which is different from the DNA profile of the person in question. This in turn means that person cannot be the source of the DNA and by extension is not the source of the evidence material.
  • "Inconclusive" means that the tests were not able to shed any light on what might or might not have happened. Possibly the evidence sample was degraded and no profile was developed or the profile developed was not foreign to the victim. Therefore no information could be discerned about the perpetrator of the crime.
  • An "inclusion" means that the genetic profile from the biological evidence and the genetic profile of the standard sample from an individual are the same. If the genetic profile is sufficiently rare in the population, this can mean that the source of the biological evidence can be attributed to that one individual alone to a reasonable degree of scientific certainty. If the genetic profile developed is not rare enough to limit it to originating from one individual, statistics can be calculated to determine how many individuals in a population could have contributed the biological evidence.
Probability of a Random Match
DNA analysis utilizes random match probabilities to sometimes state that an identified match is established. This is done using well-defined criteria that places importance on the size of the group of possible DNA sources. It allows for single source attribution to a reasonable degree of scientific certainty when random match probabilities are rare. The size of this group can be used to estimate the probability of a random match. In other words, if a person is taken at random from the population what is the probability that he or she will have the same DNA profile as that found in the evidence? The smaller the group of people included as possible sources of the evidence DNA, the lower the probability of a random match. DNA is such a powerful tool that most often evidential biological material can be deemed to originate from one specific individual to a "reasonable degree of scientific certainty."