How DNA Evidence Works

DNA samples and DNA testing involve comparing biological material against other specimens. This creates a profile to identify a unique individual in a criminal case.

DNA evidence is one of the best examples of how technology has altered the criminal justice system. Particularly, it can help exonerate the falsely convicted.

This article explores the use of DNA databases in criminal investigations and trials.

What Is DNA?

DNA (deoxyribonucleic acid) is a basic building block of life. The information encoded in an organism's DNA acts as a blueprint for the organism's biological development and functioning.

DNA exists in the cells of all living organisms. By testing the DNA found in a person's cell, scientists can develop a DNA profile for them. Only one-tenth of 1% of human DNA differs from one person to the next. Although estimates vary, forensic DNA analysis is roughly 95% accurate.

But the development of forensic DNA evidence and DNA profiling didn't exist until the mid-1980s. Scientists discovered that certain areas of the DNA strand contain patterns that repeat many times. They developed a test to measure the variation in the length of these repetitions. The number of these repetitions varies between people (except identical twins with the same DNA). The test developed to measure repetitions became known as restriction fragment length polymorphism (RFLP).

RFLP is an accurate and reliable test, but it requires a relatively large amount of DNA to work. Laboratories now use tests based on the polymerase chain reaction (PCR) method, which allows testing on tiny amounts of DNA from biological samples.

What Is DNA Evidence?

Similar to fingerprints, each individual has a unique DNA profile. There is an exception for identical twins, who share the same genetic code. Unlike fingerprints, only a minuscule amount of genetic material is needed to identify a suspect for a criminal trial.

One of the most reliable forms of evidence in many criminal cases is in our genes. A person's DNA (deoxyribonucleic acid) is biological evidence that can be analyzed in crime laboratories. DNA evidence can be collected from blood, hair, skin cells, and other bodily substances. It can even be used to solve old crimes that occurred prior to the development of DNA-testing technology.

This article provides a basic overview of DNA as a source of evidence in the criminal justice system.

Forensic DNA Evidence: A Brief History

The science of DNA testing was developed in 1985 by British scientist Alec Jeffreys. Genetic evidence was first tested by a forensic laboratory using his method one year later to solve a double homicide in England. It allowed law enforcement officers to link the suspect to other previously unsolved rapes and murders in the area. In 1987, a Florida rapist became the first criminal defendant in the United States to be convicted through DNA.

As DNA became the gold standard for identifying criminal suspects, law enforcement agencies took note. The FBI and police departments throughout the U.S. started assembling DNA databases such as the Combined DNA Index System (CODIS). 

In addition, sex offenders in all states are now required to submit DNA samples to their local police department. Many states make use of DNA evidence even in criminal charges involving misdemeanors, although many crime labs are overwhelmed with backlogs of genetic samples and may be unable to process them in a timely fashion.

Other Uses of Genetic Evidence

Genetic material preserved in evidence lockers has many applications. In addition to criminal investigations, DNA analysis can also be used to exonerate wrongly accused individuals. That includes those who were victims of wrongful convictions for violent crimes like sexual assault. This is important for those convicted solely on the basis of eyewitness testimony, which is not always reliable. More than 575 people have been exonerated through post-conviction DNA tests.

DNA court orders can also be used to determine paternity in child support cases. In the world of forensic sciences, DNA technology can also be used to:

• Identify the remains of crime and accident victims
• Conduct genealogical research

Keep in mind the disclaimer that a DNA match will not necessarily result in securing a convicted offender. Like all other evidence, DNA testing is subject to chain of custody and tampering issues.

How DNA Is Sourced and Analyzed by Law Enforcement

Investigators can collect DNA from several different sources, such as bedding, clothes, weapons, or sexual assault testing kits. Almost any biological evidence can contain DNA, although not every sample contains enough DNA to enable DNA profiling.

Common examples of biological specimens containing DNA include the following:

• Blood
• Semen
• Saliva
• Hair
• Skin Cells

Forensic laboratories will analyze the biological samples to get a DNA profile of the person(s) that the samples came from.

Suppose investigators already have a suspect in mind. In that case, they can collect samples from them to compare to the evidence collected at the scene. There are also databases of DNA profiles that investigators can use to identify suspects.

In 1994, the Federal Bureau of Investigation (FBI) received funding to compile DNA evidence into a central database. Accredited forensic crime laboratories can submit DNA profiles to the National DNA Index System (NDIS). The Combined DNA Index System (CODIS) is the software system that analyzes such evidence. The use of DNA evidence from convicted offenders, unsolved crime scene evidence, and missing persons is all stored there and helps solve cold cases.

How Does DNA Testing Work?

A sufficient amount of DNA may be found in virtually any type of biological material. For violent crimes, such evidence typically comes from blood or other bodily fluids. Hair and skin cells left at the crime scene also may provide investigators with enough DNA for testing purposes.

DNA evidence is analyzed using the polymerase chain reaction (PCR) method. This allows for very small samples to be tested and identified. Once the sample is tested, it may be cross-referenced with DNA profiles already in a database or with genetic data provided by a suspect.

While DNA testing is not completely foolproof, it is more than 99% accurate. In fact, there is only a one in one billion chance that the DNA test results of two individuals will match. Errors in testing are often the result of mix-ups in the lab or the contamination of samples. In addition, each state has specific rules for DNA sample collection and handling. Courts might not allow the use of genetic evidence in court if these requirements are not met.

Accuracy of DNA Evidence Test Results

If investigators properly collect and handle biological evidence and forensic scientists employ accepted methods and conduct the analysis correctly, DNA evidence is extremely accurate. The chances of one person's DNA profile matching another person's is minimal—about one in a billion by some estimates.

DNA evidence is a more highly effective way to match a suspect to biological samples collected during a criminal investigation than fingerprinting or eyewitness testimony, both of which have inherent flaws and inaccuracies.

Because of its accuracy, criminal lawyers rely on this evidence to prove a defendant's guilt or innocence. Defense lawyers use this type of evidence as an exoneration tool for wrongful convictions through postconviction analysis of biological samples.

Reexamining evidence collected during older investigations can also reveal that the convicted person's DNA does not match the biological samples collected at crime scenes.

Human Error and Admissibility

DNA evidence is not ironclad proof. Errors in collecting or handling biological samples can present legal issues that result in their exclusion at trial. A judge may reject DNA evidence if a lab:

• Contaminates the biological sample
• Doesn't maintain quality control
• Uses unreliable methods

Defense attorneys usually focus on investigators and forensic analysts when challenging this type of evidence. They'll try to cast doubt on the results of DNA profiles rather than attack the reliability of DNA profiling as a whole.

A well-known example is the defense counsel strategy used in the O.J. Simpson trial. Failure to comply with state requirements can result in a refusal of the court to examine DNA evidence.

Each state also has different rules regarding the expungement of evidence. For example, in California, if you plead guilty or are convicted of a felony, the state can retain your DNA profile. Other jurisdictions require convicted sex offenders or arsonists to submit a sample of their DNA.

There is also a rise in the use of familial DNA to narrow down potential suspects. Law enforcement agencies access popular consumer DNA databases such as 23andMe and Ancestry.com to find partial matches to suspect's DNA profiles. Regulations authorizing familial searching took effect in New York in 2017.

Have More Questions About How DNA Evidence Works? Talk to a Lawyer

Juries find DNA evidence compelling and convincing. But a close examination sometimes reveals flaws in collecting and processing evidence of this kind. For instance, improperly labeled evidence may incriminate the wrong suspect.

Speak with a criminal defense attorney if you're facing criminal charges and have questions about using DNA evidence in your case.