No two people have the exact same DNA. Even identical twins have different DNA due to behavioral and environmental factors. DNA evidence is an invaluable tool for obtaining convictions and exonerating people facing false accusations. However, when using DNA evidence in criminal cases, investigators must collect, store, test, and analyze it properly.
Additionally, attorneys must provide sufficient supporting evidence rather than relying on DNA alone.
What Makes Up DNA?
Almost every cell in the human body contains deoxyribonucleic acid (DNA). A person’s DNA is the same whether it comes from the skin, hair, saliva, bone, or other body tissue, and DNA does not change throughout a person’s life. Surprisingly, the DNA of any two individuals is 99.6% the same.
However, it is the 0.4% that is different that accounts for many of the genetic variations between individuals. This uniqueness gives DNA evidence its usefulness in identifying or ruling out suspects in criminal cases.
Collection and Preservation of DNA Evidence
The American Bar Association established standards for collecting and using DNA evidence in 2007. These standards help ensure that collecting DNA evidence does not violate anyone’s privacy or due process rights.
DNA Evidence in Criminal Cases History
Forensic scientists in the United Kingdom first used DNA evidence in a criminal case in 1986 to prove that a suspect who confessed to two rape and murder accusations did not commit the crimes. The first person to receive a conviction based on DNA evidence was U.K. resident Robert Melias in 1987. Later that same year, Tommy Lee Andrews became the first United States citizen convicted of a crime based in part on DNA evidence.
In 1989, the New York Supreme Court found that a lab improperly analyzed DNA for a case, which resulted in many of the current standards that ensure the proper collection, storage, and testing of DNA evidence in criminal cases. The exoneration of Glen Dale Woodall in 1992 and the conviction of Timothy Wilson Spencer in 1994 based on DNA evidence helped cement the use of DNA as an invaluable tool in criminal investigations in the United States.
Identification of Potential Sources of DNA
Crime lab scientists can develop a DNA profile by examining biological material from a variety of sources:
Scientists may also analyze hair, bone, teeth, cells, and body tissue.
Investigators may obtain the biological material for DNA testing by collecting objects that the person may have touched or worn, such as clothing, tools, weapons, bedding, cups or bottles, and fingernail scrapings.
Investigators typically look for objects in an area where criminal activity may have happened that the investigators can verify would not usually be in that location. For example, they might collect a gun found in a home where a crime occurred that the homeowners identified as not belonging to anyone who lives in the house.
Proper Collection, Labeling, and Storage of DNA Samples for DNA Profiles
DNA profiling is the process of collecting, labeling, and storing DNA samples. Investigators begin the process by collecting objects at the crime scene that may contain DNA evidence. They might then ask persons of interest to provide DNA samples voluntarily.
If a person refuses to give a sample, investigators can seek a court order to force the person to provide a sample if there is strong enough evidence that the person has committed a crime. Forensic scientists then extract DNA from the collected samples and use it to generate a profiling definition that law enforcement professionals can read.
Importance of Avoiding Contamination or Degradation of Evidence
When investigators fail to properly collect, preserve or store evidence, it can become contaminated. It may also deteriorate to the point where it is no longer usable as evidence or loses significant value as a source of information.
For this reason, investigators must follow evidence preservation protocols:
- Limits on the number of people who handle evidence
- Proper collection documentation
- Safeguards to minimize contamination
- Evidence-appropriate storage
- Adequate chain-of-evidence documentation
When improper collection, storage, or contamination compromises evidence, it may not be admissible in court.
Forensic scientists must analyze DNA evidence that law enforcement collects to determine whether it is useful in proving or disproving a criminal case. To accomplish this, they use technology, such as the process of polymerase chain reaction, to generate a DNA profile.
The Process of Polymerase Chain Reaction
PCR is a technology that allows a forensic scientist to amplify small segments of DNA by copying them in a lab. This technology makes it possible to analyze tiny amounts of DNA that may not have been usable in the past.
Comparison of DNA Profiles
DNA evidence allows investigators to link a suspect to a crime scene by comparing DNA profiles from samples taken at the scene to DNA profiles from samples taken from persons of interest and profiles stored in the FBI’s Combined DNA Index System.
Use of the FBI’s Combined DNA Index System (CODIS)
The FBI activated CODIS in October 1998. Participating forensic laboratories can compare DNA profiles from samples they analyzed to profiles stored in the database that were analyzed in other criminal cases or taken from persons convicted of crimes.
The CODIS is a hierarchy of DNA indexes from state, local, and national laboratories. Local labs upload DNA profiles to state indexes, and state indexes upload profiles to the FBI-maintained national index. The labs do not upload every profile; each lab decides which profiles to upload.
Evaluation of the Match Probability
Even though every person has unique DNA, most of it is the same as everyone else’s. DNA evidence is valuable because specific polymorphic regions vary highly between different people. These polymorphisms are what forensic scientists analyze to create DNA profiles.
Some polymorphisms are more common than others. The rarity of the polymorphisms that match between DNA profiles determines the probability that the two profiles came from the same source. The rarer the polymorphism is, the higher this probability.
Analysts use statistical formulas to determine the probability that two DNA profiles match because the profiles came from the same source rather than because of mere coincidence. Courts then consider this probability along with other evidence to determine the strength of the DNA evidence.
Interpreting the Results
DNA testing produces results that fall into three types: inclusion, exclusion, and inconclusive. When a DNA profile from a crime scene matches a profile from a known individual, this is an inclusion result, meaning that the known individual is included as a possible source of the DNA evidence. The strength of this result depends on the number of examined DNA locations and the match probability.
When the DNA profile from the known person does not match the DNA profile from the crime scene, this is an exclusion result. This means that the known person is excluded as the potential source of the DNA evidence.
When the results from DNA testing can neither include nor exclude the known person as the potential source of the DNA evidence, this is an inconclusive result. This can happen for various reasons, including the amount of DNA evidence being too small or the sample containing a mixture of DNA from multiple individuals.
Role of the Forensic Scientist in Interpreting the Results
Forensic scientists are not law enforcement officers. They do not arrest or interrogate suspects and usually do not investigate crime scenes. Instead, forensic scientists analyze DNA evidence and interpret the results. They may also testify as expert witnesses in court cases.
Importance of Accurate and Thorough Analysis
DNA evidence is only useful when properly collected, preserved, and analyzed. Advances in technology make it possible to analyze even small amounts of genetic material accurately. However, investigators and scientists must follow proper procedures to ensure that samples are free from contamination and deterioration and that analysts follow established best practices.
Admissibility in Court
The ABA established DNA evidence as admissible in court when the evidence meets one of the following standards:
- Expert testimony based on a valid scientific theory, valid implementation of that theory, and testing and interpretation that correctly applied the theory and technique
- Facts related to DNA evidence that are not subject to reasonable dispute
- Testimony from a qualified witness with skills, knowledge, education, or training relevant to the matters the witness is testifying about
The ABA standards also state that whenever possible, participants should resolve issues with the admissibility of DNA evidence before the trial.
Challenges to Admissibility and Overcoming Objections
When collected, stored, tested, and analyzed properly, DNA evidence is highly accurate. However, mistakes and contamination can impact the reliability of the results. Attorneys may choose to challenge DNA evidence on several grounds.
Crimes do not usually happen in sterile environments. DNA from individuals other than the person or persons who committed the crime can easily contaminate crime scenes. Additionally, law enforcement officers who mishandle evidence could introduce contamination.
To overcome this challenge, the prosecution or defense may need to produce evidence that documents proper handling procedures or proves why a match is likely accurate even though contamination might be present.
Lack of Supporting Evidence
DNA evidence by itself is not usually sufficient to prove that someone did or did not commit a crime. For example, when a suspect’s DNA matches samples from the crime, the defense can try to prove that the suspect could not possibly have been at the crime scene when the crime occurred. In such a case, DNA evidence may not be enough to secure a conviction.
To overcome this challenge, the opposition must present supporting evidence. For example, the prosecution could use video from a surveillance camera that shows the suspect at the crime scene to prove that the suspect had access to the scene when the crime occurred.
Improper Lab Analysis
Labs handle many samples, and if personnel does not follow proper procedures, mistakes with the storage, labeling, or analysis of evidence can occur. Labs can overcome this challenge by presenting documentation that proves the lab followed the proper procedures.
The Significance of DNA Evidence in the Prosecution of a Criminal Case
DNA is a powerful criminal justice tool because it is a source of empirical evidence that is not subject to biases and mistakes in the same way that witness testimony often is. Without DNA evidence, many cases come down to the victim’s word versus the accused. Additionally, because DNA evidence can last for years when properly stored, DNA can be a valuable tool for solving even very old cases or exonerating the falsely convicted.
Because DNA evidence is so compelling when properly produced, it may sway a judge or jury in ways that less reliable evidence might not. In this way, DNA evidence can make criminal cases fairer by providing a scientific way to prove or disprove an accusation.
However, human error can still impact the usefulness and reliability of this tool, so though DNA evidence is powerful, it is not enough on its own to result in conviction or exoneration.
How Often Is DNA Evidence Used in Criminal Cases?
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