DNA evidence has become one of the most powerful tools in modern criminal investigations. It plays a critical role in linking suspects to crimes, exonerating innocent individuals, and securing convictions. However, understanding DNA evidence can be complex. This post will explain the basics of DNA, how it’s collected, how it’s analyzed, and its role in criminal cases. By the end, you will have a clear understanding of how DNA evidence works and why it is so important in the criminal justice system.

What is DNA Evidence?

Deoxyribonucleic acid (DNA) is the molecule that carries genetic information in living organisms. It is found in every cell of the human body and is unique to every individual (except by identical twins). DNA is composed of four nucleotides: adenine (A), cytosine (C), guanine (G), and thymine (T). The sequence of these nucleotides forms a code that determines the genetic traits of an organism.

In criminal investigations, DNA evidence is invaluable because of its ability to uniquely identify individuals. When biological material (blood, hair, saliva, etc.) is found at a crime scene, it often contains DNA that can be matched to a suspect or excluded from involvement in the crime.

Like a fingerprint, DNA remains the same no matter how many times blood is drawn and tested and a DNA profile can be used to inculpate or exculpate suspects in other investigations without additional invasive procedures. See: Pace v. State, 524 S.E.2d 490 (Ga. 1999).

How is DNA Evidence Collected?

Understanding DNA evidence begins with an understanding of how it’s collected. The collection of DNA evidence requires some vital steps to ensure its integrity and reliability. These proper collection techniques are essential to avoid contamination or degradation, which could lead to incorrect conclusions.

• Crime Scene Investigation: Investigators at the crime scene carefully collect biological samples such as blood, semen, saliva, skin cells, hair, and other bodily fluids. These samples are often found on items like clothing, weapons, or surfaces at the scene.
• Preservation: Collected samples are stored in sterile containers and keep it in controlled conditions to prevent contamination and deterioration. For instance, blood and bodily fluids are often stored in cool, dry conditions to preserve the DNA.
• Chain of Custody: Maintaining a proper chain of custody is critical to ensure that the evidence has not been tampered with. Every person who handles the evidence must document their interaction with it, and it must be stored in a secure location until analysis.
• Collection From Suspects: DNA samples can also be collected from suspects, typically through a buccal swab (from inside the cheek) or a blood sample. These samples are compared to the DNA recovered from the crime scene to determine if there is a match.

How is DNA Evidence Analyzed?

DNA evidence analysis involves several steps to isolate and identify the genetic material from the biological samples. The process varies slightly depending on the laboratory and the specific techniques used, but generally includes the following:

• Extraction: The first step is to extract DNA from the collected biological material. For example, in a blood sample, cells are broken open to release the DNA, which is then separated from other components like proteins and cell debris.
• Amplification (PCR): The next step is to amplify (make many copies of) specific regions of the DNA using a technique called Polymerase Chain Reaction (PCR). This is particularly important because the DNA collected from a crime scene is often present in very small quantities.
• Analysis of STRs: DNA is made up of both coding regions (genes) and non-coding regions. For the forensic analysis, scientists focus on short tandem repeats (STRs), which are non-coding regions that vary greatly among individuals. These regions contain repeating sequences of nucleotides that differ in length between people. By analyzing the number of repeats at specific locations, forensic scientists can create a DNA profile for an individual.
• Comparison: Once a DNA profile is created from a crime scene sample, it is compared to a suspect’s DNA profile or to databases of known DNA profiles (such as the FBI’s Combined DNA Index System or CODIS). If there is a match, the evidence can link the suspect to the crime scene.
• Statistical Analysis: In many cases, forensic scientists use statistical models to determine the likelihood that the DNA found at the crime scene belongs to the suspect. These models take into account the rarity of the DNA profile in the general population. For example, if a DNA profile is found to match a suspect’s, the analysis might reveal there is a 1 in a billion chance that someone else could have the same profile.

What Are the Sources of DNA Evidence?

DNA evidence can come from various sources, and understanding those sources is crucial to interpreting the evidence. DNA evidence is contained in the following:

• Biological Fluids (Blood, Semen, Saliva): These are the most common types of DNA samples found at crime scenes. Blood is often collected from clothing, weapons, or surfaces, while saliva and semen are frequently found in cases of sexual assault.
• Hair: Hair samples can be analyzed for DNA, but it must contain a root to be useful for analysis. If the hair is simply a strand without a root, only mitochondrial DNA (which is inherited from the mother) can be analyzed.
• Touch DNA: Even if a suspect did not leave blood or other biological fluids at the crime scene, DNA can still be obtained from skin cells left behind when they touch objects. This type of DNA evidence is sometimes found on weapons, door handles, or other surfaces.
• Other Sources: Other less common sources of DNA include sweat, urine, saliva, and even the skin cells that are shed from the body.

How Accurate is DNA Evidence?

DNA evidence is generally considered highly reliable due to the uniqueness of each individual’s DNA profile. However, there are several factors that can influence the reliability of DNA evidence:

• Contamination: If the evidence is not collected or handled properly, it could become contaminated with DNA from someone else, leading to erroneous conclusions. This is why maintaining the chain of custody is crucial.
• Degradation: DNA can degrade over time due to exposure to environmental conditions such as heat, moisture, or sunlight. In cases where DNA is degraded, it may be more difficult or impossible to obtain a usable profile.
• Mixtures: In some cases, DNA samples may come from more than one person, such as in cases of sexual assault or violent confrontations. These mixed DNA profiles can be challenging to interpret, especially if the sample contains DNA from multiple contributors in equal or unequal proportions.
• Statistical Errors: While the statistical likelihood of a match is very high, there is still a small chance of error, especially if the database used for comparison is not sufficiently comprehensive or if the DNA sample is degraded.

Understanding DNA Evidence in Criminal Cases: A Case Study

To better understand how DNA evidence plays a role in criminal investigations, let’s consider a hypothetical case:

A woman is found murdered in her apartment, and investigators recover a small blood sample from a knife left at the scene. The sample is sent to a forensic laboratory, where it undergoes DNA extraction and analysis. The crime lab generates a DNA profile from the blood sample and compares it to a database of known offenders. A match is found with a suspect who has a prior conviction for assault. The suspect’s DNA is then collected through a buccal swab and compared again to the blood found at the crime scene.

In this case, the DNA evidence played a key role in linking the suspect to the crime and providing investigators with the necessary lead to pursue a conviction. Without this evidence, the case could have remained unsolved.

Understanding DNA Evidence in the Justice System

DNA evidence has revolutionized the criminal justice system, offering a level of accuracy and precision that was previously unavailable. It can be used to:

• Exonerate the Innocent: One of the most significant uses of DNA evidence is exonerating individuals who have been wrongfully convicted. Since DNA is so specific to each individual, it can be used to prove that someone was not at the crime scene or involved in the crime.
• Convict the Guilty: On the flip side, DNA evidence can also help secure convictions against guilty parties by definitively linking them to the crime scene.
• Cold Cases: DNA evidence has also been instrumental in solving cold cases, where crimes that occurred years or even decades ago remained unsolved due to lack of leads or evidence. With advances in DNA technology, cold cases are increasingly being reopened and solved.
• Preventing Future Crimes: DNA databases, such as CODIS, allow law enforcement agencies to compare DNA samples from new cases to those from previous investigations, potentially linking serial offenders to multiple crimes.

Ethical and Legal Considerations of DNA Evidence

While DNA evidence is highly valuable, its use raises ethical and legal concerns. For example:

• Privacy Issues: There are concerns about the potential for DNA databases to be misused. The collection and storage of DNA raises questions about individuals’ privacy rights and the possibility of wrongful inclusion in a database.
• Wrongful Convictions: Although rare, there have been cases where DNA evidence has been mishandled, misinterpreted, or misused, leading to wrongful convictions. Ensuring the proper handling and interpretation of DNA evidence is vital to maintaining public trust in the justice system.
• Informed Consent: In some situations, individuals may not be fully aware of the consequences of providing a DNA sample, especially if it is taken without their knowledge or consent.

My Final Thoughts

By understanding DNA evidence, you can recognize how this evidence has transformed the way criminal investigations are conducted. It has become one of the most reliable tools available to law enforcement, offering the ability to link a suspect to a crime, exonerate the innocent, and help solve cold cases.

However, the collection, analysis, and interpretation of DNA evidence requires careful attention to detail, as mistakes or contamination can compromise the results. With ongoing advancements in DNA technology, its role in criminal justice will continue to grow, further enhancing the accuracy and fairness of the justice system.

If you, or someone you know, will be proceeding to trial, challenging a criminal sentence, pursuing post-conviction relief, or litigating a Civil Rights Section 1983 lawsuit, our books are crucial resources in the pursuit of justice. Purchase your copy today, or on behalf of someone incarcerated.