How DNA Evidence is Collected, Analyzed, and Used

Deoxyribonucleic acid, or DNA, is the genetic blueprint for every living organism. Found in nearly every cell, DNA is unique to each individual, with the exception of identical twins. This uniqueness makes it a powerful method for identification in forensic science. Its use has fundamentally changed how criminal investigations and legal cases are approached.

Sources and Collection of DNA Evidence

DNA can be recovered from biological materials like blood, semen, saliva, skin cells, and hair follicles. At a crime scene, this evidence might be found on weapons, clothing, or under a victim’s fingernails. It can also be located on any surface an individual may have touched.

Investigators use sterile tools and methods like swabbing surfaces or cutting stained fabric to collect biological material. These procedures are designed to prevent the loss or degradation of the sample. Preventing contamination at the scene and during collection is a primary focus to ensure the integrity of the evidence.

For comparison, investigators collect reference samples from known individuals, such as victims and suspects, often using a non-invasive buccal (cheek) swab. Elimination samples are also taken from individuals with legitimate access to the crime scene, like first responders, to account for their DNA being present.

A documented chain of custody is maintained for all samples. This paper trail details every person who has handled the evidence from collection to laboratory analysis. The chain of custody is a requirement for the evidence to be admissible in court.

Laboratory Analysis of DNA Samples

Once evidence arrives at a forensic laboratory, it undergoes a multi-step process to generate a DNA profile. The first step is extraction, where the DNA is chemically isolated from the biological material it is contained within. This process purifies the DNA, removing other cellular components so that it can be accurately analyzed.

Following extraction is quantitation, where scientists measure the amount of human DNA present. Subsequent analysis steps work optimally with a specific quantity of DNA. Too little or too much can make results difficult to interpret.

The next step is amplification, using a technique called Polymerase Chain Reaction (PCR) to make millions of copies of specific DNA regions. Forensic analysis targets short, repeating sequences known as Short Tandem Repeats (STRs), not the entire genetic code. The number of repeats in these STR regions varies significantly among individuals, making them ideal for identification.

After amplification, the STR regions undergo separation and detection using capillary electrophoresis, which sorts the copied fragments by size. A laser detects the separated fragments, generating a visual DNA profile called an electropherogram. This profile appears as a series of peaks corresponding to the different STR fragments.

In cases with degraded or limited evidence, such as a hair shaft without a root, scientists may analyze mitochondrial DNA (mtDNA). Inherited solely from the mother, mtDNA is not as discriminating as the nuclear DNA used for STR analysis. It can, however, provide links when nuclear DNA is unavailable.

Interpreting DNA Profiles and Matches

A DNA profile is a set of numbers representing the specific lengths of STRs at multiple locations, or loci. This profile, generated from a crime scene sample, is compared to profiles from known reference samples from a suspect or victim.

The comparison results in one of three outcomes. An “inclusion,” or match, occurs when the DNA profiles are consistent, and the reference individual cannot be excluded as the source. An “exclusion” means the profiles are different, ruling out the individual as the source. An “inconclusive” result may occur if the sample quality is poor or the data is too complex to interpret.

When a match is declared, it is accompanied by a statistical calculation to convey its significance. This statistic, the Random Match Probability (RMP), estimates how common or rare the DNA profile is within a given population. An RMP might be reported as 1 in 10 billion, but this statistic does not represent the probability of a person’s guilt.

Interpretation becomes more complex with mixtures, which are samples containing DNA from two or more individuals. The resulting electropherogram shows a combination of peaks from all contributors. Analysts use specialized software to separate the individual profiles, a process that can be challenging and may lead to less definitive conclusions.

DNA Evidence in Criminal Investigations and Trials

In criminal investigations, DNA evidence serves multiple functions. It can link a suspect to a crime scene, connect separate crimes to one perpetrator, or help identify human remains. DNA has also been instrumental in exonerating individuals who were wrongly convicted.

DNA databases, like the Combined DNA Index System (CODIS) in the United States, are a component of modern investigations. CODIS allows law enforcement to compare crime scene DNA profiles against a database of profiles from convicted offenders and some arrestees. A match, or “hit,” can provide a lead in cases without a known suspect.

At trial, a qualified expert witness presents the DNA evidence. The forensic scientist explains the collection and analysis process, the resulting profile, and the statistical significance of a match. Organizations like the Innocence Project have used post-conviction DNA testing to overturn hundreds of wrongful convictions.

Recent developments have expanded how DNA generates leads. Investigative genetic genealogy uses public genealogy websites to identify potential relatives of an unknown suspect by tracing family trees. Familial searching involves searching law enforcement databases for partial matches that could indicate a close relative of the perpetrator. These techniques have helped solve numerous cold cases.

Accuracy and Ethical Considerations in DNA Use

The accuracy of DNA evidence can be influenced by several factors. Contamination can occur at any stage, from the crime scene to the laboratory, introducing extraneous DNA that can confuse the results. Environmental factors can also cause DNA to degrade, making it difficult to obtain a full profile.

Low-template DNA samples, often called “touch DNA,” consist of very small amounts of genetic material left behind by touching an object. While technology can analyze these minute samples, determining how and when the DNA was deposited is challenging. The possibility of secondary transfer—where DNA is moved from one surface to another—adds another layer of complexity to interpretation.

The expansion of DNA collection and databasing raises ethical questions about genetic privacy and the potential for misuse of personal information. Debates continue over the legality and ethics of collecting DNA from individuals who have been arrested but not yet convicted of a crime.

Advanced techniques like familial searching and investigative genetic genealogy introduce further ethical dilemmas. These methods can draw relatives into investigations without their consent, creating privacy implications for entire families. The use of consumer-facing genealogy databases for law enforcement purposes has also sparked debate about data ownership and consent.