Deoxyribonucleic acid (DNA) serves as the fundamental instruction set for nearly every living organism, carrying the unique genetic information that makes each individual distinct. Forensic science has harnessed this biological blueprint, moving beyond visible fluids like blood and semen to analyze minuscule, invisible quantities of genetic material left behind at a crime scene. This extremely small amount of material is referred to as trace DNA. Its successful recovery and analysis have profoundly changed modern criminal investigations, allowing investigators to connect individuals to objects or locations in ways previously unimaginable. Understanding the limitations and the highly sensitive techniques required to process this evidence is necessary to properly evaluate its role in the justice system.
Defining Trace DNA and Its Sources
Trace DNA is defined as genetic material present in such minute quantities that it is often invisible to the naked eye, sometimes called Low-Template DNA (LT-DNA) or Low Copy Number (LCN) DNA. This material is distinct from bulk samples, such as large bloodstains or semen, which yield robust DNA profiles with standard methods. A trace sample may contain less than 100 picograms of DNA, which equates to the genetic material from only a few dozen human cells.
The primary source of trace DNA is the constant shedding of human skin cells (epithelial cells) through contact with surfaces. This form of evidence, often called “touch DNA,” is deposited when a person handles an item like a tool, a weapon, or clothing. Trace DNA can also originate from other biological sources, including minute droplets of saliva left from speaking or coughing, or from the residue of sweat.
The amount of DNA an individual deposits varies greatly, a phenomenon known as “shedder status.” This status is influenced by factors like skin condition, recent hand washing, and the duration and pressure of contact. Saliva, in particular, contains buccal epithelial cells and white blood cells, which are excellent sources of DNA.
Gathering and Analyzing Minute Samples
The process of recovering and analyzing trace DNA begins with specialized collection techniques designed to maximize the yield. Forensic technicians typically employ moist swabbing with a sterile cotton or nylon swab, often pre-wetted with a preservative solution to enhance recovery. For porous or textured surfaces, analysts might use a tape lift method, applying adhesive tape to the surface and then transferring the adhering cells to a testing tube.
Once collected, the minute quantity of DNA is made analyzable using the Polymerase Chain Reaction (PCR). PCR is a molecular photocopying technique that amplifies specific, targeted regions of the DNA strand, known as Short Tandem Repeats (STRs), millions of times over. For trace samples, a variation called High Sensitivity PCR (HS-PCR) or LCN analysis is often utilized, which increases the number of thermal cycles (e.g., from the standard 28 to 34 cycles).
This increased amplification allows a genetic profile to be generated from a sample containing as little as 30 picograms of DNA. However, this high sensitivity introduces greater risk of “stochastic effects,” which are random fluctuations in amplification that can lead to missing data points or “allele dropout.” The resulting profile may therefore be weaker, partial, or contain a mixture of DNA from more than one individual.
The Challenge of DNA Transfer
The central limitation in interpreting trace DNA evidence is determining precisely how and when the genetic material was deposited. The high sensitivity of modern testing means DNA can be detected even after being transferred indirectly, complicating the determination of contact.
Types of Transfer
Primary transfer occurs when a person directly touches an item and leaves their DNA behind, which is the scenario most often assumed in court. Secondary transfer occurs when a person’s DNA is moved from one surface to another via an intermediate object or person. For example, DNA might transfer when a person shakes hands with another, and the second person subsequently touches a weapon. Even less frequent but still possible is tertiary transfer, where the DNA is moved across three or more surfaces before landing on the item of evidence.
This ease of movement means the mere presence of a person’s DNA on an object does not definitively prove they were involved in a crime or were the last person to handle it. Contamination remains a risk, where foreign DNA can be inadvertently introduced by investigators using unsterilized equipment or handling multiple pieces of evidence. The inability to distinguish between these different types of transfer must be considered when evaluating the weight of touch DNA evidence.
Interpreting Trace Evidence in Legal Settings
The nature of trace DNA often results in partial or complex mixed profiles, containing genetic material from two or more people, making traditional interpretation difficult. To analyze these challenging samples, forensic laboratories increasingly rely on sophisticated statistical tools known as Probabilistic Genotyping Software (PGS), such as STRmix. This software uses mathematical modeling to calculate the likelihood of a given individual being a contributor to the mixed DNA profile.
The output of PGS is the Likelihood Ratio (LR), which quantifies the strength of the evidence by comparing two opposing hypotheses. The software calculates how much more likely the DNA evidence would be if the prosecution’s hypothesis (the suspect contributed to the sample) is true, versus the defense’s hypothesis (an unknown, unrelated person contributed). For instance, an LR of 100,000 means the evidence is 100,000 times more likely under the prosecution’s scenario.
The LR represents only a comparison of two possibilities, not the probability of guilt, and its meaning is often misunderstood in legal settings. The existence of a suspect’s trace DNA on an object only proves contact occurred at some point, speaking to association, not necessarily the specific time or nature of the activity. Therefore, courts and juries must consider the LR within the broader context of the case, acknowledging the potential for innocent transfer and the limitations of the sample.