Trace DNA refers to the minute quantities of genetic material that individuals unknowingly leave behind. This biological residue is transferred to objects or people through simple contact, making it a resource in forensic investigations, especially when substantial evidence like blood or saliva is absent. The ability to analyze these nearly invisible samples has changed how crime scenes are processed.
Understanding Trace DNA Sources and Transfer
Trace DNA is a term for any small quantity of genetic material recovered. The most common source is skin cells shed from the outer epidermal layer, often called “touch DNA”. An individual can deposit hundreds of cells with a brief touch. Other sources include microscopic droplets of saliva from speech, sweat residue, and hair fragments that no longer contain the root.
The transfer of this genetic material is a primary concept in its forensic application. Primary transfer happens when an individual’s DNA is deposited directly onto an object or another person. For instance, a suspect’s skin cells left on a weapon handle is a form of primary transfer, providing a direct link between the individual and the item.
Secondary transfer occurs when DNA moves from one surface to another indirectly. For example, if a person shakes hands with another, and the second person then touches a doorknob, the first person’s DNA can be transferred to it. This mechanism can complicate investigations, as it may place an individual’s DNA at a location they never visited. Factors like a person’s “shedder status,” surface texture, and humidity all influence how much DNA is deposited and how long it persists.
Essential Techniques for Trace DNA Collection
Collecting trace DNA requires meticulous techniques to gather evidence while preventing contamination. A common method is swabbing, where a sterile cotton or foam swab, sometimes moistened with purified water, is rubbed over a surface to lift cellular material. This is effective on smooth, non-porous surfaces like countertops or weapon handles.
For textured surfaces such as clothing or upholstery, taping is often used. Investigators press specialized, DNA-free adhesive tape against the surface, lifting skin cells onto the adhesive. The tape is then secured for transport to the lab. In other situations, direct cutting of a portion of an item, like a shirt collar, is performed to secure an area likely to contain DNA. All collection must be done with sterile instruments and personal protective equipment to avoid introducing external DNA.
Laboratory Procedures for Analyzing Trace DNA
Once a sample arrives at the laboratory, it undergoes a multi-step process to generate a DNA profile. The first step is extraction, where chemicals break open the collected cells and release the DNA. Following extraction is quantification, which measures the amount of human DNA present. This is a determining factor for the success of subsequent steps, especially with low-level samples.
The next stage is amplification, using a technique called Polymerase Chain Reaction (PCR). PCR acts as a molecular copy machine, making millions of copies of specific regions of DNA. Forensic analysis focuses on Short Tandem Repeats (STRs), which are repeating segments of DNA that vary in length between individuals. After amplification, the copied STR fragments are separated by size using capillary electrophoresis, allowing scientists to create a unique genetic profile.
Complexities in Trace DNA Profile Interpretation
Interpreting profiles from trace DNA presents challenges due to the small quantity of starting material. When working with Low Copy Number (LCN) DNA, random amplification effects can occur. These effects can lead to “allelic drop-out,” where a part of the DNA profile fails to amplify and is missing from the result. The opposite issue, “allelic drop-in,” can also happen, where contaminant DNA is amplified and appears as an extra peak in the data.
Another complexity is the prevalence of DNA mixtures, where a sample contains DNA from two or more individuals. Distinguishing the individual contributions within a complex mixture can be difficult, especially if the contributors are not present in equal amounts. Environmental degradation can also break down the DNA, resulting in partial profiles. To address these issues, labs use probabilistic genotyping software, which applies statistical models to assess the likelihood of different contributor combinations within a mixture.
Trace DNA’s Impact on Forensic Science
Trace DNA analysis has greatly impacted forensic science, enabling investigators to link individuals to crime scenes with high precision, even without visible biological fluids. DNA recovered from items like weapon grips, steering wheels, or clothing can place a suspect at a scene or in contact with a victim. This has helped solve numerous cases, including cold cases where evidence was preserved for decades.
Beyond securing convictions, trace DNA has also been used for exonerating the wrongly accused. It can provide evidence that points away from a suspect and toward another individual. However, the presence of DNA does not explain the context of its deposition. Finding someone’s DNA on an object does not automatically prove their involvement in a criminal act, a limitation that requires the evidence be considered within the full context of an investigation.