Forensic biology is the scientific discipline that applies the study of living organisms and their components to matters of law and criminal justice. This field focuses heavily on the cellular material left behind at a crime scene, which acts as a silent witness in an investigation. Deoxyribonucleic acid, or DNA, is the complex molecule within most cells that contains the instructions for building and maintaining the human body, and it is the primary focus of forensic analysis. The specific sequence of this genetic material is unique to every individual (except identical twins), providing a powerful means of identification and allowing forensic scientists to establish a definitive link between a suspect, a victim, and the crime location. By processing minute traces of biological matter, they generate a detailed genetic profile that directs investigations and provides objective evidence in court.
Cellular Evidence Sources
Biological evidence is collected from a wide range of substances found at a crime scene, all of which contain cells that house the identifying DNA. Common sources include body fluids such as blood, semen, and saliva, which are frequently deposited during violent or sexual crimes. Blood is an excellent source of DNA, specifically from the white blood cells, because mature red blood cells lack a nucleus and therefore do not contain nuclear DNA for standard profiling. Saliva contains buccal epithelial cells shed from the inner lining of the mouth, making items like cigarette butts, drinking glasses, or envelope seals useful sources.
Even evidence that is less obvious can yield cellular material. Hair that retains the root or follicle contains the nucleated cells necessary for obtaining a complete DNA profile. Skin cells, often referred to as “touch DNA,” are left behind when a person contacts an object like a weapon or clothing. Modern techniques allow for the analysis of these trace amounts, which is important when no fluids are present. In cases involving unidentified or decomposed human remains, hard tissues such as bone and teeth are collected because they offer a protected source of DNA.
Essential DNA Analysis Techniques
Once cellular material is secured, the laboratory process begins with DNA extraction, a procedure designed to isolate the DNA molecule from other cellular components and debris. Following extraction, the DNA must be quantified to determine the exact amount and quality of human DNA recovered, which informs subsequent steps. This quantification ensures the correct amount of DNA is used in the next process, helping to avoid issues like incomplete profiling or wasted sample material.
The fundamental technique in forensic DNA analysis is the Polymerase Chain Reaction (PCR), which serves to amplify specific regions of the isolated DNA. PCR involves repeated cycles of heating and cooling, multiplying minute traces of DNA into millions of copies. This amplification is necessary because the original crime scene sample may contain only a tiny or degraded amount of genetic material. This technology enables the analysis of samples that were once considered unusable.
Following amplification, the DNA profile is generated using Short Tandem Repeat (STR) analysis. STRs are short sequences of DNA, typically two to seven base pairs, that are repeated multiple times at specific locations (loci) on a chromosome. The number of these repeats varies significantly between individuals, making them highly polymorphic and useful for identification. Forensic laboratories generally analyze a standardized set of 22 or more autosomal STR locations to create a unique genetic fingerprint. The resulting fragments are separated and measured using capillary electrophoresis, producing the final DNA profile.
The Process of Identification and Comparison
The purpose of generating a DNA profile is to compare it against other profiles to identify or exclude individuals involved in a crime. This comparison involves matching the profile obtained from the crime scene evidence against a known sample from a suspect, a victim, or other individuals. If a match is found, the genetic profile is given statistical weight to describe its rarity in the general population. The random match probability (RMP) is the statistic used for single-source profiles, expressing the likelihood that a randomly chosen, unrelated person would have the exact same DNA profile.
For samples containing DNA from multiple sources (mixtures), the Combined Probability of Inclusion (CPI) is calculated. This indicates the probability that a random person could not be excluded as a contributor to the mixture. This comparative process is aided by forensic databases, such as the Combined DNA Index System (CODIS) in the United States. CODIS is FBI software that allows federal, state, and local laboratories to electronically compare DNA profiles.
The database contains profiles categorized into indexes, including those from convicted offenders, arrestees, and unknown profiles collected from crime scenes. A search can result in a “hit” linking a crime scene profile to an individual, or a “case-to-case” link connecting two previously unrelated crime scenes. CODIS functions as a tool to link information and generate investigative leads for law enforcement. The data stored in CODIS is de-identified, meaning the searcher only sees the genetic markers, which helps minimize potential bias during comparison.
Proper Handling of Biological Evidence
The integrity of cellular evidence rests heavily on the procedures followed during its collection and storage before it ever reaches the laboratory. Proper handling is necessary to prevent DNA degradation and ensure the evidence remains admissible in court. The most important element is the “chain of custody,” a chronological record documenting every person who has possessed the evidence from collection until final disposition.
Maintaining a complete chain of custody establishes the authenticity of the evidence and assures that no tampering or substitution has occurred. Contamination is a persistent risk, often involving the transfer of foreign DNA from personnel or other evidence items. To prevent contamination, personnel must use appropriate personal protective equipment (PPE), and each item of evidence must be packaged separately.
Biological samples must be preserved correctly to slow the natural degradation process caused by moisture, heat, and microbial growth. Items containing wet biological fluids should be allowed to air-dry before packaging them in breathable containers, such as paper bags, rather than sealed plastic. For long-term storage or transport, the evidence is kept in a cool, dark, and dry environment, often involving refrigeration or freezing, to maintain the viability of the cellular material.