A comprehensive review of forensic science — key terms, evidence analysis, DNA profiling, toxicology, and entomology review questions.
Forensic science is the application of scientific principles and techniques to matters of criminal and civil law. It encompasses a broad range of disciplines — from biology and chemistry to digital analysis — all directed at collecting, preserving, and interpreting physical evidence. The goal is to reconstruct events objectively and present findings that can withstand scrutiny in court.
Modern forensic practice rests on Locard's Exchange Principle: every contact between two objects results in a transfer of material. This underpins evidence collection at crime scenes, laboratory analysis of trace materials, DNA profiling, toxicological screening, and the estimation of time of death through pathological and entomological indicators. Together, these methods allow investigators to link suspects, victims, and scenes with a high degree of scientific certainty.
Forensic Science
The application of scientific methods and principles to criminal and civil investigations, aimed at collecting and analysing evidence for use in legal proceedings.
Locard's Exchange Principle
The principle that every contact between two objects results in a mutual transfer of material — the foundation of trace evidence analysis.
Chain of Custody
The documented, unbroken record of who handled a piece of evidence, when, and under what conditions — essential for admissibility in court.
Physical Evidence
Tangible objects found at a crime scene, such as fingerprints, fibres, glass fragments, tool marks, and footwear impressions.
Biological Evidence
Evidence of biological origin — blood, saliva, semen, hair, and skin cells — often used for DNA analysis to identify individuals.
DNA Profiling
A technique that analyses specific regions of DNA (short tandem repeats) to produce a unique genetic profile for identification purposes.
PCR (Polymerase Chain Reaction)
A laboratory method used to amplify small quantities of DNA into millions of copies, enabling analysis from minute or degraded samples.
STR (Short Tandem Repeat)
Repeating sequences of 2–6 base pairs in DNA that vary between individuals. STR analysis at multiple loci forms the basis of modern DNA profiling.
CODIS
The Combined DNA Index System — the FBI's national DNA database used to store and compare DNA profiles from crime scenes and convicted offenders.
Electrophoresis
A technique that separates DNA fragments by size using an electric field through a gel matrix, allowing visualisation and comparison of genetic profiles.
Fingerprint Patterns
The three main fingerprint patterns: loops (~60–65 % of prints), whorls (~30–35 %), and arches (~5 %). Each person's ridge detail is unique.
AFIS
Automated Fingerprint Identification System — a computerised database that stores and rapidly compares fingerprint records for identification.
Latent Print
An invisible fingerprint left by natural skin oils and sweat on a surface, revealed through powdering, chemical treatment, or alternative light sources.
Forensic Pathology
A medical specialty focused on determining the cause and manner of death through autopsy and examination of body tissues.
Autopsy
A systematic post-mortem examination of a body to determine the cause, mechanism, and manner of death, and to collect evidence.
Manner of Death
The circumstances surrounding death, classified as natural, accident, suicide, homicide, or undetermined (sometimes abbreviated NASH-U).
Rigor Mortis
Post-mortem stiffening of muscles caused by the depletion of ATP and cross-linking of actin and myosin. Begins 2–6 hours after death and is fully established by approximately 12 hours.
Livor Mortis
The settling of blood in the lowest parts of the body due to gravity after circulation ceases. Visible within 1–2 hours and becomes fixed at approximately 8–12 hours.
Algor Mortis
The gradual cooling of the body after death toward ambient temperature. Typically estimated at roughly 1.5 °F (0.83 °C) per hour under standard conditions.
Forensic Toxicology
The study of drugs, poisons, and other toxic substances in biological specimens to determine their role in injury, illness, or death.
GC-MS (Gas Chromatography–Mass Spectrometry)
An analytical technique that separates chemical mixtures (GC) and identifies individual compounds by their mass spectra (MS). The gold standard for confirmatory drug testing.
Immunoassay
A rapid screening test that uses antibody–antigen reactions to detect the presence of drugs or other substances in biological samples. Used as a presumptive test before confirmatory analysis.
Forensic Entomology
The study of insects associated with decomposing remains. Insect species, life stages, and succession patterns are used to estimate the post-mortem interval.
PMI (Post-Mortem Interval)
The time elapsed since death. Estimated using a combination of body changes (rigor, livor, algor mortis), decomposition stage, and entomological evidence.
Insect Succession
The predictable sequence in which different insect species colonise decomposing remains over time, used to help determine PMI.
Blowfly (Calliphoridae)
Typically the first insects to arrive at remains, often within minutes. Their known developmental rates at specific temperatures make them valuable PMI indicators.
Forensic Botany
The application of plant science to legal investigations — using plant growth, pollen, or algae to link suspects to locations or estimate time frames.
Palynology
The study of pollen and spores. In forensics, pollen found on clothing or remains can link a person to a specific geographic location or environment.
Diatom Analysis
Examination of microscopic algae (diatoms) in tissues or water samples. Used in drowning investigations — diatoms in bone marrow or organs suggest ante-mortem submersion.
| Type | Examples | Collection Method |
|---|---|---|
| Physical | Fingerprints, fibres, glass, tool marks | Lifting tape, tweezers, casting |
| Biological | Blood, saliva, hair, skin cells | Swabbing, cutting, plucking |
| Chemical | Drugs, gunshot residue (GSR), accelerants | Sampling, swabbing |
| Digital | Phone records, CCTV, computer files | Imaging, extraction |
| Feature | DNA Profiling | Fingerprinting |
|---|---|---|
| Evidence Source | Blood, saliva, hair, skin cells | Ridge patterns on fingers |
| Uniqueness | Unique to each individual (except identical twins) | Unique to each individual, including identical twins |
| Database | CODIS (Combined DNA Index System) | AFIS (Automated Fingerprint Identification System) |
| Technology | PCR amplification, STR analysis, electrophoresis | Powder dusting, chemical fuming, digital scanning |
| Sample Size | Can work with very small or degraded samples | Requires a visible or recoverable print |
| Family Linking | Can identify biological relatives via partial matches | No familial linking capability |
| Indicator | What It Measures | Timeline |
|---|---|---|
| Algor mortis | Body cooling toward ambient temperature | Begins immediately after death |
| Livor mortis | Blood pooling in lowest body parts | Visible 1–2 hrs, fixed 8–12 hrs |
| Rigor mortis | Muscle stiffening from ATP depletion | Begins 2–6 hrs, full by ~12 hrs |
| Insect activity | Colonisation and succession patterns | Minutes to months |
Q1: Explain Locard's Exchange Principle and its significance in forensic science.
Locard's Exchange Principle states that whenever two objects come into contact, a mutual transfer of material occurs. In forensic science this means that a perpetrator will inevitably leave traces at a crime scene (e.g. fibres, hair, skin cells, fingerprints) and carry traces away. This principle underpins the entire discipline of trace evidence analysis and justifies the meticulous collection and preservation of even microscopic materials from crime scenes.
Q2: Describe the steps involved in DNA profiling.
DNA profiling involves several key steps: (1) Sample collection — biological material such as blood, saliva, or hair is gathered from the crime scene. (2) DNA extraction — DNA is isolated from the cells. (3) PCR amplification — the polymerase chain reaction copies specific STR loci millions of times, even from minute samples. (4) Electrophoresis — the amplified fragments are separated by size on a gel or through capillary electrophoresis. (5) Analysis and comparison — the resulting profile is compared against suspect samples or searched in databases like CODIS to find matches.
Q3: What are the three types of fingerprint patterns?
The three main fingerprint patterns are loops, whorls, and arches. Loops are the most common (~60–65 % of fingerprints) and feature ridges that enter from one side, curve around, and exit from the same side. Whorls (~30–35 %) have circular or spiral ridge patterns. Arches (~5 %) have ridges that enter from one side and flow out the other in a wave-like pattern. Despite falling into these three broad categories, every individual's fingerprint has unique minutiae (ridge endings, bifurcations) that allow positive identification.
Q4: How is time of death estimated using forensic indicators?
Time of death is estimated using multiple indicators: Algor mortis measures the rate of body cooling toward ambient temperature (roughly 1.5 °F / 0.83 °C per hour under standard conditions). Livor mortis tracks blood pooling, becoming visible within 1–2 hours and fixed at 8–12 hours. Rigor mortis (muscle stiffening) begins at 2–6 hours and is fully established by approximately 12 hours. Entomological evidence — the species, life stage, and succession of insects colonising the remains — can estimate intervals from hours to months. Investigators use these indicators in combination, as environmental factors (temperature, humidity, clothing) affect each one.
Q5: What is the role of forensic entomology in criminal investigations?
Forensic entomology uses the study of insects — particularly their predictable patterns of colonising decomposing remains — to aid criminal investigations. Blowflies (family Calliphoridae) are typically the first to arrive, often within minutes of death. Because their developmental stages (egg, larva, pupa, adult) proceed at temperature-dependent rates, entomologists can calculate the minimum post-mortem interval (PMI). Insect succession — the sequence of different species arriving over days, weeks, and months — extends this timeline further. Entomological evidence can also indicate whether a body has been moved, if toxins are present (through analysis of larvae), or the season of death.
Master the glossary first. Forensic science exams rely heavily on precise terminology. Use the key terms above as flashcards — cover the definition and try to recall it from the term alone.
Learn the DNA profiling workflow end to end. Sketch the steps from sample collection through CODIS matching. Being able to diagram the entire process is one of the most common exam requirements. Review the DNA Profiling chapter for details.
Compare and contrast evidence types. Practice describing the differences between physical, biological, chemical, and digital evidence — including how each is collected and analysed. Tables like the ones above are excellent revision aids.
Practise time-of-death calculations. Given a body temperature and ambient temperature, estimate time of death using algor mortis. Then cross-reference with rigor mortis and livor mortis stages. Check the Forensic Pathology chapter for worked examples.
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