Forensic science Study Guide

📖 Core Concepts Forensic Science – Application of scientific methods to support legal decisions in criminal and civil cases. Locard’s Exchange Principle – “Every contact leaves a trace”; the basis for collecting trace evidence. DNA Profiling – Uses individual genetic variation to identify persons; pioneered by Alec Jeffreys (1984). Fingerprint Uniqueness – Traditional assumption that no two fingerprints are alike (false‑positive 1 in 64 billion per Galton). Chain of Custody – Documented, unbroken handling of evidence from collection to courtroom to guarantee admissibility. Admissibility Standards – Evidence must meet legal thresholds (e.g., Daubert/Rule 702, Melendez‑Diaz requirement for expert testimony). 📌 Must Remember Locard’s Principle (1910) → Every interaction transfers material → search for trace evidence. DNA Match Probability – Typically expressed as 1 in several million to billions; depends on loci used. Fingerprint False‑Positive – 1 in 64 billion (Galton). Key Legal Cases – Melendez‑Diaz v. Massachusetts (2009): expert testimony must be based on generally accepted science. Wrongful Conviction Stats – 39‑46 % involve forensic errors (Innocence Project). Discredited Techniques – Comparative bullet‑lead analysis, bite‑mark analysis, unvalidated genetic genealogy use. 🔄 Key Processes Crime‑Scene Evidence Collection Secure scene → document → photograph → create 3‑D point cloud (laser scanner/drones) → collect samples with proper packaging. DNA Profiling Workflow Extract DNA → amplify loci (STRs) → electrophoresis → compare profile to database → calculate match probability. Fingerprint Development & Comparison Locate latent prints → enhance (powder, ninhydrin, lasers) → digitize → compare minutiae patterns to known prints using classification systems (e.g., Henry). Genetic Genealogy Investigation Upload crime‑scene DNA to public database → identify close relatives via shared DNA segments → narrow suspect pool → obtain direct sample for confirmation. Digital Forensics Extraction Seize device → create forensic image (hash verification) → analyze file system, logs, communications, SIM data. 🔍 Key Comparisons DNA Profiling vs. STR Typing – DNA profiling is the overall concept; STR (short tandem repeat) typing is the common laboratory method used for profiling. Fingerprint Dactyloscopy vs. Document Examination – Dactyloscopy compares latent prints to known prints; document examination compares whole documents (ink, paper, typeface). Bite‑Mark Analysis vs. Dental DNA Analysis – Bite‑mark relies on pattern matching (discredited); dental DNA uses genetic material from teeth (scientifically validated). Traditional Ballistics vs. 3‑D Crime‑Scene Scanning – Traditional examines physical markings on bullets; 3‑D scanning captures scene geometry for trajectory reconstruction. ⚠️ Common Misunderstandings “All fingerprints are unique” – While highly distinctive, statistical validation is still debated; over‑reliance can mask error potential. “DNA always gives a definitive match” – Matches are probabilistic; low‑quantity or degraded samples can yield partial profiles or mixed DNA. “Forensic evidence is always admissible” – Must satisfy legal standards (relevance, reliability, proper methodology). “Genetic genealogy is unrestricted” – Privacy concerns and varying legal rulings limit its use; not all jurisdictions permit it. 🧠 Mental Models / Intuition Trace‑Transfer Model – Imagine two hands shaking; each leaves microscopic skin cells on the other. Any contact leaves a “handshake” of material you can later detect. Probability Lens – Treat DNA/fingerprint matches like weather forecasts: a 99.999% probability is strong, but not absolute certainty. Chain‑Link Analogy – Evidence custody is a chain; a weak link (missing documentation) can break admissibility. 🚩 Exceptions & Edge Cases Low‑Copy DNA Samples – May require mitochondrial DNA analysis or next‑generation sequencing; higher risk of contamination. Partial Fingerprints – May be sufficient for a “supporting” identification but not a definitive match. Mixed DNA Profiles – Require statistical software (e.g., likelihood ratios) to interpret contributors. Legal Exceptions – Certain “exigent” circumstances can bypass standard chain‑of‑custody rules (e.g., imminent danger). 📍 When to Use Which Identify a Person → Use DNA profiling (high discriminative power) or fingerprints (rapid field comparison) depending on sample availability. Link Weapon to Crime → Apply ballistic comparison when recovered projectiles exist; otherwise, use gunshot residue testing. Investigate Cyber Incident → Deploy digital forensics (mobile device > computer) based on the primary media involved. Assess Time Since Death → Choose forensic entomology (insect succession) for later stages; forensic pathology (rigor mortis, livor mortis) for early stages. 👀 Patterns to Recognize Bloodstain Shapes – High‑velocity spatter (fine mist) → gunfire; low‑velocity (large drops) → blunt force. DNA Mixture Ratios – Peaks of similar height often indicate equal contributors; one dominant peak suggests a major contributor. Fingerprint Ridge Patterns – Loops, whorls, arches → guide classification and comparison. Digital Artifact Timeline – File creation → modification → access timestamps often reveal user activity sequence. 🗂️ Exam Traps “All DNA evidence is conclusive” – Distractor; ignore the probabilistic nature and potential for contamination. Choosing “Bite‑Mark” as a reliable method – Wrong; the technique is widely discredited. Assuming any trace evidence meets Locard’s principle automatically – Trap; must demonstrate transfer and relevance. Mixing up “ballistics” with “trajectory analysis” – Ballistics examines markings; trajectory analysis reconstructs bullet path (different skill sets). Over‑valuing a single fingerprint match without statistical support – Exams may present a match without citing false‑positive rates; remember to consider the statistical backing.