Forensics Practice Tests

evidence analysis, toxicology, DNA analysis, document examination, and crime scene procedures.

Fingerprints

• Patterns: Loop (60-65%, most common), Whorl (30-35%), Arch (5%, rarest)
• Loop types: Ulnar (opens toward pinky), Radial (opens toward thumb)
• Whorl types: Plain, Central pocket, Double loop, Accidental
• Arch types: Plain, Tented
• Minutiae: Ridge endings, bifurcations (splits), dots - used for matching
• Latent prints: Invisible, need powder/chemicals to develop
• Patent prints: Visible, made in blood/ink/paint
• Plastic prints: 3D impressions in soft material
• AFIS: Automated Fingerprint Identification System

Blood evidence

• Blood types: A, B, AB (universal recipient), O (universal donor)
• Rh factor: + or - (Rh+ can receive Rh-, Rh- cannot receive Rh+)
• Antigens: A has A antigens, B has B antigens, AB has both, O has neither
• Antibodies: A has anti-B, B has anti-A, AB has neither, O has both
• Presumptive tests: Kastle-Meyer (pink), Luminol (glows), detect possible blood
• Confirmatory tests: Precipitin test confirms human blood
• Blood spatter:
• High velocity (gunshot): Fine mist, <1mm droplets
• Medium velocity (beating): 1-4mm droplets
• Low velocity (dripping): >4mm droplets
• Direction: Tail points in direction of travel
• Angle of impact: Width/Length = sin(angle)
• Point of origin: Trace trajectories back using strings

Time of death

• Algor mortis: Body cooling, ~1.5°F (0.8°C) per hour until ambient
• Formula: Hours since death ≈ (98.6 - body temp) / 1.5
• Livor mortis (lividity): Blood pooling in lowest areas
• Begins immediately, becomes fixed after 8-12 hours
• If moved after fixation, shows two patterns
• Rigor mortis: Muscle stiffening from ATP depletion
• Begins 2-4 hours, complete at 6-12 hours, peaks at 12 hours
• Dissipates 24-48 hours (in same order: jaw, neck, arms, legs)
• Decomposition stages: Fresh → Bloat → Active decay → Advanced decay → Dry
• Entomology: Insect succession helps estimate time of death

Dna analysis

• Nuclear DNA: Inherited from both parents, unique (except identical twins)
• Mitochondrial DNA (mtDNA): Inherited from mother only, useful for degraded/hair samples
• STR (Short Tandem Repeats): 13+ loci analyzed, basis of DNA fingerprinting
• PCR (Polymerase Chain Reaction): Amplifies small DNA samples
• CODIS: Combined DNA Index System (FBI database)
• Gel electrophoresis: Separates DNA fragments by size

Hair and fiber

• Hair structure: Cuticle (outer), Cortex (middle, pigment), Medulla (inner core)
• Medulla patterns: Absent, Fragmented, Continuous, Stacked
• Hair can show: Species, body location, race, treatments, drugs
• Natural fibers: Cotton, wool, silk (from plants/animals)
• Synthetic fibers: Polyester, nylon, rayon (manufactured)
• Cross-section, color, diameter used for comparison

Toxicology

• LD50: Dose lethal to 50% of test population
• Drugs metabolized mainly by liver
• Detection: Blood (recent), Urine (days), Hair (months)
• Common poisons: Cyanide (bitter almonds), Arsenic (garlic), Carbon monoxide (cherry red)
• Blood alcohol: 0.08% legal limit, 0.30%+ potentially fatal

Document examination

• Handwriting analysis: 12+ characteristics compared
• Ink analysis: Chromatography separates components
• Paper analysis: Watermarks, fiber content
• Indented writing: ESDA (electrostatic detection) reveals impressions

Crime scene

• First responder: Secure scene, render aid, call for backup
• Chain of custody: Document every person who handles evidence
• Evidence collection: Photograph, sketch, collect, package, label
• Locard's Exchange Principle: "Every contact leaves a trace"

Common mistakes

• Confusing fingerprint pattern types
• Getting blood type compatibility wrong (who can receive from whom)
• Mixing up the three mortis types and their timing
• Forgetting that mtDNA is maternal only