Forensic Science Notes

Inheritance of DNA Characteristics

  • DNA is located in every nucleated cell: skin cells, spermatozoa, white blood cells.

Bloodstain Pattern Analysis

  • Bloodstain patterns on surfaces offer insights into violent crimes.
  • Location, distribution, and appearance of bloodstains help reconstruct events.
  • Interpretation reveals:
    • Direction of blood origin
    • Angle of impact
    • Victim's position during the bloody event

Impact Bloodstain Spatter Patterns

  • Impact spatter: Object impacts a blood source.
  • Forward spatter: Blood projected outward and away from the source.
  • Back spatter (blow-back): Blood projected backward from the source.

Classifying Impact Spatter by Velocity

  • Droplet size classifies impact patterns, giving general crime nature insight.
    • Low Velocity Spatter: Drops > 4 mm, force up to 5 ft/sec.
    • Medium Velocity Spatter: Drops 1-4 mm, force 5-25 ft/sec.
    • High Velocity Spatter: Drops <1< 1 mm, force >100> 100 ft/sec.
  • Classifications are descriptive only and don't illuminate specific events.

Area of Convergence and Origin

  • Area of convergence: 2D point of origin of impact pattern drops. Determined by drawing lines through long axes of bloodstains.
  • Area of origin: 3D space of blood projection, indicating victim/suspect position during the event. String method approximates this using impact angles.

Specific Spatter Types

  • Gunshot Spatter: Fine forward spatter from exit wound, back spatter from entrance wound. No exit = only back spatter.
  • Cast-off Spatter: Blood-covered object flings blood onto nearby surface, common with bloody fists/weapons. Pattern affected by object size, blood amount, direction. Forward/backward patterns indicate minimum blows delivered.
  • Void Patterns: Object blocks blood spatter deposition, revealing size/shape of missing object/person. Useful for establishing body position.
  • Drop Trail Patterns: Series of drops separate from other patterns, forming a line made by suspect after injury.

Documenting Bloodstain Evidence

  • Note, study, and photograph each pattern/drop to record location and distinguish lab samples.

DNA in Non-criminal Cases

  • Used in paternity, probate, immigration cases, and victim identification in mass disasters (e.g., September 11, Hurricane Katrina).

Individualizing Bloodstains

  • DNA analysis associates blood to a single individual.

CODIS

  • CODIS (Combined DNA Index System): FBI software with local, state, national DNA databases of offenders, crime scene evidence, and missing persons. Contains ~470,000 unsolved case profiles.

Collection of DNA Evidence

  • Sources: Blood, semen, saliva, skin cells, hair, bone.
  • Found on bottles, cans, glasses, cigarettes, bite marks, envelopes.

Blood Typing

  • Population distribution: 43% type O, 42% type A, 12% type B, 3% type AB.
  • Type A: anti-B, no anti-A. Type B: anti-A, no anti-B. Type AB: neither anti-A nor anti-B. Type O: both anti-A and anti-B.
  • A-B-O and Rh systems are most important. Type A has A antigens, type B has B antigens, type AB has both, type O has neither.

Blood Origin Testing

  • Precipitin test determines human vs. animal origin using antisera from rabbits injected with known animal blood.
  • Gel diffusion: Antibodies/antigens diffuse on agar plate. Bloodstain and human antiserum placed in separate holes. Precipitation line forms if blood is human.

Packaging Biological Evidence

  • Each stained article packaged separately in paper bag or ventilated box. Refrigerate/cool storage until lab delivery.

Glass Fragment Analysis

  • Compare density and refractive index.
  • Radial fractures radiate outward, concentric fractures encircle the hole. Fracture patterns determine impact direction.

Flotation Method

  • Precise method for comparing glass densities by immersing glass particle in liquid.

Collection of Glass

  • Package fragments in solid containers to prevent breakage. Wrap suspect's shoes/clothing in paper for lab analysis.

Comparison of Soil

  • Forensic geologists compare mineral content, considering variety of minerals, rocks, and artificial debris for comparison points.

Analyzing Cracks

  • Sequence of impacts determined by fracture termination at existing fracture lines.

Morphology of Hair

  • Hair grows from follicle, extending from root/bulb to shaft to tip end.
  • Shaft layers: cuticle, cortex, medulla - intensely examined.

Root

  • Follicle produces hair and continues growth in anagen, catagen, and telogen phases.
  • Follicular tag: Translucent tissue around hair shaft near root. DNA analysis can individualize hair.

Methods for Paint Comparison

  • Pyrolysis gas chromatography and infrared spectrophotometry distinguish paint binder formulations.
  • Crime labs identify car make/model using color charts or PDQ database.

Paint Collection and Preservation

  • Paint chips found on persons/objects in hit-and-run incidents. Collect uncontaminated reference paint from undamaged area near damage.

Glass Composition

  • Laminated glass in windshields has plastic layer between two glass pieces.

Paint

  • Automobile coatings: electrocoat primer, primer surfacer, basecoat, clearcoat.

Types of Fibers

  • Natural fibers: animal/plant sources (wool, mohair, cashmere, furs, cotton).
  • Man-made fibers: regenerated (rayon, acetate, triacetate) or synthetic (nylons, polyesters, acrylics).
  • Synthetic fibers: polymers (macromolecules) of repeating monomer units.

Fiber Evidence

  • Microscopic comparison of color and diameter using comparison microscope.

Fire Triangle

  • Fire requires fuel, oxygen, and heat.

Hair and DNA

  • DNA profiling individualizes human hair.
  • DNA detection more likely in anagen phase.
  • Follicular tag is a rich source of nuclear DNA.

Comparing Strands

  • Presence/absence of medulla, pigment granule distribution, shape, and color intensity.

High Explosives

  • Nitroglycerin-based dynamite replaced by ammonium nitrate-based explosives.
  • Primary explosives: ultra-sensitive to heat, shock, or friction, found in blasting caps/primers.

Low Explosives

  • Decompose by deflagration, causing container fragmentation. Black powder and smokeless powder are common.
  • Black powder: potassium/sodium nitrate, charcoal, sulfur. Smokeless powder: nitrated cotton or nitroglycerin and nitrocellulose.

Laboratory Recovery of Flammable Residues

  • Heat airtight container to trap volatile residues in airspace.

Collection of Fire Scene Evidence

  • Vapor detector (sniffer) or trained canine locates flammable liquid residues. Collect uncontaminated control specimens (substrate control).

Indicators of Arson

  • Fire origin usually at lowest point with most intense burning.

DNA in Forensic Analysis

  • Short Tandem Repeat (STR): FBI began using STRs in 1998, still in use today.
  • STR testing uses a multiplex PCR process to test multiple genetic markers at the same time as well as to determine gender.
  • Focused on short repeating sequences within DNA rather than the entire genome, as in RFLP.
  • Automated, color-coded, rapid, computerized

Short Tandem Repeats (STRs)

  • STR analysis is the most successful and widely used DNA profiling procedure.
  • STRs are locations on the chromosome that contain short tandem repeats that repeat themselves within the DNA molecule.

The Fire Scene

  • Focus search on fire's origin for accelerant or ignition device.

Introduction

  • Criminalist detects and identifies chemical materials, reconstructs igniter mechanisms. Fire investigator determines cause of fire.

FORENSIC VIDEO ANALYSIS

  • Examination of images and sound captured by video camera and recorded to videotape or to a computer hard drive or to other digital media.

Methods for Fiber Comparison

  • Infrared spectrophotometry identifies generic fiber class. Molecules in fiber absorb infrared light to form characteristic pattern.