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Trace Evidence I: Hair and Fibers

Trace Evidence Overview

  • Definition: Trace evidence is any evidence that can be collected in small amounts and undergoes laboratory analysis.
  • Common Types:
    • Hairs
    • Fibers
    • Paints
    • Glass
    • Soil

Hair Evidence

  • Significance: Hair can serve as physical evidence in various crimes, providing corroborative evidence even though individualization is not always possible. It can link a suspect to a crime scene or victim.
  • Collection Importance: Proper collection and submission of hair samples, alongside reference samples from known individuals, enhances evidential strength and increases reliability in forensic analysis.

Morphology of Hair

  • Structure:
    • Composed of three layers:
    1. Cuticle: This outer layer has a scaled structure that points toward the tip, important for distinguishing between species and identifying the origin of the hair.
    2. Cortex: The middle layer contains pigment granules that give hair its color and provide the primary evidence for forensic hair analysis.
    3. Medulla: The central canal that varies in presence and characteristics between species; it may impact species identification in forensic cases.
  • Hair Follicle Structure:
    • Extends from the epidermis, includes various structures necessary for hair growth, such as the bulb where the hair originates, and associated glands.
Cuticle
  • Features:
    • Provides species differentiation rather than individualization; uniquely structured for animals versus humans, aiding forensic analysis in determining the source of hair samples.
Cortex
  • Key Points:
    • Main body of the hair, responsible for color due to embedded pigment granules.
    • Color is determined by melanin levels; absence of melanin results in gray or white hair, while varying concentrations yield different shades.
Medulla
  • Characteristics:
    • Ratio of medulla diameter to hair shaft is known as the medullary index.
    • Typically larger in animals (over 50% of diameter) compared to humans (less than 1/3), providing a key measurement in forensic comparisons.
    • Variability in presence; often absent or fragmented in human hair, complicating identification.

Hair Phases of Growth

  1. Anagen Phase: Active growth; characterized by a flame-shaped bulb appearance which is most likely to yield DNA evidence.
  2. Catagen Phase: Transitional phase; hair undergoes changes in shape and growth ceases; elongated root appearance.
  3. Telogen Phase: Resting phase; hair is at its final stage before shedding with a club-shaped root appearance.

Hair Comparisons

  • Analysis Criteria:
    • Comparison of length and diameter of hair strands.
    • Examination of scale structure and characteristics of pigment granules (shape, color).
    • Assessment of the medullary index and pattern as they relate to species differentiation.
  • Challenges:
    • Comparisons are subjective and need careful analysis; many distinctions exist within individual hair characteristics, and the reliability of match conclusions can vary.

DNA Analysis in Hair

  • Nuclear DNA: Most effective from anagen phase hair with follicular tags as it can yield profiles necessary for standard identification processes.
  • Mitochondrial DNA: Found in hair shaft, primarily matters for maternal lineage, and is typically less individualized than nuclear DNA, offering a broader yet less targeted familial connection.

Hair Evidence Collection

  • Collection Protocol:
    • Collect multiple full-length head and pubic hairs when possible to maximize the chances of successful analysis.
    • Samples from deceased individuals during autopsies are also necessary to ensure potential evidence is preserved and analyzed.

Fiber Evidence

  • Types of Fibers:
    • Natural Fibers: Sourced from plants (cotton, flax) or animals (wool, silk).
    • Synthetic Fibers: Include plastics and textiles made from chemicals (nylon, polyester), often contaminating crime scenes and providing forensic links.
Fiber Comparisons
  • Comparison Techniques:
    • Use a comparison microscope for assessing color, diameter, and other physical properties to establish matches.
    • Ultimate analysis requires matching morphological characteristics and confirming findings through chemical composition analysis.
Fiber Examination Tools
  • Microspectrophotometer: A key tool utilized for profiling fibers using various light spectrums; helps identify characteristics for comparison with known standards.

Evidence Handling

  • Collection Methods:
    • Use clean forceps for fiber removal, ensuring not to transfer outside contaminants during the collection process.
    • Package fibers carefully to prevent contamination and preserve their physical integrity during transport to analysis laboratories.

Importance of Analytical Techniques

  • Such analyses can connect fibers or hair to a suspect or scene, although it is crucial to confirm findings with robust methods such as DNA analysis to establish reliable connections in a forensic context.