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.