Forensics - Hair, Fibers, and Biometrics

How to calculate MI

Medullary diameter divided by hair shaft diameter

How to use MI to determine if a hair is human or not

MI < 1/3 ➡ human

MI > 1/3 ➡ Non-human hair

Types of medulla

Continuous (unbroken line), fragmented (uneven segments), interrupted (regularly spaced), absent (no medulla)

Types of cuticle

1. Coronal (Crown-like) “Scales look like stacked cups or crowns”

• Found in small rodents and bats

2. Spinous (Petal-like) “Scales are pointed and resemble flower petals”

• Found in cats, seals, and minks

3. Imbricate (Flattened and Overlapping) “Scales are flat and lie close together”

• Common in human hair and some animal hairs

Which type hair can you test for DNA

You can test the root for nuclear DNA or the hair shaft for mitochondrial DNA (mDNA)

4 stages of hair growth

Anagen - The growth phase, when hair grows from the hair follicle  

Catagen - The transition phase, when hair follicles shrink and hair growth slows

Telogen - The resting phase, when hair is no longer growing

Exogen - The shedding phase, when old hair is shed and usually replaced with new hair

What is the significance of hair that is forcibly removed to an investigation

If hair is forcibly removed, it may contain follicular tissue, which provides nuclear DNA for identification. Helps determine if a struggle or violent crime occurred.

Three Main Types of Fibers

• Natural - From plants or animals

• Regenerated - Chemically altered natural materials (e.g., rayon from cellulose)

• Synthetic - Made from petroleum-based chemicals (e.g., nylon, polyester)

Know your sources

• Cotton - Derived from the seed fibers of the cotton plant

• Silk - Produced by the silkworm as it spins its cocoon

• Asbestos - A mineral fiber found naturally in the earth

• Angora - Comes from the Angora rabbit’s fur

• Wool - Obtained from the hair of sheep

• Cashmere - Comes from the undercoat of Cashmere goats

Know the monomers

Proteins ➡ Amino acids

Starches (Carbohydrates) ➡ Monosaccharides (e.g., glucose)

Nucleic Acids (DNA, RNA) ➡ Nucleotides

Hair and fibers evidence

Class Evidence - Hair and fibers alone cannot identify a specific person, but they can narrow down a group (e.g., type of hair, type of fiber)

Individual Evidence - If the hair root is attached, nuclear DNA can make it individual evidence

What are biometrics

A way to identify people using their physical or behavioral characteristics

Requirements for biometrics

• Universality - Every person must have the trait

• Uniqueness - The trait must be different for every person

• Permanence - The trait should remain unchanged over time

• Measurability - It must be possible to measure and record the trait

• Ease of Use - Must be easy, quick, and reliable

Retina

The layer at the back of the eye with blood vessels. Unique but rarely used in biometrics

Iris

Controls the amount of light allowed in the eye and is responsible for eye color.

Commonly used in biometrics because they are easier to capture and dont change over time