Trace Evidence l: Hairs and Fibers

Chapter 10
Trace Evidence l:
Hairs and Fibers
Introduction
• Hair
– encountered as physical evidence
– wide variety of crimes
– not yet possible to individualize a human hair to any
single head or body through its morphology
• Strong corroborative evidence
– placing an individual at a crime scene
– properly collected and submitted to the lab
– accompanied with adequate number of reference
samples
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Morphology of Hair
• Hair
– appendage of the skin that grows out of the
hair follicle
– length of hair extends from its root (bulb
embedded in the follicle), into a shaft, and
terminates at a tip (end)
• Shaft
– three layers—the cuticle, cortex, and medulla
– subjected to the most intense examination
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Anatomy of Single Strand of Hair
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• Cuticle
Cuticle and Cortex
– scale structure covering the exterior of the hair
• always point towards the tip of the hair
• pattern is useful in species identification
• Cortex
– main body of the hair shaft
– embedded with pigment granules that give hair
color
– The color, shape, and distribution of these
granules provide points of comparison among the
hairs of different individuals
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Cuticle Scale Patterns
Cat/ Fox
Rabbit/
Mouse
Humans
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Medulla
• Cellular column running through the center of the
hair
– Medullary index = diameter of medulla / diameter of
hair shaft
– For humans, the medulla generally occupies < 1/3 the
diameter of the shaft, while for animals it is >1/2.
– continuous, interrupted, fragmented or absent
– The presence varies from individuals and even among
hairs of a given individual.
– different shapes, depending the species
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Medulla Patterns in Humans
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Animal Medulla Patterns
Dog
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Cow
Rabbit
Deer
FoxMouse
Raccoon
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Root
• Provides the tools necessary to
produce hair and continue its growth
• Follicular tag
– translucent tissue surrounding the hair’s
shaft near the root when forcibly
removed
– Using DNA analysis, the hair may be
individualized
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Comparing Strands
• Use the comparison microscope
• Matching the color, length, and diameter
• Microscopic examination will reveal
morphological features that distinguish
human and animal hair
• Important for identification
– Scale structure
– Medullary index
– Medullary shape
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Comparing Strands
• Other important features for comparing human hair
are:
– the presence or absence of a medulla.
– the distribution, shape, and color intensity of the pigment
granules present in the cortex.
• Most common request  determine if hair
recovered at crime scene matches hair removed
from suspect
• Microscopic hair examinations tend to be subjective
and highly dependent on the skills and integrity of
the analyst.
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Questions
• Can the body area from which a hair originated be
determined?
• Can the racial origin of hair be determined?
• Can the age and sex of an individual be determined
from a hair sample?
• Is it possible to determine if a hair was forcibly
removed from the body?
• Are efforts being made to individualize human hair?
• Can DNA individualize a human hair?
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Mongoloid (Asian Descent)
Hairs will
– be very coarse
– contain larger pigment granules than Caucasoid hair
– contain a thick medulla
– be straight
– have completely round
cross section
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Caucasoid (European Descent)
• Hairs will
– be fine to slightly coarse
– range in color from very light to dark
– be generally straight or with slight waves
– have slightly oval shaped cross section
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Negroid (African Descent)
• Hairs will
– be medium coarse
– contain the largest pigment granules
– Have flatter cross section
– often have split ends
– be kinky or curly
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Hair and DNA
• DNA profiling has extended to the individualization
of human hair
• Hair in the anagen phase has a higher probability of
containing DNA in hair roots than hair in the catagen
or telogen phases
• The follicular tag may still be present if hair was
forcibly removed
– This has proven to be a rich source of nuclear DNA
associated with hair.
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Phases of Hair Growth
Anagen
• Actively growing
• 85% of hair
• Forcibly removed
• Generally retains
follicular tag
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Telogen
• Hair stopped growing
• 12-15% of hair
• Hair may readily fall out
• Does not retain follicular tag
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Hair and Mitochondrial DNA
• Mitochondrial DNA can be extracted from the
hair shaft.
– Mitochondrial DNA is found in cellular material
located outside of the nucleus and it is
transmitted only from the mother to child.
• All positive microscopical hair comparisons
must be confirmed by DNA analysis.
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Collection and Preservation
• Forensic hair comparisons involve either head hair or
pubic hair.
• 50 full-length hairs from all areas of the scalp will
ensure a representative sampling of head hair
• 24+ full-length pubic hairs should cover the range of
characteristics present in pubic hair.
• Hair samples are also collected from the victim of
suspicious deaths during an autopsy.
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Types of Fibers
• Natural fibers are derived in whole from animal or
plant sources.
– Plants – seeds, fruits, stems, leaf
•
•
•
•
Seeds – cotton
Fruits – coir
Stems – hemp, jute, flax (linen)
Leaf – manila sisal
– Animals – wool, mohair, cashmere, angora, furs, silk
– Minerals – asbestos, fiberglass
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Types of Fibers
• Man-made fibers are manufactured.
– Regenerated fibers are manufactured from natural raw
materials
– Ex: rayon, acetate, triacetate
– Synthetic fibers are produced solely from synthetic
chemicals
– Ex: nylons, polyesters, acrylics, olefins
• Polymers are synthetic fibers composed of a large
number of atoms arranged in repeating units
known as monomers.
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Fiber Evidence
• Class evidence
• The quality of fiber evidence depends on
– ability to identify the origin of the fiber
– at least narrow the possibilities to a limited
number of sources
• Individual evidence
– If the examiner is presented with fabrics that
can be exactly fitted together at torn edges
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Fiber Evidence
• Microscopic comparisons between questioned
and standard/reference fibers
– initially for color and diameter characteristics
– weave pattern
• Other features in comparing fibers are:
– Lengthwise striations on the surface of the fiber
– Delustering particles that reduce shine
– The cross-sectional shape of the fiber
• Compositional differences may exist in the dyes
applied during manufacturing process
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Methods for Fiber Comparison
• The visible light microspectrophotometer
– convenient way to compare the colors of fibers through
spectral patterns.
• Chromatographic separation
– more detailed analysis of the fiber’s dye composition
• Infrared spectrophotometry
– rapid and reliable method for identifying the generic class
of fibers
• Polarizing microscope.
– rapid and reliable method for identifying the generic class
of fibers
– Depending on the class of fiber, each will have a
characteristic index of refraction
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Collection and Preservation
• The investigator’s task of looking for minute
strands of fibers often becomes one of identifying
and preserving potential “carriers” of fiber
evidence.
• Relevant articles of clothing should be packaged
carefully in separate paper bags.
• If it is necessary to remove a fiber from an object,
the investigator must use clean forceps, place it in
a small sheet of paper, fold and label the paper,
and place the paper packet inside another
container.
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
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Silk
• Comes from the silk worm’s cocoon
•
http://www.micktravels.com/china/silk.html
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