Biological Evidence - Hair

Hair as crime scene evidence

usefulness in forensic science is limited

it is not possible to determine if a hair originated from a particular person on the bases of morphology - colour + structure

hair has a poor discriminatory power

if DNA is obtained from cells attached to the hair root, it becomes valuable evidence

Hair transfer is limited

people shed hair continuously and sometimes hair transfers to another person or left at a scene

absence of hair matching a suspect does not mean contact did not occur

even in sexual assault cases where there is considerable contact, hair may not transfer

studies have shown hat transferred head hairs is found on the outer clothing o victims or suspects in 13% pf sexual assault cases

hairs matching suspects pubic hair or head hair found on the underwear of 3-4% of victims of sexual assaults

hair as evidence

if a balaclava was lest at a crime scene, a forensic scientist would try and obtain a DNA profile from kin cells or saliva from the inner surface

it is possible to distinguish different types of hair - scalp hair, pubic hair and beard hair also human vs animal hair

examination of the roots with a microscope can establish if hair fell out or was pulled

sample required for hair analysis

items normally searched for hair include clothing, balaclavas and motor cycle helmets

reference samples are taken from suspects for comparison purposes by combing the hair of a suspect until a representative sample of hair - pulled hair is obtained too

only a medical examiner should take reference samples of public hair

3 layers of the hair shaft

cuticle

cortex

medulla

cuticle

outside covering of hair

hair is resistant to chemical decomposition and retains its structural features over a long period of time due to the cuticle

made of scales that point towards the tip of hair

scales form from cells that have hardened or keratinised and flattened in progressing from the follicle of the skin

variety of scale patterns seen by scanning electro microscopy SEM used in species identification

cortex

layer of cells within the cuticle

contains pigment granules which give hair its colour

colour, shape and distribution of granules used in comparing hair from different individuals

examined microscopically in liquid medium to allow maximum light penetrate hair

medulla

collection of cells that looks like a central canal running through a hair

diameter in animal cells, the medulla is more than half the hairs diameter

for humans, the diameter is less than a third

in human head hair, the medulla is fragmented or absent

hair growth phases

anagen

catagen

telogen

anagen phase

initial growth phase when hair follicle produced hair - root bulb has flame shaped appearance

may last up to six years

when pulled from the root, some hairs have a follicular tag from which DNA can be extracted

follicular tag

a translucent piece of tissue surrounding the hair’s shaft near the root

catagen phase

hair growing at decreasing rate

2-3 weeks root bulb shrinks and is elongated as it is being pushed out of the follicle

telogen phase

hair growth has ended and root takes on club shaped appearance

2 to 6 months hair pushed out of follicle and sheds

human vs human hair comparison

look for matches in - colour, length and diameter

presence or absence of medulla and distribution

shape and colour intensity of the pigment granules of cortex

dyed colour is present in cuticle as well as cortex

Hair DNA analysis

successful by extracting DNA from the follicular tag/scalp cells in anagen phase

naturally shed hair in telogen phase are poor for DNA typing - shrivelled root with no adhering cells

forcibly pulled out hair collected often yield DNA profile

Hair DNA analysis

mitochondrial DNA analysis of hair shaft can be used where nuclear/genomic DNA is limited / no follicular tag

less discriminatory than genomic DNA typing as it can’t individualise hair on this basis but could exclude many individuals based on mitochondrial profile

collection and preservation of hair evidence

hair samples from crime scene must always be accompanied by standard reference samples from victim and suspects

approx 50 full length hairs from head or 24 from pubic area gives representative sample covering range of characteristics in particular area

hair samples always taken routinely in autopsy where suspicious death has occurred and retained a reference samples that may be required later during investigation

high profile case involving hair analysis

central park jogger - microscopic hair analysis used to wrongly convict five teenagers