8 th Grade Forensic Science Hairs & Fibers http://media.popularmechanics.com/images/PMX0706FORENSICSHairSmall.jpg Presentation developed by T. Trimpe 2006 http://sciencespot.net/
Hair Evidence The first step in the hair examination process is to determine whether the hair in question originated from an animal or a human . If the hair originated from an animal, it is possible to further identify it to a particular species of animal. It is not possible to identify hairs to a specific animal to the exclusion of other similar animals. An example of this occurs when dog hairs can be associated to a particular breed but cannot be identified to a specific dog within that breed. Hair is one of the most common forms of trace evidence The examination of human hairs in the forensic laboratory is conducted through the use of light microscopy . The examination involves the identification of questioned hairs and the comparison of questioned and known hairs. http://www.fbi.gov/hq/lab/fsc/backissu/july2000/deedric1.htm#Index%20(Hairs) A scientist using a comparison-light microscope View of matching hairs using a comparison-light microscope What characteristics help investigators identify hair samples?
Hair shape (round or oval) and texture (curly or straight) is influenced heavily by genes . Nutritional status and intentional alteration (heat curling, "perms") can affect the physical appearance of hair. Sources: http://library.thinkquest.org/04oct/00206/lesson.htm#t_hair & http://www.fbi.gov/hq/lab/fsc/backissu/july2000/deedric1.htm#Index%20(Hairs) Biology of Hair Hair is an outgrowth of the skin and is produced from a structure called the hair follicle . Humans develop hair follicles during fetal development, and no new follicles are produced after birth . Hair is composed of the protein keratin . Keratin is also the primary component of finger and toe nails . Hair color is mostly the result of pigments , which are chemical compounds which reflect certain wavelengths of visible light. Hair color may also be influenced by the optical effects of light reflecting and bouncing off the surfaces of the different hair. The body area from which a hair originated can be determined by the sample’s length, shape, size, color, stiffness, curliness, microscopic appearance, pigmentation, and the appearance of the medulla. Hairs that exhibit similar characteristics from different areas on the body are often referred to as body hairs and include hairs found on the upper legs, lower abdomen, and back.
Hair Structure Hair is composed of three principal parts: The structure of hair has been compared to that of a pencil with the medulla being the lead , the cortex being the wood and the cuticle being the paint on the outside. http://library.thinkquest.org/04oct/00206/lesson.htm#t_hair Cuticle – outer coating composed of overlapping scales Cortex – protein-rich structure around the medulla that contains pigment Medulla – central core (may be absent)
The cortex is perhaps the most important component in determining from which individual a human hair may have come . Microscopic examination can also reveal the condition and shape of the root and tip .
Continuity - one continuous structure or broken into pieces
Opacity - how much light is able to pass through it
It may also be absent in some species.
Like the cuticle, the medulla can be important for distinguishing between hairs of different species , but often does not lend much important information to the differentiation between hairs from different people .
Hair is one of the most common forms of trace evidence, but is not the best type of physical evidence for establishing identity. It is not possible to show with any certainty that two hairs came from the same person or animal.
However, hair can be used to rule out certain suspects or scenarios. It can also be used to corroborate (support) other physical evidence if it is consistent with the rest of the evidence.
A strand of hair has three distinctive parts: cuticle, cortex, and medulla. By examining these parts as well as pigmentation and other features an investigator can compare hair samples from a crime scene.
Hair can be used for DNA testing.
The hair follicle, which is in the base of hair, contains live cells with DNA material and must be present for DNA testing.
For using it in analysis of DNA (paternity tests, crime investigations, etc), the hair must be uprooted.
The hair which falls as breakage isn't useful since it doesn’t have a root.
Fiber Evidence A fiber is the smallest unit of a textile material that has a length many times greater than its diameter . Fibers can occur naturally as plant and animal fibers, but they can also be man-made . A fiber can be spun with other fibers to form a yarn that can be woven or knitted to form a fabric. The type and length of fiber used, the type of spinning method, and the type of fabric construction all affect the transfer of fibers and the significance of fiber associations. This becomes very important when there is a possibility of fiber transfer between a suspect and a victim during the commission of a crime. http://www.fbi.gov/hq/lab/fsc/backissu/july2000/deedric3.htm#Fiber%20Evidence Matching dyed synthetic fibers or dyed natural fibers on the clothing of a victim to fibers on a suspect’s clothing can be very helpful to an investigation, whereas the matching of common fibers such as white cotton or blue denim cotton would be less helpful. The discovery of cross transfers and multiple fiber transfers between the suspect's clothing and the victim's clothing dramatically increases the likelihood that these two individuals had physical contact.
Many different natural fibers originating from plants and animals are used in the production of fabric. http://www.fbi.gov/hq/lab/fsc/backissu/july2000/deedric3.htm#Fiber%20Evidence Types of Fibers More than half of all fibers used in the production of textile materials are man-made . Some man-made fibers originate from natural materials such as cotton or wood; others originate from synthetic materials. Cross-section of a man-made fiber Cotton fibers are the plant fibers most commonly used in textile materials The animal fiber most frequently used in the production of textile materials is wool , and the most common wool fibers originate from sheep.