(AHFBM) Collection of Hair Evidence Lesson

Collection of Hair Evidence

Recall that morphological characteristics are not considered individual evidence, and thus they can't be used to link a human hair to any single head or body. When a suspect is available, the collection of several hairs as standards/reference samples can provide strong circumstantial evidence, but not positive identification. Most often, scale structure, medullary index, and medullary shape are used for comparison. The value of hair evidence relates to the degree of probability associated with a questioned hair and a particular individual.

Body Area

The structure of hair differs on different parts of the body. There are several subtle characteristics that can help investigators determine which part of the body the hair originated. Hairs from the scalp have consistent diameters and uniform distribution of pigment. Pubic hairs typically have continuous medullae. Facial or beard hairs have triangular cross-sections and eyebrow hair decreases in diameter from the root to the tip. In addition, there are other various types of hairs found on the human body at various points in growth and development. Lanugo, a coat of delicate, downy hairs, is typically found on human fetuses prior to birth. Babies born prior to full term gestation may retain the lanugo hair after birth for a short period.

Another type of hair that is associated with growth and development is Vellus hair. Vellus hair is the fine hair present on the body after birth and before puberty. Once a person reaches adulthood, they typically have what is known as terminal hair.

Terminal hair is the larger, coarser hair in the adult that replaces the vellus hair. You may have wondered why hair seems to change during puberty; it is due to hormonal fluctuations which change the type of hair that grows on the body. As the body changes, so does the hair structure.

Race

It is not always possible to determine race from hair samples. Some characteristics are common in each race, but there are always variations and exceptions.

People of Asian descent typically have continuous medullae. The pigment granules are generally very dense and occur in large patches or streaks. In cross-section, the hairs are almost always circular. Hair color is almost exclusively black.

People of European descent have an even distribution of pigments in cortex and may have fragmented or completely absent medullae. In cross-section, the hairs appear to be irregularly shaped ovals. Hair color varies from blonde to black; this group of people have the most variation in hair color.

People of African descent are more likely to have dense and unevenly distributed pigment granules. In cross section, the hair is often thin and appears somewhat flattened. The medulla is typically fragmented or absent. Hair color is generally black with some variances noted in individuals with albinism or European ancestry.

Hair Growth

Hair grows about one centimeter per month. The growth of human hair occurs in three developmental stages called anagen, catagen, and telogen. An easy way to remember the order of this process is to remember "ACT"!

The anagen phase is the initial growth phase during which the hair follicle actively produces hair and hair grows. The Catagen phase is a transition phase between the anagen and telogen phases of hair growth; hair is at rest and not actively growing. The telogen phase is when the follicle is dormant or resting. During the telogen, hair routinely falls from the skin. Hair in the telogen phase is the most common type of hair sample found in crime scenes because hairs in this phase fall out with little to no provocation.

Identification & Comparison of Hair

It is usually very easy to compare animal hair to human hair, but comparing human hair to other human hair is difficult. Most commonly, forensic analysis is used to determine whether or not hair recovered from a crime scene is comparable to hair removed from a suspect. The analyst will look at morphological characteristics such as color, length, diameter, medulla, and scale pattern. If hair has been forcibly removed the follicular tag may be present which is a rich source of Mitochondrial DNA which can then be used to identify an individual. Nuclear DNA is DNA that is present in the nucleus of a cell and that is inherited from both parents. Mitochondrial DNA differs from Nuclear DNA in that it is found inside the Mitochondria of the cell rather than in the Nucleus. Mitochondrial DNA, or mtDNA, is only passed down by a mother to her children, hence it is also colloquially called "Maternal DNA". The reasoning that mtDNA from the father is not passed down to the child is that the mtDNA from the father does not enter the egg cell during fertilization; it remains with a small portion of the sperm cell outside the egg. Mitochondrial DNA is very important in Forensics because it can be used in analysis of very old, degraded or small samples unlike with Nuclear DNA analysis. A strand of hair with no follicular tag, or "root", would then be class evidence since it can't be linked to a specific individual. A hair with a follicular tag can be considered individual evidence if mtDNA can be extracted from the hair and analyzed to link it to a specific person.

[CC BY 4.0] UNLESS OTHERWISE NOTED | IMAGES: LICENSED AND USED ACCORDING TO TERMS OF SUBSCRIPTION