There are three phases of mammalian hair follicle growth. In the anagen phase, the hair is actively growing; the catagen phase is the transitional period between the other two phases; and in the telogen phase, the follicle is dormant .
Anagen Hair Follicle
In the anagen phase, mitotically active cells around the dermal papilla of the follicle grow upward to form the medulla, cortex, cuticle, and root sheath (see Figure 8).
These actively growing hairs remain in this phase for several years. Approximately 80% of all follicles will be in the anagen phase. The cells and resulting sheath of this phase are DNA-rich in comparison to the other sections of the hair, making anagen hairs ideal for nDNA analysis .
Catagen Hair Follicle
After a period of growth, the hair stops growing and initiates the catagen or transitional phase, which lasts several weeks. Less than 1% of follicles are in the catagen phase. During this phase, the melanocytes in the follicle contract and stop producing and distributing pigment granules. The root and root sheath shrink and the base of the hair rounds and becomes surrounded by a brush-like capsule called the club (see Figure 9) .
Telogen Hair Follicle
At the final stage of the growth, a human hair will enter the telogen phase, where the follicles are at the mature and stable stage and the hair is fully developed (see Figure 10). Approximately 20% of follicles are in this phase. A hair in this phase will either be released by mechanical means, such as brushing, or it may be forced from the skin by an emerging hair. Approximately 90-95% of shed hairs are telogen phase hairs. On a scalp containing approximately 100,000 follicles, at least 100 scalp hairs per day will be shed . Therefore, a majority of scalp hairs collected at crime scenes will be telogen phase hairs. Telogen hairs are typically not good candidates for nDNA analysis because of the lack of cellular material, such as the sheath, due to degradation
 Bisbing, R.E. “The Forensic Identification and Association of Human Hair.” Trans. Array Forensic Science Handbook, Volume I. Richard Saferstein. 2nd ed. Upper Saddle River: Prentice Hall, 2002. 389-428..