Hair is a common biological material found in various settings. Within this seemingly simple structure lies deoxyribonucleic acid, or DNA, which serves as the unique genetic blueprint for every individual. Understanding DNA’s location in hair is fundamental to its significance, especially in forensic science.
DNA in the Hair Follicle
The hair follicle, also known as the hair root, is the living component of the hair structure embedded within the skin. It is a dynamic organ responsible for hair growth and contains a rich supply of actively dividing cells. These living cells are abundant in nuclear DNA (nDNA).
Nuclear DNA is highly distinctive, providing a unique genetic profile for each individual, with the exception of identical twins who share the same nuclear DNA. The presence of intact cellular structures within the follicle makes it an optimal source for recovering this type of DNA. Consequently, a hair that has been forcibly removed, typically retaining its follicle, is considered a superior sample for individual identification. The follicle also contains mitochondrial DNA (mtDNA), but nDNA is the primary focus for individual identification.
DNA in the Hair Shaft
In contrast to the follicle, the hair shaft is the visible portion of the hair that extends beyond the skin’s surface. This part of the hair is largely composed of dead, keratinized cells and structural proteins. Due to its non-living nature, the hair shaft lacks nuclear DNA, as its cells have completed their life cycle and no longer possess intact nuclei.
Despite the absence of nuclear DNA, the hair shaft does contain mitochondrial DNA (mtDNA). Mitochondria are small organelles responsible for energy production, possessing their own distinct DNA. Mitochondrial DNA is inherited exclusively from the maternal line. While useful for tracing maternal lineages or excluding individuals, mtDNA cannot uniquely identify a single person, as all individuals within the same maternal line share identical mitochondrial DNA sequences.
Implications for DNA Identification
The distinct locations of DNA within hair have significant implications for identification purposes, particularly in forensic investigations. A hair sample that includes the root, indicative of a hair that was pulled or forcibly removed, is highly valuable because it contains nuclear DNA. Recovering nuclear DNA from such a sample allows for a precise individual match, offering strong evidence in identification cases.
Conversely, hair shafts, which are commonly shed hairs found at crime scenes, present a greater challenge for DNA analysis. These samples only yield mitochondrial DNA, which provides less specific information than nuclear DNA. While mtDNA can help narrow down a pool of suspects to a maternal lineage, it cannot pinpoint a single individual. Therefore, the presence or absence of the hair follicle dictates the type of DNA available and, consequently, the level of individual identification that can be achieved.