Hair is a protein filament composed primarily of keratin, a tough structural protein that forms the outer layer of the skin, nails, and hair. The viability and location of DNA within a hair sample depend entirely on which part of the hair is collected. The majority of the hair fiber is biologically inactive. The genetic material needed for individual identification is concentrated in specific, living cells at the base of the strand, which is crucial for forensic and genetic testing.
The Structure of Hair and the Follicle
The structure of a hair strand is divided into two main parts: the shaft and the root. The hair shaft is the visible portion extending above the skin surface, composed of dead, keratinized cells. The shaft is made up of three distinct layers: the outer cuticle, the cortex, and the inner medulla.
The hair root is the living structure embedded within the skin, encased by the hair follicle. At the base of the root is the hair bulb, which surrounds the dermal papilla that supplies the hair with blood and nutrients.
As cells in the hair bulb divide and push upward, they undergo keratinization, hardening and losing their internal cellular structures, including the cell nucleus. This transformation means the hair shaft, once fully formed, is biologically inert.
Nuclear DNA: The Genetic Fingerprint Location
The most reliable source for a full, unique genetic profile is found exclusively in the hair root. This genetic material is nuclear DNA (nDNA), located within the cell nucleus, containing an individual’s complete genome inherited from both parents. For nDNA to be recovered, the hair must have been forcibly removed, such as by plucking, rather than naturally shed.
When a hair is forcefully removed, it often pulls along the follicular tag, a translucent piece of tissue surrounding the root sheath. These follicular tag cells are living cells from the hair follicle that retain their nuclei, making them rich with high-quality nDNA. This tissue is used for creating a full genetic fingerprint using techniques like Short Tandem Repeat (STR) analysis.
The hair shaft, composed of dead cells, lacks the intact nuclei necessary for profiling. While modern techniques can sometimes detect fragmented nDNA within the shaft, the concentration and quality are significantly lower than what is found in the follicular tag. Therefore, the root with its attached tissue is necessary for obtaining a complete, individual-specific nDNA profile.
Analyzing the Hair Shaft: Mitochondrial DNA
When a hair is found without the root, it is still possible to recover genetic information in the form of mitochondrial DNA (MtDNA). This DNA is located in the mitochondria, the organelles responsible for energy production that reside in the hair’s cortex cells, not the cell nucleus. MtDNA is much more numerous, with hundreds to thousands of copies per cell, compared to only two copies of nDNA.
This high copy number makes MtDNA resistant to degradation, allowing it to be recovered from old, chemically treated, or severely degraded hair shafts. Because it is found throughout the keratinized shaft, MtDNA analysis is often the only option when the root is unavailable. However, MtDNA is inherited exclusively from the mother, meaning all individuals along the same maternal line will share the identical MtDNA sequence.
Why the Difference Matters in Testing
The distinction between nDNA and MtDNA dictates the type of genetic testing performed and the certainty of the result. Nuclear DNA analysis offers a highly specific result, capable of linking a sample to a single individual with confidence. This power of individualization is why nDNA from a follicular tag is preferred in forensic and paternity testing.
Mitochondrial DNA analysis, while less powerful for individual identification, is valuable for specific scenarios. Since it only establishes a maternal lineage, it can include or exclude an entire family line from a sample. In cases involving highly degraded or ancient samples, where nDNA has broken down, the stable MtDNA can provide a link to a known maternal relative.