The question of whether DNA can be recovered from cut hair is complex, and the answer depends entirely on the specific part of the hair strand being examined. Hair is a protein filament primarily composed of keratin, which is a dead, hardened protein structure with no living cells in the visible shaft. A full, unique DNA profile is most successfully obtained if the hair was forcibly removed and still retains the attached root tissue or follicle. While cut hair—meaning the hair shaft without the root—does contain genetic material, it is a different type of DNA that provides less specific information than the full genetic blueprint found in the root.
The Critical Difference: Nuclear DNA in the Root vs. Mitochondrial DNA in the Shaft
The biological structure of hair dictates the type and quality of DNA that can be recovered. The hair strand consists of two main parts: the root, which is the living portion embedded in the skin, and the shaft, which is the dead, visible length of hair. The root contains the hair follicle, which is rich in living, nucleated cells that house Nuclear DNA (nDNA). This nDNA is the high-quality genetic material that is unique to an individual, providing the full genetic profile used for individual identification.
In contrast, the hair shaft is a highly keratinized structure where the living cells have died, and their nuclei, which contain the nDNA, have degraded during the growth process. The shaft does not contain viable nDNA but instead retains many copies of Mitochondrial DNA (mtDNA). This mtDNA resides in the mitochondria, the small organelles responsible for energy production within the cell. The high copy number of mtDNA means it is more resistant to degradation and can survive in the tough keratin structure of the hair shaft.
Mitochondrial DNA differs from nuclear DNA in that it is inherited exclusively from the mother, meaning all maternally related individuals share the same mtDNA sequence. Because of this inheritance pattern, mtDNA cannot provide the unique identifier that nDNA offers, but rather links an individual to a specific maternal lineage. The quantity of mtDNA in a single hair shaft is substantial, which is why it can be recovered even from ancient or highly degraded samples where nDNA is no longer present.
How Forensic Scientists Extract DNA from Hair
Extracting genetic material from hair, particularly from the shaft, requires a specialized and sensitive approach due to the low quantity and degraded nature of the DNA. The first step involves physical and chemical preparation, where the hair is cleaned to remove surface contaminants and then subjected to chemical lysis. This lysis process typically uses a digestion buffer and an enzyme, such as Proteinase K, to break down the keratin structure of the hair shaft and release the DNA.
Following the breakdown of the hair structure, the DNA molecules are purified from the chemical mixture using methods such as silica-bead extraction or NaI treatment, which separate the DNA from the cellular debris and chemical reagents. The recovered DNA quantity from a hair shaft is often minuscule, making it impossible to analyze directly, so the next necessary step is amplification. Forensic scientists employ the Polymerase Chain Reaction (PCR) technique to create millions of copies of the specific mtDNA target regions, known as hypervariable regions 1 and 2 (HV1 and HV2).
Limitations and Uses of Hair DNA Evidence
The type of genetic information recovered from a hair sample defines its practical use in forensic and genealogical investigations. If the hair contained the root tissue, the recovered nuclear DNA provides a unique match to an individual, allowing for definitive identification. However, if the sample is a rootless hair shaft, the mitochondrial DNA analysis only allows for a match to a maternal line, meaning the profile could be shared by a mother, siblings, or other relatives along the direct maternal chain. This limitation means mtDNA is primarily used to confirm or exclude a person as being part of a group, rather than identifying them as the sole source.
Despite this limitation in individualization, mtDNA analysis is valuable when traditional nuclear DNA testing is not possible, such as with old or degraded samples. Its stability and high copy number make it an effective tool for analyzing hair evidence recovered from cold cases or ancient remains. Furthermore, the maternal inheritance pattern of mtDNA is used extensively in genealogy and ancestry tracing to map ancient human migration patterns and establish maternal relationships across generations. The ability to recover this genetic material from a rootless hair shaft makes it an investigative tool for linking evidence to a suspect’s family line, even if a direct identification is not possible.