Hair is a common source of biological evidence at crime scenes. While it offers clues like color or general origin, its ability to yield DNA is often misunderstood. Successful DNA analysis, especially for individual identification, depends on specific hair parts.
Understanding Hair Structure and DNA Location
A hair strand consists of two primary parts: the hair shaft and the hair follicle. The hair shaft is the visible portion extending from the skin, composed of dead, keratinized cells. These cells lack a nucleus, where most nuclear DNA is stored.
The hair follicle is the living structure embedded within the skin. It contains actively growing cells in the hair bulb, the base of the follicle. These cells are nucleated and contain nuclear DNA. Blood vessels nourish these cells, supporting hair growth and providing material for DNA extraction.
The Essential Part for Nuclear DNA
The hair follicle, specifically the root, is the most valuable part for obtaining nuclear DNA. Nuclear DNA is unique to an individual (except identical twins) and inherited from both parents, making it highly discriminating for identification. For DNA analysis, hairs must be plucked to ensure the root, with its cellular material, is present.
Hair undergoes a growth cycle with three main phases: anagen, catagen, and telogen. The anagen phase is the active growth period when the hair follicle actively produces the hair shaft and contains abundant nuclear material. Hairs forcibly removed during this anagen phase often have a translucent tissue surrounding the root, known as a follicular tag, which is a rich source of nuclear DNA. Hairs in the telogen phase are resting hairs that have naturally shed and typically lack sufficient nuclear DNA in the shaft. Keratinization during hair growth can degrade nuclear DNA in the hair shaft, making recovery challenging from rootless hairs.
Distinguishing DNA Types from Hair
Hair can provide two types of DNA: nuclear DNA (nDNA) and mitochondrial DNA (mtDNA). Nuclear DNA is found in the nucleated cells of the hair follicle/root and is highly specific to an individual. Mitochondrial DNA is located in the mitochondria, which are present in numerous copies within each cell and can be found throughout the hair shaft, even when the root is absent.
Mitochondrial DNA is inherited solely from the mother, meaning all individuals along the same maternal lineage share identical mtDNA profiles. This makes mtDNA less discriminating than nuclear DNA, as it cannot differentiate between maternally related individuals. However, mtDNA analysis is useful for highly degraded samples or when only hair shafts without roots are available, due to its high copy number and protection by the mitochondrial membrane.
Collecting and Analyzing Hair for DNA
For effective DNA profiling, forensic scientists prioritize collecting hairs that include the root or follicular tag. The presence of this cellular material significantly increases the likelihood of recovering sufficient nuclear DNA for robust analysis. While hair analysis can involve examining hair structure under a microscope, DNA extraction and amplification techniques, such as Polymerase Chain Reaction (PCR), are used to generate a genetic profile.
The success rate of obtaining a full DNA profile depends on the quality and quantity of nucleated cells. Although some advanced methods can recover nuclear DNA from rootless hair shafts, the most comprehensive and individual-specific results are achieved when the hair root is intact. Proper collection techniques that ensure the hair is plucked, rather than cut, are important for maximizing the potential for DNA evidence.