Hair is a frequently encountered form of evidence at crime scenes. Its utility for DNA analysis relies on the presence of specific biological components. Obtaining DNA from a hair sample for forensic investigation is a complex process, highly dependent on the hair’s condition and type.
Key Hair Components for DNA Analysis
Hair consists of two main parts: the visible hair shaft and the hair root, embedded within the skin’s follicle. For DNA analysis, the most important component is the follicular tag, a translucent tissue often surrounding the hair’s shaft near the root, especially if forcibly removed. This tag contains nucleated cells, the primary source of an individual’s nuclear DNA. Without it, such as in naturally shed hairs, obtaining nuclear DNA is much less likely. While historically believed that rootless hair contained no nuclear DNA, recent advancements show fragmented nuclear DNA can be recovered from hair shafts, though it is highly degraded.
Nuclear DNA Versus Mitochondrial DNA in Hair
Two types of DNA can be recovered from hair: nuclear DNA (nDNA) and mitochondrial DNA (mtDNA). Nuclear DNA, found within the cell’s nucleus, is inherited from both parents, making it unique to an individual (except identical twins). This makes nDNA the preferred source for individual identification in forensic cases. The follicular tag provides its richest source, allowing for highly discriminatory profiles.
In contrast, mitochondrial DNA is located in the mitochondria, structures in the cell’s cytoplasm, and is inherited exclusively from the mother. Many mtDNA copies exist per cell, making it more abundant and often recoverable even from highly degraded samples or the hair shaft itself, where nDNA is scarce. While mtDNA cannot uniquely identify an individual because all maternally related individuals share the same sequence, it can exclude suspects or link evidence to a maternal lineage when nDNA is unavailable.
Factors Influencing DNA Quality and Yield
Even when necessary hair components are present, external factors can impact DNA extraction and analysis. Environmental exposure, such as heat, moisture, and ultraviolet (UV) light, can degrade DNA over time. For instance, DNA degrades rapidly in moist environments, while cold, dry conditions can preserve it. Chemical treatments like hair dyes, bleaches, and perms can also damage hair structure and reduce recoverable DNA quantity and quality. The sample’s age also plays a role, with older samples generally having more degraded DNA, though well-preserved ancient hair can still yield genetic material.
Forensic Significance of Hair DNA
Hair DNA evidence holds value in forensic investigations, primarily for establishing associations between individuals and crime scenes. Nuclear DNA, when obtainable from the hair root or follicular tag, offers a powerful tool for individual identification. However, its availability is often limited, as many hairs found at crime scenes are naturally shed and lack these cellular components.
When nuclear DNA is not viable, mitochondrial DNA analysis serves as an important alternative. Although less specific, mtDNA can link a hair sample to a maternal lineage, useful for exclusionary purposes or narrowing down potential suspects. Advances in DNA analysis techniques continue to improve the ability to extract and interpret genetic information from hair, even from degraded samples.