Deoxyribonucleic acid (DNA) contains the genetic instructions for all organisms. Though often considered fragile, DNA found in hair samples is surprisingly durable. Hair’s unique structure provides a protective environment, preserving DNA for extended periods and making it a valuable biological sample.
DNA’s Resilience in Hair
DNA’s longevity in hair is largely due to its unique structure, primarily composed of keratin. This tough, fibrous protein forms the hair shaft, shielding the DNA within.
Hair contains two types of DNA: nuclear DNA (nDNA) and mitochondrial DNA (mtDNA). Nuclear DNA, providing an individualized genetic profile, is found mainly in the hair root or follicle.
Mitochondrial DNA, conversely, is present in hundreds to thousands of copies throughout the entire hair shaft, even without the root. This higher copy number makes mtDNA more resistant to degradation than nDNA. Thus, mtDNA is often analyzed in older or degraded hair samples when nDNA is not viable. However, mtDNA is maternally inherited, meaning individuals from the same maternal lineage share identical sequences, limiting its ability to uniquely identify an individual compared to nDNA.
Factors Affecting DNA Preservation
While hair’s keratin matrix offers inherent protection, several external and environmental factors influence DNA viability. Temperature extremes, whether hot or cold, accelerate DNA degradation. High humidity and moisture also promote DNA breakdown by encouraging microbial growth and chemical reactions.
Ultraviolet (UV) radiation from sunlight is another factor, directly damaging DNA and degrading protective hair components. Chemical exposure, such as strong acids or alkalis, can also rapidly degrade DNA, making extraction difficult. The presence of the hair root is also important, as its living cells contain a more abundant and intact source of nuclear DNA, which is more susceptible to degradation in the hair shaft alone.
Why Hair DNA Matters
The ability to recover DNA from hair is significant across various fields. In forensic science, hair frequently serves as evidence at crime scenes, linking individuals to locations or victims. It can provide associative evidence in violent crimes where physical contact is likely.
Beyond forensics, hair DNA contributes to historical research, allowing analysis of ancient remains or artifacts for insights into past populations or individuals. Genealogical studies also utilize hair samples for tracing maternal ancestry through mitochondrial DNA, offering a non-invasive way to explore family lineages. Hair’s widespread presence and ease of transfer make it a valuable source of genetic information.
Limitations in Hair DNA Analysis
Despite its potential, analyzing DNA from hair presents challenges. Hair shafts, especially those without the root, often contain minute, highly fragmented quantities of nuclear DNA. This low quantity and degraded state make obtaining a complete, reliable nuclear DNA profile difficult using traditional methods.
Contamination poses another risk during collection and analysis. External DNA, such as from skin cells or microbes, can easily mix with the sample, complicating interpretation. Extracting and amplifying such small, degraded DNA fragments requires highly sensitive, specialized laboratory techniques, increasing analysis difficulty and cost.