DNA testing using a hair sample is possible, but success and analysis type depend heavily on the presence of the follicle. The follicle contains most viable genetic material; without it, a comprehensive DNA profile is significantly more challenging.
Hair Structure and DNA Location
Hair consists of two parts: the shaft and the follicle (or root). The shaft, the visible portion extending from the skin, is primarily composed of dead cells packed with keratin protein. While the shaft does not contain nuclear DNA, it does contain mitochondrial DNA (mtDNA). MtDNA is distinct from nuclear DNA, existing in hundreds to thousands of copies per cell and inherited solely from the mother.
In contrast, the hair follicle, embedded within the skin, contains living cells with a nucleus. These cells are rich in nuclear DNA, the complete genetic blueprint unique to an individual, inherited from both parents. This nuclear DNA is what laboratories typically analyze for specific identification, such as paternity or individual profiling. Therefore, the hair follicle is crucial for most detailed DNA analyses.
Types of DNA Tests from Hair
DNA extracted from hair samples can be used for various purposes, depending on the type of DNA available. Forensic analysis frequently utilizes hair found at crime scenes. If a hair sample includes the root, nuclear DNA can be extracted for Short Tandem Repeat (STR) analysis, providing a highly discriminatory genetic profile for identification. This method is crucial for linking individuals to crime scenes or specific evidence.
When only the hair shaft is available, mitochondrial DNA (mtDNA) analysis can be performed. This testing is useful for degraded samples or when nuclear DNA is scarce, providing information about maternal lineage. While mtDNA analysis can help exclude individuals or establish maternal familial connections, it does not offer the same individual specificity as nuclear DNA, as all maternally related individuals share the same mtDNA profile. Paternity and ancestry testing are also possible, relying on nuclear DNA from the follicle for specific identification or a broader profile, or mitochondrial DNA from the shaft for maternal lineage insights.
Challenges and Considerations
Despite its utility, DNA testing from hair samples presents several challenges. The most significant difficulty arises when a hair sample lacks an intact follicle, making a complete genetic profile for identification or paternity problematic. While advanced techniques can sometimes recover nuclear DNA from rootless hair shafts, success rates are generally lower compared to samples with follicles.
Sample degradation is another common issue, as environmental exposure (heat, moisture, sunlight) can damage DNA. Older or improperly stored hair samples may yield insufficient or highly fragmented DNA, making analysis difficult or impossible. Contamination is also a concern, as foreign DNA from skin cells, other individuals, or handling errors can mix with the sample, leading to ambiguous or inaccurate results. Hair samples often contain limited DNA quantities compared to blood or saliva, requiring highly sensitive laboratory techniques for successful analysis.
Collecting and Storing Hair Samples
Proper collection and storage are essential for successful DNA extraction from hair. To ensure the presence of the DNA-rich follicle, hair samples should be pulled from the scalp or body, rather than cut. Aim for 5-10 hairs, ensuring each strand has a visible root, which often appears as a small, light-colored bulb at the end. Using clean gloves during collection helps prevent contamination from external DNA sources.
After collection, avoid touching the root end directly. Place samples in a paper envelope, as plastic bags can trap moisture and promote DNA degradation. Label the envelope clearly with relevant information, such as source and date of collection. If immediate submission to a laboratory is not possible, store samples in a cool, dry place away from direct sunlight to preserve DNA integrity. While DNA in hair can remain viable for years if stored correctly, prompt analysis is recommended.