DNA testing analyzes an individual’s unique genetic blueprint, used for applications ranging from personal curiosity, such as ancestry or paternity, to legal and forensic investigations. Hair is a common source of biological evidence, yet its utility for generating a full genetic profile is often misunderstood. Successfully using a hair sample for genetic analysis depends entirely on specific biological components and precise collection methods. The inherent structure of the hair strand presents a challenge, making it a less straightforward sample than a simple cheek swab.
The Essential Requirement for Viable Hair DNA
The success of a DNA test hinges on the presence of nuclear DNA, which contains the comprehensive genetic information inherited from both parents. This complete genetic profile is found almost exclusively in the living cells of the hair root, or follicle, which is the bulbous tissue at the base of the hair strand. When a hair is shed naturally or cut, this follicle tissue is not present, severely limiting the potential for a full DNA analysis.
The main body of the hair strand, known as the shaft, is composed of the protein keratin and lacks a nucleus, meaning it contains little to no nuclear DNA. Instead, the hair shaft is rich in mitochondrial DNA (mtDNA). While laboratories can analyze mtDNA, it is only inherited from the maternal line and provides a less specific genetic signature. Therefore, for standard relationship or identity testing, securing a hair with an intact root is necessary.
Step-by-Step Collection and Submission Protocol
To obtain a viable sample, the hair must be forcefully extracted from the scalp or body to ensure the root remains attached, rather than simply being cut at the shaft. The collected hairs must have the small, translucent bulb visible at the end, which is the DNA-rich follicle tissue. Naturally shed hairs, which typically lack the follicle, are unsuitable for this type of analysis.
Laboratories recommend submitting a minimum of five to ten hair strands that have been manually plucked with the root intact. Handle the samples as little as possible to prevent contamination from foreign DNA or environmental substances. The collected hairs should be placed in a clean, dry paper envelope, as plastic bags can trap moisture and accelerate the degradation of the genetic material.
Analyzing Hair Samples in the Laboratory
Once the sample arrives at the testing facility, the first laboratory step is to excise the follicular tissue from the hair shaft to extract the nuclear DNA. Because the amount of DNA recovered from a single hair root is often extremely small, the Polymerase Chain Reaction (PCR) technique is used to create millions of copies of the target DNA sequences. This amplification process is accomplished by placing the sample through cycles of heating and cooling.
During the PCR cycles, the double-stranded DNA is separated by heat, and an enzyme, such as Taq polymerase, is introduced to synthesize two new strands from each original strand. This exponential copying process produces a sufficient quantity of DNA for analysis. If the sample lacks a root and only the hair shaft is available, the lab must switch to analyzing the mitochondrial DNA, which involves a different sequencing process. Melanin, the pigment that gives hair its color, can sometimes interfere with the PCR process, posing an additional technical challenge.
Reliability and Limitations of Hair DNA Testing
Compared to the standard buccal swab, which reliably yields high-quality nuclear DNA from cheek cells, hair is considered a secondary and less reliable source for relationship testing. The primary factor limiting the success of hair testing is the absence of the follicle, which prevents the recovery of the specific nuclear DNA required for identity and paternity analysis. Poor collection, such as cutting the hair, or improper storage that introduces moisture and contamination, can severely degrade the sample, leading to inconclusive results.
When nuclear DNA testing fails, the analysis may be limited to mitochondrial DNA, which can only confirm a maternal lineage, not an individual identity. Even when a root is present, the DNA may be degraded due to the hair’s keratinization process, resulting in a low template sample that is difficult to amplify. Laboratories caution that hair samples have a higher likelihood of resulting in a failure to obtain a full genetic profile compared to a straightforward cheek swab.