Hair, a common biological sample, often sparks curiosity about its genetic material. Many wonder if a single strand can reveal an individual’s unique blueprint. Understanding DNA in hair involves examining its structure and cellular components.
The Source of DNA in Hair
The hair follicle, also known as the root, is the most reliable source for obtaining a complete DNA profile. This is because the follicle contains living cells with nuclei, rich in nuclear DNA. When a hair is forcibly removed, a translucent follicular tag may be present around the hair’s shaft near the root, providing a rich source of nuclear DNA.
The hair shaft, the visible part extending from the scalp, primarily contains mitochondrial DNA (mtDNA). While once believed to contain little nuclear DNA, recent research indicates it is present, even in rootless hair, though highly degraded. This nuclear DNA is fragmented due to the keratinization process, which degrades the cell nucleus. Despite this, nuclear DNA can comprise a significant portion of the total human DNA in hair shafts, sometimes exceeding 95%.
Types of DNA from Hair
Two main types of DNA can be recovered from hair: nuclear DNA and mitochondrial DNA. Nuclear DNA is highly individual, providing a unique genetic fingerprint for each person, except for identical twins. This type of DNA is often analyzed using Short Tandem Repeat (STR) profiling, a highly discriminating method.
Mitochondrial DNA (mtDNA) is located in the mitochondria, organelles found in the cytoplasm of cells, including those in the hair shaft. Unlike nuclear DNA, mtDNA is inherited solely from the mother, meaning all individuals along a maternal line share the same mitochondrial DNA sequence. While less individualizing, mtDNA is present in hundreds to thousands of copies per cell, making it more abundant and often recoverable even from degraded samples or hair shafts without roots. Its high copy number makes it useful for older or challenging samples where nuclear DNA may be too degraded.
Applications of Hair DNA Analysis
DNA extracted from hair samples has several practical applications, particularly in forensic science. Hair is common evidence at crime scenes, and its DNA can link individuals to a location or establish associations, especially in cases involving physical contact. Forensic analysis prioritizes nuclear DNA profiling if root material is present for highly discriminatory results. If the root is absent, mitochondrial DNA analysis provides valuable information, though it cannot uniquely identify an individual.
Hair DNA can also be used in paternity testing, though it is less reliable than cheek swab samples and typically requires the hair root. While some companies offer hair-based paternity tests, DNA quality from hair varies, and traditional cheek swabs consistently yield higher quality DNA for more accurate results. Mitochondrial DNA from hair can also be used for ancestry tracing, helping individuals learn about their maternal lineage and geographical origins.
Factors Affecting DNA Recovery
Several factors can significantly impact the successful recovery and analysis of DNA from hair. The condition of the hair sample plays a large role; broken or highly degraded hairs may yield less DNA. Environmental exposure, such as heat, moisture, and certain chemicals, can degrade DNA over time, making extraction more difficult. For instance, UV radiation can cause oxidative DNA damage in hair follicles and shafts.
The age of the hair sample also influences DNA recovery, with older samples having more degraded DNA. The presence or absence of the hair root is a major determinant; intact roots or follicular tags provide a richer source of nuclear DNA. Hair shafts, especially rootless ones, present challenges due to low DNA quantity and high degradation. Despite these challenges, hair shafts can be surprisingly resistant to external contamination, suggesting the keratin structure may protect the internal DNA.