Hair found at a crime scene or in a paternity dispute often raises questions about DNA analysis. Many wonder if the entire hair, including its root, is necessary to extract genetic information. The answer depends on the type of DNA and testing goals.
Hair Structure and DNA Location
A hair strand consists of two parts: the hair shaft and the hair follicle, also known as the root. The hair shaft is the visible portion above the skin, composed mainly of dead, hardened protein called keratin. This part of the hair does not contain living cells with intact nuclei.
The hair follicle is the living part embedded within the skin. This region contains actively dividing cells rich in nuclear DNA. When a hair is forcibly removed, a small piece of tissue, known as a follicular tag, may be present around the root, providing a valuable source of these DNA-rich cells.
The Two Types of Hair DNA
Hair can yield two types of DNA: nuclear DNA (nDNA) and mitochondrial DNA (mtDNA). Nuclear DNA is found within the nucleus of living cells and is unique to an individual, except for identical twins. This type of DNA provides a highly specific genetic fingerprint and is primarily located in the hair root or any attached follicular tissue.
Mitochondrial DNA resides in the mitochondria, the cell’s energy-producing organelles. It is more abundant and robust than nuclear DNA, especially in degraded samples like hair shafts. mtDNA is inherited maternally, meaning all individuals in the same maternal lineage share the same mitochondrial DNA sequence. This makes mtDNA less individualizing than nuclear DNA, though it can still be informative for lineage tracing or exclusionary purposes.
Hair DNA in Forensic and Paternity Testing
The presence or absence of the hair root influences the utility of hair samples in forensic and paternity testing. For individual identification, nuclear DNA from the hair root is preferred. The living cells in the root provide the necessary quality and quantity of nuclear DNA for robust profiling. Without the root, obtaining sufficient nuclear DNA for a complete profile becomes challenging, though advancements are making it increasingly possible to extract some nuclear DNA even from rootless hair shafts.
Hair samples without roots primarily yield mitochondrial DNA. While mtDNA analysis can be valuable for linking a sample to a maternal lineage or excluding individuals, it cannot uniquely identify a person or confirm paternity on its own. DNA from hair for paternity testing, even with roots, is often more expensive and less reliable than using blood or cheek swab samples. Contamination and limited DNA quantity can further complicate analysis, sometimes requiring multiple hair samples or retesting.