To understand if DNA can be obtained from a cut hair without its root, it’s important to know hair’s basic biological components. The type of DNA available depends significantly on which part of the hair strand is examined.
Hair Structure and DNA
A single hair strand consists of two main parts: the hair shaft and the hair follicle. The shaft is the visible part above the skin, primarily composed of hardened keratin protein. The follicle, or root, is embedded in the skin where hair growth originates and contains actively dividing cells.
Most nuclear DNA, an individual’s unique genetic blueprint, is found within the nucleated cells of the hair follicle. These cells contain a full set of chromosomes, inherited equally from both parents. The hair shaft, composed of keratinized dead cells, contains very little nuclear material as cells lose their nuclei during keratinization. However, the hair shaft does contain mitochondrial DNA (mtDNA).
The Crucial Difference: Cut vs. Pulled Hair
The presence or absence of the hair follicle makes a significant difference in obtaining viable DNA. When hair is cut, the follicle is typically not present on the detached strand. This means the primary source of nuclear DNA is absent from the cut hair sample. Consequently, obtaining a complete and unique nuclear DNA profile from cut hair is highly challenging for standard analysis.
In contrast, pulled or forcibly removed hair often retains the follicle or a small piece of tissue attached to the root. This follicular material is rich in nucleated cells, making it a reliable source for extracting nuclear DNA. Nuclear DNA analysis, such as Short Tandem Repeat (STR) profiling, provides highly individualized results, making it the preferred method for personal identification in forensic cases.
What Can Be Learned from Hair Without a Follicle?
Even without the follicle, cut hair can still yield genetic information through mitochondrial DNA (mtDNA) analysis. Mitochondrial DNA is found in mitochondria, present in many copies within each cell and throughout the hair shaft. Unlike nuclear DNA, mtDNA is inherited almost exclusively from the mother, so all individuals along a maternal lineage share nearly identical mtDNA sequences.
The abundance of mtDNA copies makes it readily recoverable from hair shafts, even from degraded or older samples. This allows its use in establishing maternal lineage or identifying individuals through maternal relatives when nuclear DNA is unavailable. While mtDNA analysis can exclude suspects or provide ancestry clues, its maternal inheritance means it cannot distinguish between maternally related individuals, offering less discriminatory power than nuclear DNA. Recent advancements are improving nuclear DNA extraction from rootless hair shafts, though the DNA is often degraded and fragmented.