You cannot determine a person's biological sex by looking at a single strand of hair.

Human hair is primarily composed of alpha-keratin, a fibrous structural protein that is the same regardless of biological sex. At the microscopic level, forensic experts examine the cuticle, cortex, and medulla to determine species or ethnicity, but these structures do not vary by gender. A 2011 study published in the Journal of Forensic Sciences confirms that physical characteristics like diameter and scale patterns are influenced by age and health rather than sex.The hair shaft itself is technically dead tissue and lacks the nuclear DNA required for chromosomal analysis. While mitochondrial DNA (mtDNA) can be extracted from the shaft, it only provides maternal lineage and cannot distinguish between a brother and a sister. To determine sex, scientists must have the hair follicle or 'root bulb' which contains living cells with 23 pairs of chromosomes.In these root cells, the presence of the SRY gene on the Y chromosome or the ratio of X to Y chromosomes provides a definitive biological identification. Without this root material, any perceived differences in hair texture or shine are usually the result of cosmetic treatments, diet, or environmental factors. Modern forensic labs, such as those operated by the FBI, require at least a small portion of the follicular tag to perform successful nuclear DNA profiling.