Genetic factors appear to play a major role in determining hair texture—straight, wavy, or curly—and the thickness of individual strands of hair. Studies suggest that different genes influence hair texture and thickness in people of different ethnic backgrounds. For example, normal variations (polymorphisms) in two genes, EDAR and FGFR2, have been associated with differences in hair thickness in Asian populations. A polymorphism in another gene, TCHH, appears to be related to differences in hair texture in people of northern European ancestry. It is likely that many additional genes contribute to hair texture and thickness in various populations.
Several genetic syndromes are characterized by unusual hair texture. These syndromes are caused by variants in genes that play roles in hair structure and stability, including genes associated with specialized cell structures call desmosomes that hold hair cells together, proteins called keratins that provide strength and resilience to hair strands, and chemical signaling pathways involving a molecule called lysophosphatidic acid (LPA), which promotes hair growth. Genetic syndromes that feature altered hair texture include:
- Autosomal recessive hypotrichosis (caused by variants in the DSG4, LIPH, or LPAR6 gene)
- Keratoderma with woolly hair (caused by variants in the JUP, DSP, DSC2, or KANK2 gene)
- Monilethrix (caused by variants in the DSG4, KRT81, KRT83, or KRT86 gene)
- Uncombable hair syndrome (caused by variants in the PADI3, TCHH, or TGM3 gene)
Researchers speculate that the genes associated with these disorders probably also contribute to normal variations in hair texture and thickness.
Factors other than genetics can also influence hair texture and thickness. Hormones, certain medications, and chemicals such as hair relaxers or perms can alter the characteristics of a person’s hair. These changes can be temporary or permanent. Hair texture and thickness can also change with age.