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:
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.
Scientific journal articles for further reading
Fujimoto A, Kimura R, Ohashi J, Omi K, Yuliwulandari R, Batubara L, Mustofa MS, Samakkarn U, Settheetham-Ishida W, Ishida T, Morishita Y, Furusawa T, Nakazawa M, Ohtsuka R, Tokunaga K. A scan for genetic determinants of human hair morphology: EDAR is associated with Asian hair thickness. Hum Mol Genet. 2008 Mar 15;17(6):835-43. Epub 2007 Dec 8. PubMed: 18065779.
Fujimoto A, Nishida N, Kimura R, Miyagawa T, Yuliwulandari R, Batubara L, Mustofa MS, Samakkarn U, Settheetham-Ishida W, Ishida T, Morishita Y, Tsunoda T, Tokunaga K, Ohashi J. FGFR2 is associated with hair thickness in Asian populations. J Hum Genet. 2009 Aug;54(8):461-5. doi: 10.1038/jhg.2009.61. Epub 2009 Jul 10. PubMed: 19590514.
Medland SE, Nyholt DR, Painter JN, McEvoy BP, McRae AF, Zhu G, Gordon SD, Ferreira MA, Wright MJ, Henders AK, Campbell MJ, Duffy DL, Hansell NK, Macgregor S, Slutske WS, Heath AC, Montgomery GW, Martin NG. Common variants in the trichohyalin gene are associated with straight hair in Europeans. Am J Hum Genet. 2009 Nov;85(5):750-5. doi: 10.1016/j.ajhg.2009.10.009. Epub 2009 Nov 5. PubMed: 19896111. Free full-text from PubMed Central: PMC2775823.
Shimomura Y, Christiano AM. Biology and genetics of hair. Annu Rev Genomics Hum Genet. 2010;11:109-32. doi: 10.1146/annurev-genom-021610-131501. Review. PubMed: 20590427.