Why do Asians have straight hair?

gbhealthwatch.com - Is it better to have curly or straight hair? - Genes and Hair Curl

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Human hair comes with all sorts of colors, textures and shapes. Notably, African hair is more coiled and dry; Asian hair is straighter and thicker; and Caucasian hair is somewhere in between with around 45% having straight hair, 40% having wavy hair, and 15% having curly hair. These variations are determined by many genes and genetic polymorphisms in combination with environmental factors (and not in the least by hair stylists!). At this point there is much more that we don’t know than what we do know. Here are some interesting research results that shed some light in this area:

The gene for curly hair in Caucasians

It has been long established that curly hair is a dominant trait in Caucasians and straight hair is recessive. This means two things: 1) if a person carries one allele for curly hair and another for straight hair, this person will have curly hair; 2) curly hair is a simple trait and is most likely determined by one single gene. However, a single gene has yet to be found to be solely responsible for the curly (or straight) hair trait in Caucasians. A recent genome wide association scan has found a SNP (single nucleotide polymorphism) called rs11803731 in the TCHH gene which accounts for about 6% of hair curliness. The TCHH gene encodes a protein called trichohyalin, which is known to be expressed at high levels in hair follicles and has been shown to be involved in the cross-linking of the keratin filaments found in hair. The ancestral allele of this SNP (the A-allele) is present in the worldwide population. Sometime during human history, a mutation lead to the emergence of the T-allele (called the derived allele in Fig. 1A). The T-allele causes an amino acid to change from leucine to methionine at position 790 of the TCHH gene. The function of this change is not clear. Nevertheless, Caucasians carrying the T-allele are more likely to have straight hair (about 70%) than those without the T-allele (about 50%) [1]. From this we can infer that curly hair is the ancestral trait while straight hair evolved much later.

The genes for thick straight hair in Asians

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Most people of East Asian descent have thick, straight hair. This corresponds with a SNP (rs3827760) in the EDAR gene which is involved in hair follicle development. The ancestral allele of this SNP is the A-allele. The G-allele is the newly derived allele that leads to the thick, straight hair. In certain parts of Asia, almost all people have the G-allele (see Fig.1B). People with the GG genotype at this SNP have thicker hair compared to those with the AA genotype due to the modification of a single amino acid in the protein. Those with the AG genotype have hair slightly thinner than those with GG, but still thick when compared to Europeans and Africans (likely AA) [2, 3]. Figure 1. The allele distribution of SNP rs11803731 of the TCHH gene (A) and SNP rs3827760 of the EDAR gene (B) Another gene that contributes to the thick-hair trait in the Asian population is FGFR2. The T-allele of the SNP rs4752566 in the FGFR2 gene increases the expression level of FGFR and leads to an increased hair follicle growth and thicker hair [4]. Does the thickness of the hair make it difficult to curl? Or do genes make the hair thick AND straight? We will have to wait for more research results to answer these questions.

Does straight hair provide any selection advantage?

Straight hair seems to be a new trait in human evolution. It is associated with the newly emerged alleles in both the TCHH gene and EDAR gene. The distributions of these straight hair related alleles (see figure) support the hypothesis that our human ancestors had curly hair; the straight hair found in East Asians and Caucasians likely developed independently. So what is so good about straight hair? It must provide some biological advantage for hair health. It has been observed that oily hair is typically straight and that individuals with both curly and oily hair are extremely rare. One theory is that the oils secreted into the hair shaft by the sebaceous glands can travel down the shaft of straight hair more easily. Studies indicate that the G-allele of the SNP rs3827760 in the EDAR gene responsible for the East Asian hair type arose during the past 65,000 years, when early humans were migrating ‘Out of Africa’ into Europe, then Asia. During the Ice Age, thicker hair might have been advantageous to protect against the cold. The EDAR gene is also involved in skin gland function. A study found that increased EDAR activity can cause enlarged skin glands (sebaceous glands found in abundance on face and scalp) and enlarged eyelid glands (meibomian gland) among other effects. These enhanced gland functions, may result in increased skin lubrication/protection and decreased evaporation which might have been positively selected during the cold, dry Ice Age [5].

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1. Medland SE et al. (2009). “Common variants in the trichohyalin gene are associated with straight hair in Europeans.” Am. J. Hum. Genet.85(5):750-5. 2. Fujimoto, A; Kimura, R; Ohashi, J; Omi, K; Yuliwulandari, R; Batubara, L; Mustofa, MS; Samakkarn, U et al. (2008). “A scan for genetic determinants of human hair morphology: EDAR is associated with Asian hair thickness.” Human Molecular Genetics 17 (6): 835– 43. doi: 10.1093/hmg/ddm355. PMID 18065779. 3. Mou, C; Thomason, HA; Willan, PM; Clowes, C; Harris, WE; Drew, CF; Dixon, J; Dixon, MJ et al. (2008). “Enhanced ectodysplasin-A receptor (EDAR) signaling alters multiple fiber characteristics to produce the East Asian hair form.” Human Mutation 29(12): 1405–11. doi:10.1002/humu. 20795. PMID 18561327. 4. 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, and J Ohashi (2009). “FGFR2 is associated with hair thickness in Asian populations.” J Hum Genet 54: 461-465. PMID:19590514 5. Chang SH, Jobling S, Brennan K, Headon DJ (2009) Enhanced Edar Signalling Has Pleiotropic Effects on Craniofacial and Cutaneous Glands. PLoS ONE 4(10): e7591. doi:10.1371/journal.pone.0007591

gbhealthwatch.com - Is it better to have curly or straight hair? - Genes and Hair Curl

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