TCHH rs1180373: Hair Texture and Curl Variation

What each genotype means

Genomic Location and the Nature of rs1180373

The single nucleotide polymorphism designated rs1180373 resides within the trichohyalin (TCHH) gene, situated in the Epidermal Differentiation Complex on the long arm of human chromosome 1 (1q21.3). Trichohyalin belongs to a family of structural proteins that play vital roles in the terminal differentiation and mechanical reinforcement of keratinizing epithelia. At the molecular level, rs1180373 represents a common single nucleotide change (alleles A and T) that alters the coding sequence of the TCHH transcript, resulting in an amino acid substitution (often characterized as Leu790Phe) within the highly repetitive central alpha-helical rod domain. Because this domain is integral to how the protein forms multimeric filaments and interacts with nearby molecules, alterations at this locus can influence the mechanical stability and spatial arrangement of follicular cells during hair shaft extrusion.

The Biological Function of Trichohyalin

The TCHH gene encodes trichohyalin, an intermediate filament-associated structural protein expressed abundantly within the inner root sheath (IRS) and the medulla of developing hair follicles. In cellular biology, trichohyalin acts as a cross-linking scaffold: it associates extensively with keratin intermediate filaments, guided by enzymatic modifications such as citrullination and transglutaminase-mediated cross-linking. As detailed in resources like [MedlinePlus Genetics: TCHH](https://medlineplus.gov/genetics/gene/tchh/), normal trichohyalin function is critical for conferring rigidity and regular cylindrical geometry to the growing hair fiber. Extreme disruptions, such as severe loss-of-function mutations in TCHH, can compromise this cross-linking framework, leading to irregular, non-round hair shaft cross-sections seen in conditions like uncombable hair syndrome. Under normal physiological conditions, subtle variations in protein conformation subtly alter the mechanical resistance of the inner root sheath, modulating the curvature and fiber cross-section.

Research Associations with Hair Curl and Morphology

Multiple genome-wide association studies (GWAS) have established that rs1180373 is one of the primary common genetic contributors to hair morphology in populations of European descent, as documented across repositories such as the [GWAS Catalog](https://www.ebi.ac.uk/gwas/). Seminal research investigating hair texture in European cohorts demonstrated that the derived allele is significantly correlated with straight hair, whereas the alternative ancestral allele correlates with increased wave, curliness, or frizz. However, scientific evidence also underscores the polygenic nature of hair texture; rs1180373 explains only a modest portion of total phenotypic variance. Furthermore, association studies reveal that the predictive strength of rs1180373 is ancestry-specific. While it plays a prominent statistical role in individuals of European heritage, it does not reliably account for hair curliness or coiled textures observed in African or East Asian populations, where distinct genetic variants—such as EDAR for hair thickness—predominate.

Population Distribution and Ancestry Patterns

Allele frequencies for rs1180373 show notable geographic differentiation across global populations. In European cohorts, the minor allele frequency (MAF) is high, typically hovering around 0.40 to 0.42, ensuring that homozygous and heterozygous genotypes are both widely represented across the general population. In contrast, population genomics databases show that the allele distribution varies dramatically among non-European groups, being found at considerably lower frequencies or showing different linkage patterns in indigenous populations across East Asia and sub-Saharan Africa. This divergence illustrates how evolutionary pressures, sexual selection, and genetic drift have shaped visible physical diversity across human groups over tens of thousands of years. As a result, commercial genetic testing algorithms cannot generalize findings from rs1180373 across all ancestral backgrounds with equal fidelity.

Practical Implications and Limitations for Consumers

Finding rs1180373 in personal direct-to-consumer genetic testing results offers an interesting scientific window into the genetics of physical traits, but it carries no clinical urgency. Because this variant reflects normal human morphological variation rather than pathology, your genotype cannot be used to diagnose, prevent, or treat any medical condition. Readers should understand that genetics is only one part of the equation: environmental influences, personal hair styling, humidity, heat application, chemical treatments, and age-related hormonal shifts all strongly impact how hair appears and behaves. You cannot alter your underlying genotype, nor should you modify hair care products based on this SNP alone. If an individual experiences unusual, sudden hair breakage, scalp irritation, or unmanageable texture changes, they should consult a board-certified dermatologist rather than relying on genetic trait reports.

Frequently asked questions

Sources & further reading