TCHH rs11803733: Hair Morphology and Curl Structure
The genetic variant rs11803733 is a single nucleotide polymorphism located in the TCHH gene on chromosome 1. It belongs to a functional cluster of missense variations in TCHH that influence the inner root sheath of the hair follicle and the mechanical curl structure of scalp hair. Evidence indicates that alleles in this locus are statistically associated with variations between straight, wavy, and curly hair phenotypes, primarily studied in European populations.
What each genotype means
Higher likelihood of straight hair
You carry two copies of the A allele in the trichohyalin (TCHH) gene, which is statistically associated with straight hair texture in populations of European ancestry. Research shows that this variant influences the mechanical cross-linking of keratin intermediate filaments within the hair follicle. However, hair texture is a polygenic trait shaped by multiple interacting genes and environmental styling factors, meaning this genotype does not guarantee straight hair.
Found in approximately 10% to 16% of individuals of European ancestry; it occurs at markedly lower frequencies in East Asian and African populations.
Intermediate hair texture likelihood
You carry one straight-associated allele (A) and one ancestral allele (G) at this position in the TCHH gene. In European association studies, this heterozygous genotype is typically linked to intermediate hair morphology, such as wavy hair, or a moderate probability of straight hair. Because overall hair curliness is influenced by multiple genes across the genome, your actual hair pattern reflects combined genetic and non-genetic factors.
Carried by roughly 40% to 50% of individuals of European ancestry; it is much less common in East Asian populations.
Higher likelihood of wavy or curly hair
You carry two copies of the ancestral G allele in the TCHH gene, which is associated with a lower probability of naturally straight hair and a higher likelihood of wavy or curly hair among Europeans. Trichohyalin acts as a structural stabilizer within the growing hair sheath, and the absence of the A allele preserves ancestral hair morphology patterns. Hair texture remains complex and polygenic, so this genotype alone does not determine your exact curl pattern.
Present in roughly 35% to 45% of individuals of European ancestry and represents the predominant genotype in most East Asian and African populations.
Genomic Context and Variant Architecture
The single nucleotide polymorphism rs11803733 is located on human chromosome 1 within the epidermal differentiation complex at 1q21.3. This genomic region houses the TCHH gene, which encodes trichohyalin. The variant represents a single base-pair substitution within the coding sequence of TCHH, positioning it as part of an extensively studied haplotype block alongside other well-characterized missense SNPs such as rs11803731. Because these adjacent variants reside within the same functional locus, they exhibit linkage disequilibrium and jointly modulate the primary amino acid sequence of the structural protein trichohyalin.
Biological Function of the TCHH Gene
The TCHH gene provides instructions for producing trichohyalin, an essential structural protein expressed in high amounts within the developing inner root sheath of the hair follicle as well as the filiform papillae of the tongue. Inside developing hair follicles, trichohyalin acts as a molecular cross-linking bridge between keratin intermediate filaments and itself. This dense, organized network confers mechanical rigidity, resilience, and cylindrical shape to the growing hair shaft as it keratinizes and emerges from the skin. Alterations in the trichohyalin sequence can modify the structural framework of the inner root sheath, altering the degree of hair curvature.
Research Findings and Evidence Strength
Genome-wide association studies (GWAS) have established the TCHH region on 1q21 as one of the strongest genetic determinants of hair morphology in European populations. Research conducted on cohorts of European descent revealed that common missense variants in TCHH account for approximately 6% of the variance observed between straight, wavy, and curly hair forms. The variant rs11803733 functions as part of this functional cluster affecting mechanical stability. However, the direct evidence for rs11803733 specifically, separate from neighboring markers like rs11803731, is considered limited to moderate because linkage disequilibrium across the repeat-rich region makes pinpointing the isolated effect of single missense changes challenging.
Ancestry and Population Patterns
Allele frequencies in the TCHH hair morphology cluster exhibit marked variation across global populations. Derived alleles associated with straight hair variation reach their highest prevalence in Northern European populations, where the alternate alleles commonly appear at frequencies between 30% and 40%. Conversely, these specific TCHH derived variants are found at substantially lower frequencies in East Asian populations, where hair straightness and thickness are instead predominantly driven by the EDAR and FGFR2 loci. In African populations, hair curl is a complex, multigenic trait where the European-associated TCHH haplotypes are generally rare or uninformative.
Clinical Interpretation and Practical Limitations
Having a specific genotype at rs11803733 is not a medical diagnosis and carries no direct risk of disease. Hair morphology is a complex polygenic trait shaped by multiple genetic loci alongside external factors such as chemical treatments, humidity, age, and hair length. While consumer genetic tests often report TCHH variants to describe physical traits, genotypes at rs11803733 merely reflect statistical probabilities of straight versus curly hair architecture. They cannot be used to diagnose rare structural hair disorders such as uncombable hair syndrome, which involve distinct pathogenic variants, nor do they carry pharmacogenomic implications.
How common is this variant?
The variant allele is common in populations of European ancestry, with minor allele frequencies generally observed around 30% to 40%, but it is found at markedly lower frequencies in East Asian and African populations.
Frequently asked questions
Does my rs11803733 genotype determine my exact hair type?
No. While rs11803733 is statistically linked to hair curvature, it only accounts for a fraction of total variation. Multiple genes—including EDAR, WNT10A, and various keratin loci—along with environmental factors such as humidity and hair care habits, collectively determine your observable hair texture.
Can rs11803733 cause hair loss or alopecia?
Current research does not link rs11803733 to androgenetic alopecia, alopecia areata, or general hair thinning. This variant is categorized as a trait polymorphism influencing hair shaft architecture and curvature rather than follicle survival or shedding.
Why does this variant appear mostly in European populations?
The derived alleles within the TCHH 1q21 locus appear to have expanded geographically in Europe and parts of western-central Asia through historical drift and demographic shifts. Different ancestral groups possess distinct genetic variants, such as EDAR in East Asians, that independently influence hair texture.
Is rs11803733 related to uncombable hair syndrome?
Severe, rare mutations in the TCHH gene can cause uncombable hair syndrome (characterized by spun-glass hair that cannot be flattened). However, rs11803733 is a common, benign variant associated only with standard variation in natural hair curliness, not with genetic structural disorders.
Sources & further reading
Educational information only, last refreshed 9/12/2026. Not medical advice — these associations describe population statistics, not individual predictions.
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