Hair texture is not controlled by a single gene with a clean dominant-versus-recessive relationship, so labeling either straight or curly hair as “the dominant one” oversimplifies what is actually happening. Dozens of genetic variants scattered across the genome each nudge hair shape in one direction or another, and the combined effect of all of them, along with the physical architecture of each hair follicle, determines whether your hair grows straight, wavy, or tightly coiled. The old textbook claim that curly is dominant over straight persists in classrooms and online quizzes, but genetics research over the past two decades has moved well past that framing.
Why the Simple Dominant-Recessive Story Falls Apart
The idea that one allele for “curly” dominates one allele for “straight” comes from an era when genetics classes needed tidy examples. Hair texture was lumped in with tongue-rolling and earlobe attachment as traits supposedly governed by a single gene. The problem is that hair shape exists on a continuous spectrum. You can have pin-straight hair, loose waves, tight ringlets, coils, and everything in between. Traits that fall on a smooth gradient like this almost always involve many genes, each contributing a small effect, rather than one gene flipping a switch.
A major meta-analysis of genome-wide association studies in Europeans identified twelve loci with statistically significant links to hair shape, eight of them newly discovered and four previously known. The confirmed loci include regions near the trichohyalin gene, the WNT10A gene, and the GATA3 region, among others. The novel ones span genes involved in keratin-associated proteins, growth factor signaling, and transcription factors active in the hair follicle. No single locus came close to explaining most of the variation.
What Physically Makes Hair Curl
Before the genetics, it helps to understand what a curl actually is at the level of the hair follicle. The shape of the follicle tunnel beneath your skin plays a major role. Research using three-dimensional computer-aided reconstruction of follicles showed that follicle geometry closely tracks hair form: a helical follicle produces tightly coiled hair, while a straight follicle produces straight hair. The follicle essentially acts as a mold, and the emerging fiber takes on whatever curvature the mold imposes.
Inside the follicle, cell growth is not perfectly symmetrical. In curly hair, one side of the follicle’s growth zone produces cells faster than the other. This asymmetry extends above the lower bulb region and delays the maturation of the inner and outer root sheaths on the faster-growing side. Meanwhile, a specific hair cortex keratin accumulates unevenly, concentrating on the inner curve of the bend. In straight hair, that same keratin is distributed evenly around the circular fiber cross-section. So curl is partly a mechanical consequence of lopsided growth and lopsided protein distribution inside the follicle.
Computational modeling supports this view. When researchers simulated a growing hair fiber being pushed through a curved or pinched follicle channel, different configurations of mechanical force produced different residual strains in the fiber, which in turn corresponded to two-dimensional waves or three-dimensional helical coils. The follicle’s shape sets up the forces; the fiber’s internal structure locks them in.
The Genes Behind Hair Shape
If many genes are involved, which ones matter most? The answer depends partly on ancestry, because different populations have arrived at straight or curly hair through different genetic routes.
Trichohyalin in Europeans
In people of European descent, the strongest single genetic signal for hair shape maps to the trichohyalin gene, known as TCHH. Trichohyalin is a structural protein expressed in the inner root sheath of the hair follicle, the sleeve that guides the growing fiber. A genome-wide scan across three Australian samples of European ancestry found that common variants in TCHH explained roughly six percent of the variance in hair curliness. That may sound modest, but for a complex trait influenced by dozens of genes, it is a large single-gene effect. The straightening variants in TCHH are most common in Northern Europeans.
EDAR in East Asians
East Asian populations, where straight and thick hair is extremely common, owe much of that phenotype to a variant in the EDAR gene. A study of over 1,700 individuals from Han, Tibetan, Mongolian, and Li populations found that the adaptive variant EDAR 370A was significantly associated with straight hair, with an additive effect and roughly double the odds of having straight hair per copy of the allele. EDAR is also a major contributor to hair fiber thickness across Asian populations, linking the thick, straight hair phenotype to a single well-characterized variant. Interestingly, the TCHH straightening variants prominent in Europeans and the EDAR variant common in East Asians appear to be independent evolutionary solutions to producing straight hair.
Lipid-Signaling Genes and Extreme Curl
At the far end of the curl spectrum, mutations in two lipid-signaling genes, LPAR6 and LIPH, cause a condition called autosomal recessive woolly hair. People with woolly hair have sparse, tightly curled fibers from early childhood. Unlike the common variation controlled by many genes, woolly hair follows a clear recessive inheritance pattern: you need two copies of a disrupted gene to show the phenotype. Studies have identified multiple mutations in LPAR6 across Pakistani families and biallelic LIPH variants in Chinese patients, with some evidence that specific mutations produce milder or more severe hair loss depending on which part of the protein they disrupt. These genes encode components of a lipid-signaling pathway active in the hair follicle, and their disruption leads to abnormal follicle development.
So while the common variation in hair texture is polygenic and does not fit a dominant-recessive model, certain rare extremes of curl do behave in a classically recessive way. The distinction matters: the question most people are asking when they wonder about “dominant or recessive” is about everyday variation between family members, and for that, the answer is genuinely polygenic.
Genes Shared Across Species
One reason researchers are confident that specific genes influence curl is that the same genes show up in other mammals. Variants in KRT71, a keratin gene, cause curly coats in dogs, cats, mice, and rats, and the gene also affects human hair shape. When a second curly-coat allele was identified in dogs, researchers noted that the molecular consequence of the canine variant paralleled what was already known about KRT71 disruptions in other species. This kind of cross-species conservation is strong evidence that the gene is doing something fundamental to fiber shape rather than being a statistical coincidence in one human population.
Why Your Hair Texture Can Change
Many people notice their hair becoming curlier or straighter at different life stages, which would not make sense if hair shape were locked in by a single dominant gene at birth. Puberty is a common turning point. Hormones, especially androgens like testosterone and dihydrotestosterone, act on the dermal papilla cells of the hair follicle and can convert fine, straight vellus hairs into thicker terminal hairs. The hormonal shift does not just affect whether hair grows; it can alter follicle geometry enough to change the curl pattern of the hair that emerges. Pregnancy, menopause, and thyroid changes are other hormonal events that people commonly report as triggers for shifts in hair texture.
There is also growing evidence for epigenetic regulation of hair shape. A recent study comparing gene expression in very curly versus less curly hair follicles found that all five canonical histone genes were expressed at higher levels in very curly follicles, with some histone variants showing fold changes above five. Histones are the proteins around which DNA is wound, and changes in histone expression alter how tightly or loosely genes are packaged, which in turn affects which proteins a cell produces and in what quantities. This suggests that curl is not only about which gene variants you inherit but also about how actively those genes are being read in the follicle at any given time, a process that can shift with age, hormones, and environmental conditions.
The Evolutionary Story Behind Curl
If different populations arrived at straight or curly hair through different genetic paths, the natural question is why. For tightly curled hair, one compelling explanation is thermoregulation. Scalp hair reduces the amount of solar radiation reaching the skin, but tightly curled hair does this especially well. Experimental work using thermal manikins found that hair significantly cut down solar heat gain to the scalp, and that tightly curled hair offered the most protection compared to straighter forms. The amount of sweat needed on the scalp to balance incoming solar heat was lower in the presence of hair, meaning less water lost to cooling. In equatorial environments with intense overhead sun, tightly curled hair may have been an advantage: it creates a thicker insulating canopy with more air space between strands, reducing the thermal load on the brain while conserving sweat.
The evolutionary pressures favoring straight hair in Northern European and East Asian populations are less clearly understood. Some researchers have proposed that straight hair may be better at channeling rain away from the face in wetter climates, or that the thick, straight fibers common in East Asia provided better insulation in cold environments. These hypotheses are harder to test than the thermoregulation story, and the evidence for them is thinner. What is better established is that the EDAR 370A variant in East Asians shows signatures of strong positive selection, meaning it spread rapidly through the population because carriers had some advantage, though whether that advantage was related to hair, sweat glands (EDAR also affects sweat gland density), or something else entirely remains debated.
Rare Syndromes That Affect Hair Shape
Beyond woolly hair, a few rare genetic conditions dramatically alter hair form and illustrate how specific molecular pathways control the fiber’s structure. Uncombable hair syndrome is one of the more visually striking examples. Children with this condition have dry, frizzy, silvery-blond hair that stands out from the scalp and resists being combed flat. A study of 107 individuals with uncombable hair syndrome found that about three-quarters carried mutations in one of three functionally related genes: PADI3, TGM3, or TCHH. All three encode proteins involved in cross-linking and stabilizing the inner root sheath of the hair follicle, the same structure that trichohyalin helps build. When cross-linking fails, the inner root sheath cannot properly guide the growing fiber, and the result is a hair shaft with an abnormal cross-sectional shape, often triangular or kidney-shaped, that refuses to lie flat.
PADI3 mutations are particularly interesting because the same gene is associated with central centrifugal cicatricial alopecia, a common form of scarring hair loss that primarily affects women of African ancestry. The connection hints that the molecular machinery governing hair shape and the machinery maintaining the follicle’s structural integrity overlap substantially. Disruptions in the same pathway can produce wildly different outcomes depending on the specific mutation, the genetic background, and the follicle’s baseline architecture.
Can DNA Predict Your Hair Shape?
Forensic scientists have been trying to build predictive models that could tell investigators whether an unknown person has straight or curly hair based solely on a DNA sample. The most robust model to date uses 32 genetic variants from 26 loci to classify hair as straight versus non-straight. In Europeans, the model achieved moderate accuracy, while in non-Europeans it performed somewhat better, largely because the strong effect of the EDAR variant in East Asian and admixed populations gives the model a powerful single predictor to lean on. Adding sex and age to the model only marginally improved accuracy.
The gap between “moderate” and “reliable enough for a courtroom” is worth noting. Predicting hair color from DNA has become fairly accurate for red hair and black hair, where a small number of genes have large effects. Hair shape is harder because the genetic architecture is more diffuse: many small-effect genes, population-specific variants, and environmental modifiers all muddy the prediction. Forensic DNA phenotyping for hair shape remains a work in progress, useful as an investigative lead but nowhere near definitive.
What Textbooks Could Say Instead
The persistence of “curly is dominant” in biology classes is not malicious; it is a holdover from a time when the genetics of complex traits were poorly understood, and teachers needed simple examples. A more accurate classroom shorthand would be that hair texture is influenced by many genes, each with a small effect, and that different populations have different key players. Within any single family, one parent contributing mostly “curl” variants and the other contributing mostly “straight” variants will often produce children with wavy hair, which looks superficially like incomplete dominance but is actually the blending of many independent genetic inputs.
If you and your partner both have curly hair and your child comes out with straight hair, that is not a Mendelian impossibility. It just means the child happened to inherit a combination of common variants that, together, favor straightness over curl. The same logic applies in reverse. The trait is genuinely unpredictable from parental phenotype alone in a way that eye color, for instance, is somewhat more predictable. The sheer number of contributing loci, now numbering at least twelve confirmed and likely many more, makes hair shape one of the more genetically complex visible traits humans carry around.
The Inner Root Sheath Connection
One theme running through much of this research is the outsized importance of the inner root sheath, the cylindrical sleeve inside the follicle that surrounds and molds the growing hair fiber. Trichohyalin is expressed there. The genes disrupted in woolly hair and uncombable hair syndrome encode proteins active there. The keratin asymmetry observed in curly hair originates there. The inner root sheath hardens before the hair cortex does, essentially setting the curvature template that the softer cortex is forced to adopt as it stiffens. A 2023 study of gene expression in curly versus straight follicles reinforced this by identifying strong links to trichohyalin, a copper transporter protein called CUTC, and the inner root sheath component keratin 74, all of which operate in or near this structure.
Researchers studying hair from a biomechanical angle have found that disulfide bonds, the chemical cross-links that give keratin fibers their strength, are not equally susceptible to stress throughout the protein. Some cysteine residues, especially those near the head and tail ends of type II keratins and in keratin-associated proteins, are more easily disrupted than others, and their vulnerability changes with the fiber’s water content. This helps explain why humidity can temporarily alter curl pattern: water disrupts hydrogen bonds between proteins in the cortex, allowing the fiber to relax or tighten depending on its internal stress distribution. The permanent shape, though, is set by the follicle and locked in by disulfide bonds that form as the fiber hardens above the skin surface.