Is Hair Type Genetic? How Genes Determine Your Strands

Hair type is overwhelmingly genetic. Twin studies in people of European ancestry estimate that 85 to 95 percent of the variation in hair curliness traces back to inherited factors, leaving only a small fraction attributable to environment and personal habits.¹ But the genetics behind your strands are far more intricate than a single “curly” or “straight” gene. Different populations around the world have arrived at remarkably similar hair textures through entirely different genetic routes, and the total number of genes involved keeps climbing with each new study.

How Strong Is the Genetic Influence?

Heritability estimates put the genetic contribution to hair curliness between roughly 85 and 95 percent.¹ That is an unusually high figure, comparable to traits like adult height. It means that when you see siblings with nearly identical wave patterns, that is not coincidence — shared DNA is doing most of the work. A genome-wide study of over 6,600 admixed Latin Americans reinforced the point, finding significant heritability for hair shape along with other hair features like color and greying.² That study also identified 18 genome-wide significant signals for various hair and facial hair traits, including novel loci specifically tied to scalp hair shape.

What those numbers do not mean is that your environment has zero influence. Hormones, aging, nutrition, and chemical treatments can all shift how your hair looks and feels. But those influences operate within a range that your genes set. If your DNA codes for tight coils, no amount of biotin supplements or climate change will produce pin-straight growth at the root.

Different Populations, Different Genes for Similar Hair

One of the more surprising findings in hair genetics is that populations on opposite sides of the planet can have similarly straight hair for completely different genetic reasons. In East Asian populations, the primary driver is a variant of the EDAR gene called EDAR370A. This variant increases the signaling output of the receptor it encodes, which enlarges hair follicles during embryonic development and produces fibers that are thicker, straighter, and rounder in cross-section.³ A study of over 1,700 individuals from Han, Tibetan, Mongolian, and Li populations confirmed a strong association between the 370A variant and straight hair, with the effect being additive — carrying two copies doubles the odds of straight hair compared to carrying none.⁴

In Europeans, the main gene linked to straight hair is a completely different one: trichohyalin, or TCHH, which is active in the developing inner root sheath of the hair follicle. Common variants in TCHH explain about 6 percent of the variance in hair straightness among Europeans, and these variants are most common in Northern European populations.⁵ Six percent might sound modest, but for a single gene among dozens of contributors, it is a substantial chunk. Researchers have noted that the distribution of straight-hair TCHH variants in Europe parallels the distribution of the straight-hair EDAR variant in Asia, suggesting independent selective pressures may have favored straighter hair in both regions.⁵

In admixed Latin American populations, the picture gets even more layered. The large CANDELA genome study identified a novel locus influencing hair shape that includes a substitution in the gene PRSS53, a serine protease not previously linked to hair texture.² So we have at least three distinct genetic routes to shaping hair curvature across three broad population groups, and the total count of relevant loci keeps growing with each study.

What Happens Inside the Follicle

Your genes influence hair texture by shaping the follicle itself and the structural proteins the follicle produces. The follicle’s geometry matters: a round follicle tends to produce a straight, cylindrical fiber, while a more asymmetric or curved follicle pushes the growing fiber into a spiral. But it is not just follicle shape. Inside the hair fiber, two types of cortical cells — called orthocortex and paracortex — are distributed differently depending on whether your hair is straight or curly. Straight hair tends to have these cell types arranged symmetrically around the fiber’s center, while curly hair has them distributed unevenly, creating an internal asymmetry that bends the fiber as it grows.⁶

A gene expression study comparing plucked hair follicles from people with very curly versus very straight hair found 85 genes with significantly different activity levels between the two groups. The asymmetry was striking: 68 of those genes were more active in the curly-hair follicles, while only 17 were more active in straight-hair follicles.⁷ The researchers concluded that producing a curved fiber is simply more demanding for follicle cells — it requires more genetic machinery to be turned on. Straight hair, in that sense, appears to be a more metabolically “default” state, and curliness requires extra biological effort.

This internal architecture also has mechanical consequences. Curly fibers have a distinctive springlike behavior when stretched: they store mechanical energy in an initial “toe region” of their stress-strain curve that straight fibers lack entirely. Essentially, curly hair has a built-in elastic component — springiness — on top of the strength that all hair shares.⁸ This is why a tightly coiled strand can stretch quite far before snapping, and also why curly hair is more prone to tangling and mechanical damage from brushing.

Why Your Hair Texture Changes Over a Lifetime

If hair type is so heavily genetic, why do many people notice their curls loosening after childhood, or their straight hair getting wavier after pregnancy? The answer lies in how hormones and aging interact with the genetic blueprint your follicles are working from.

Hormones, particularly androgens like testosterone and dihydrotestosterone, have a powerful influence on hair follicle behavior. They bind to receptors in the follicle’s dermal papilla and can convert small, fine vellus hairs into thicker, darker terminal hairs — which is why body and facial hair often increases at puberty.⁹ But androgens and other hormonal shifts can also alter the follicle’s shape and activity cycle, which is why some women notice a change in curl pattern during pregnancy or menopause. The genetic instructions haven’t changed — the hormonal environment reading those instructions has.

Aging adds another layer. As follicle stem cells accumulate damage and undergo epigenetic changes over the years, the follicles they support can shrink, alter their cycling, or produce finer fibers.¹⁰ Many people experience their hair becoming thinner and sometimes straighter with age, not because their DNA has changed but because the follicle’s capacity to execute those instructions degrades. Chemical treatments like relaxers and keratin straightening can temporarily override texture by breaking and reforming the protein bonds within the fiber, but the effect is cosmetic — new growth from the follicle returns to whatever your genes dictate.¹¹

When a Single Gene Mutation Changes Everything

While most normal variation in hair texture is polygenic — shaped by many genes each contributing a small amount — there are rare conditions where a mutation in a single gene dramatically overrides the system. These conditions are informative because they reveal how much a single molecular disruption can alter what comes out of the follicle.

Woolly hair, an autosomal recessive condition characterized by tightly curled, fragile scalp hair from birth, has been traced to mutations in the LPAR6 and LIPH genes.¹² These genes are part of a signaling pathway involving lysophosphatidic acid, a lipid molecule that helps regulate follicle development. Mutations in LIPH can also cause hereditary hypotrichosis — abnormally sparse hair.¹³ There is notable clinical variability, meaning that different mutations in the same gene can produce woolly hair in one family and thinning hair in another, which underscores that even “single-gene” conditions are complicated by the variant’s specific location and severity.

Uncombable hair syndrome is another striking example. Children with this condition develop dry, frizzy, wiry hair that resists any attempt to comb it flat — sometimes described as “spun glass hair” because of its silvery, translucent sheen.¹⁴ The condition has been traced to mutations in three genes: PADI3, TGM3, and, notably, TCHH — the same trichohyalin gene involved in normal straight-hair variation in Europeans.¹⁵ This is a useful illustration of how the same gene can, depending on which variant you carry, either nudge your hair toward a straighter pattern or produce a dramatic shaft abnormality. The condition typically manifests in infancy and often improves spontaneously as children grow, though the exact reason for improvement remains unclear.

Can DNA Tests Predict Your Hair Type?

Given that so much of hair texture is genetic, you might expect that a DNA sample could reliably predict whether someone has straight, wavy, or curly hair. Forensic geneticists have been working toward this goal as part of broader efforts to predict physical appearance from DNA left at crime scenes. The results so far are modest.

Early attempts using just three genetic variants could identify people with straight hair with high sensitivity — about 81 percent — but the specificity was low, meaning many non-straight-haired people were incorrectly predicted to have straight hair too.¹⁶ A more comprehensive model using 32 variants from 26 different loci improved on this, achieving prediction accuracy (measured as area under the curve) of about 0.66 in Europeans and 0.79 in non-Europeans.¹⁷ The gap between those two numbers is revealing: most of the improved accuracy in non-Europeans came from the EDAR variant, which has a large individual effect size in East Asian populations. In Europeans, where no single variant dominates and the genetic architecture is more diffusely polygenic, prediction remains harder.

A separate meta-analysis of genome-wide data identified 14 independent genetic variants that significantly contribute to hair shape variation, but even combined they produced a predictive model with a cross-validated accuracy of only about 0.66.¹⁸ For comparison, DNA-based predictions of eye color routinely exceed 0.90 accuracy for blue and brown eyes. Hair shape, it turns out, is distributed across so many small-effect genetic contributions that current science can describe the general trend but not make a confident call for any individual. Adding age and sex to the models barely improved accuracy.¹⁷

EDAR and the Thickness Question

Most conversations about hair genetics focus on curl pattern, but thickness is just as heritable and has its own genetics. The EDAR gene is the best-studied example. In multiple Asian populations — Japanese, Thai, Indonesian — the 1540C variant was strongly associated with increased hair cross-sectional area, with the association holding up across different countries when the data were combined.¹⁹ Separate work confirmed that this variant spread through Asian populations through positive natural selection, meaning that at some point in the past, thicker hair conferred a survival or reproductive advantage — though what that advantage was remains debated.²⁰

The mouse experiments on EDAR made the mechanism vivid. When researchers elevated EDAR signaling activity in transgenic mice, the animals’ coat shifted to match the typical East Asian human hair profile: individual hairs became straighter, thicker, and rounder in cross-section, all produced by enlarged follicles that developed from enlarged embryonic precursors.³ The researchers described it as “the simplest of genetic alterations” producing multiple visible changes — a clean illustration of how one well-placed variant can reshape a complex trait.

Hair Diversity in Evolutionary Context

The genes that build hair fibers have deep roots. The keratin-associated protein (KRTAP) gene family, which encodes the matrix proteins that glue keratin filaments together inside each hair shaft, diversified early in mammalian evolution. Most KRTAP subfamilies were already present in the common ancestor of modern mammals, and the family has since undergone rapid divergent evolution between species — which is part of why fur and hair look so different from one animal to the next.²¹

Within humans, the diversity of scalp hair forms appears to be relatively recent. Modern humans emerged in Africa roughly 300,000 years ago, and the EDAR370A variant associated with thick, straight East Asian hair likely originated in the region of modern central China only about 30,000 years ago.²² The genetic bases and exact timing of other hair form differences across human populations remain unclear. Yet depictions of people with long, styled scalp hair appear in Palaeolithic caves, rock paintings, and carved figurines from prehistoric populations worldwide, suggesting that long hair capable of being manipulated into hairstyles was universal across early human groups even before the specific variants we study today became common.²² The genetic fine-tuning of exactly how that hair looked — its thickness, curl, and cross-section — came later, shaped by forces that researchers are still working to identify.

1
PubMed. Estimating the heritability of hair curliness in twins of European ancestry
2
PubMed Central. A genome-wide association scan in admixed Latin Americans identifies loci influencing facial and scalp hair features
3
PubMed. Enhanced ectodysplasin-A receptor (EDAR) signaling alters multiple fiber characteristics to produce the East Asian hair form
4
PubMed. The adaptive variant EDARV370A is associated with straight hair in East Asians
5
PubMed Central. Common variants in the trichohyalin gene are associated with straight hair in Europeans
6
PubMed Central. The structure of people’s hair
7
PubMed Central. Changing human hair fibre colour and shape from the follicle
8
Journal of Investigative Dermatology. Understanding Curly Hair Mechanics: Fiber Strength
9
PubMed Central. Hormonal Effects on Hair Follicles
10
PubMed Central. Toward Elucidating Epigenetic and Metabolic Regulation of Stem Cell Lineage Plasticity in Skin Aging
11
International Journal of Cosmetic Science. An overview of chemical straightening of human hair: technical aspects, potential risks to hair fibre and health and legal issues
12
PubMed Central. Mutations in LPAR6/P2RY5 and LIPH are associated with woolly hair and/or hypotrichosis
13
PubMed Central. Autosomal recessive woolly hair/hypotrichosis with homozygous mutation in the LIPH gene: a case report
14
PubMed Central. Assessment of the Genetic Spectrum of Uncombable Hair Syndrome in a Cohort of 107 Individuals
15
American Journal of Human Genetics. Mutations in PADI3, TGM3, and TCHH Cause Uncombable Hair Syndrome
16
PubMed. Evaluation of the predictive capacity of DNA variants associated with straight hair in Europeans
17
PubMed. Towards broadening Forensic DNA Phenotyping beyond pigmentation: Improving the prediction of head hair shape from DNA
18
PubMed Central. Meta-analysis of genome-wide association studies identifies 8 novel loci involved in shape variation of human head hair
19
PubMed. A replication study confirmed the EDAR gene to be a major contributor to population differentiation regarding head hair thickness in Asia
20
Human Molecular Genetics. A scan for genetic determinants of human hair morphology: EDAR is associated with Asian hair thickness
21
PubMed Central. Molecular evolution of the keratin associated protein gene family in mammals, role in the evolution of mammalian hair
22
British Journal of Dermatology. Evolution of long scalp hair in humans