Redheads do seem to turn up with wavy or curly hair more often than you would expect by chance, and the reasons are rooted in overlapping genetics rather than any single “red-and-curly” gene. The MC1R gene that gives hair its red pigment does not directly control curl, but several genes that influence hair shape also affect hair color, and the populations where red hair is most common happen to carry curl-promoting genetic variants at high frequency. The connection is real, though subtler and more tangled than most people assume.
What Makes Hair Red in the First Place
Hair color comes down to two pigments produced by melanocytes deep inside the hair follicle. Most people’s hair is dominated by eumelanin, which ranges from brown to black. Red hair stands apart because it contains roughly equal levels of eumelanin and a reddish-yellow pigment called pheomelanin, a ratio found in no other natural hair color.1PubMed. Diversity of human hair pigmentation as studied by chemical analysis of eumelanin and pheomelanin That unusual pigment balance is driven primarily by variants of the melanocortin-1 receptor gene, MC1R. When MC1R carries certain loss-of-function variants, the melanocytes in your follicles shift production away from eumelanin and toward pheomelanin, and you end up with red hair. The effect is dose-dependent: carrying two of these variants almost always produces red hair, while carrying just one can nudge hair toward auburn or strawberry blonde without going fully red.
Those same MC1R variants also affect the skin, which is why red hair typically comes packaged with fair skin, freckles, and sun sensitivity. Melanocyte precursors migrate into the developing follicle during embryonic growth, eventually settling just above the dermal papilla, where they pump pigment into the growing hair shaft for the rest of their lives.2Journal of Investigative Dermatology Symposium Proceedings. Fate of melanocytes during development of the hair follicle pigmentary unit Because pheomelanin is chemically different from eumelanin, it changes the internal structure of the fiber itself, not just its color. That structural difference turns out to matter for the curl question.
How Hair Gets Its Curl
Hair curvature is not determined by a single gene. It is a polygenic trait, meaning dozens of genetic variants each nudge the follicle toward straighter or curlier output. The most influential gene identified so far in people of European descent is trichohyalin, or TCHH, which encodes a protein in the inner root sheath of the hair follicle. Certain TCHH variants account for roughly six percent of the overall variation in whether Europeans have straight, wavy, or curly hair.3PubMed Central. Common variants in the trichohyalin gene are associated with straight hair in Europeans Six percent may sound small, but for a complex trait shaped by many genes and environmental factors, one gene explaining that much is a strong signal.
Beyond TCHH, a large meta-analysis identified eight additional loci linked to hair shape in Europeans, including genes involved in keratin-associated proteins, growth-factor signaling, and enzymes that modify proteins in the developing hair shaft.4Human Molecular Genetics. Meta-analysis of genome-wide association studies identifies 8 novel loci involved in shape variation of human head hair Other work has pointed to a copper transporter gene called CUTC and keratin 74, both active in the inner root sheath, as additional contributors.5Experimental Dermatology. The biology and genetics of curly hair The inner root sheath acts as a mold that guides the growing hair fiber. When its composition is asymmetric, the fiber bends, and the degree of bending determines whether you get a loose wave or a tight coil.
The Genetic Overlap Between Hair Color and Hair Shape
Here is where the story gets interesting for redheads specifically. When researchers combed through genome-wide data on hair color in the UK Biobank, they discovered that seven of the loci originally identified as hair-shape genes also significantly influenced hair color. The overlapping genes include ERRFI1, FRAS1, HOXC13, PADI3, KRTAP, PEX14, and LGR4.6Nature Communications. Genome-wide study of hair colour in UK Biobank explains most of the SNP heritability In other words, variants in these genes do not just shape how curly or straight your hair grows; they simultaneously push your pigmentation in one direction or another. This kind of genetic multitasking, where a single gene affects more than one trait, is a key reason that hair color and hair texture tend to travel together in populations.
To be precise, the UK Biobank study measured the overlap with “blonde versus non-blonde” classification, not red hair directly. But the principle holds broadly: the genetic machinery that builds the hair follicle, assembles keratin fibers, and shapes the inner root sheath also feeds into how pigment gets deposited. When you inherit a particular constellation of variants across these shared loci, you don’t just get a hair color or a hair texture in isolation. You get a package deal, and some packages happen to pair red pigment with a tendency toward waves or curls.
Why Northern European Ancestry Matters
Red hair is overwhelmingly concentrated in populations of Northern European descent, peaking in Ireland, Scotland, and parts of Scandinavia, where somewhere between roughly four and thirteen percent of people are natural redheads depending on the region. The TCHH variants that promote straight hair in Europeans follow a strikingly similar geographic gradient. Those straight-hair variants reach their highest frequency in Northern Europeans.3PubMed Central. Common variants in the trichohyalin gene are associated with straight hair in Europeans This might seem like it would work against redheads having curly hair, not for it. But population genetics is rarely that tidy.
The straight-hair TCHH variants are common but not universal in Northern Europeans, and they explain only a fraction of total hair-shape variation. The remaining variation comes from those other loci, many of which overlap with hair-color genes. In a population where MC1R red-hair variants, TCHH variants, and the hair-shape/color-overlap loci are all circulating at high frequency, you get a wider range of trait combinations than you would in a population where these variants are rare. Some combinations yield straight red hair; others yield curly red hair. The point is that the opportunity for red-and-curly combinations is highest in the same Northern European gene pool where all these variants are concentrated. If red hair were equally common across all human populations, the association with curl might look quite different or might not exist at all.
The Pheomelanin Factor
There is also a physical, non-genetic dimension worth considering. Red hair fibers are chemically distinct from blonde, brown, or black fibers. While all non-red hair is dominated by eumelanin, red hair has a high pheomelanin content and relatively low eumelanin.7International Journal of Cosmetic Science. How different is human hair? A critical appraisal of the reported differences in global hair fibre characteristics and properties towards defining a more relevant framework for hair type classification Pheomelanin granules differ in size and shape from eumelanin granules, and because pigment granules are distributed throughout the cortex of the hair shaft, they affect its internal geometry. When the distribution of granules inside the cortex is uneven, it can contribute to mechanical asymmetry in the fiber, the same kind of asymmetry that causes bending and curl.
This does not mean pheomelanin alone makes hair curl. Plenty of redheads have poker-straight hair. But when the genetic tendency toward curvature is already present from TCHH, KRTAP, or any of the other shape loci, the unusual fiber composition of red hair may amplify the visible effect. Think of it this way: a hair shaft that is already slightly asymmetric from follicle architecture might look straighter when it is thick and rigid with eumelanin, but wavier when its internal composition is different. The perception of curl in redheads could be partly a matter of the same mild curvature being more visible or more pronounced in a pheomelanin-rich fiber.
Red hair also tends to have distinctive texture independent of curl. Many redheads describe their hair as coarser, thicker per strand, or more resistant to straightening than the hair of siblings or friends with other colors. While these observations are partly anecdotal, they align with what we know about how pigment chemistry alters the internal structure of hair fibers. The physical feel of the hair reinforces the impression that something about red hair is “different,” even when the actual curvature is moderate.
Ancient DNA and Hair Trait Connections
One of the more unexpected threads in this story comes from archaic human DNA. A study of Neanderthal-introgressed alleles in modern Europeans found that inherited Neanderthal variants affect skin tone and hair color, among other traits.8PubMed Central. The Contribution of Neanderthals to Phenotypic Variation in Modern Humans What surprised researchers was that multiple Neanderthal alleles at different locations in the genome contributed to both lighter and darker skin and hair, suggesting that Neanderthals themselves were probably variable in these traits rather than uniformly dark or light.
Separately, analysis of archaic introgression across different populations found a Neanderthal-introgressed variant specifically associated with red hair color in Europeans.9PubMed Central. Denisovan and Neanderthal archaic introgression differentially impacted the genetics of complex traits in modern populations This does not mean Neanderthals “gave us” red hair, since MC1R variants in modern humans appear to have arisen independently. But it does show that the genetic landscape influencing hair traits in Europeans has been shaped by ancient admixture events. If archaic introgression contributed variants that nudge both pigmentation and hair morphology, it adds yet another layer to why certain trait combinations cluster in populations with Neanderthal ancestry.
Is the Association as Strong as People Think?
Probably not. Human perception is notoriously biased by salience: red hair is rare and visually striking, so when you see a redhead with a head of curls, the combination is memorable. A brown-haired person with similar waves might not register as “curly” in your mind the same way. This kind of perceptual bias inflates the apparent strength of the association beyond what the genetics alone would predict.
No large-scale study has directly measured the correlation between red hair and hair curvature as a primary outcome. The evidence for the connection is all indirect: shared loci, overlapping population distributions, and plausible physical mechanisms. That indirect evidence is solid enough to say the association is not pure coincidence, but it falls short of proving that being a redhead makes you meaningfully more likely to have curly hair compared to, say, a dark-haired person from the same Northern European population carrying many of the same curl-associated variants.
What we can say with confidence is that the genetic architecture of hair color and hair shape is more intertwined than was appreciated even a decade ago. The old model, in which separate genes independently determined color, texture, thickness, and curliness, has given way to a picture where many of these traits share regulatory machinery inside the follicle. Red hair may be a particularly visible marker of a broader constellation of follicle-biology variants, and curl is one of the traits that comes along for the ride.
Why Red Hair Also Seems to Come with Specific Curl Patterns
Anecdotally, hairdressers and redheads themselves often report that red curly hair behaves differently from curly hair in other colors. It can be more resistant to humidity-driven frizz in some cases, or more prone to it in others, depending on the individual. The explanation likely circles back to pheomelanin. Melanin granules contribute to the mechanical stiffness and moisture-absorption properties of the hair cortex. Because pheomelanin is structurally distinct from eumelanin, red hair may respond differently to the environmental triggers that make curly hair expand, shrink, or change shape throughout the day.
None of this has been rigorously studied at scale, so take it with appropriate skepticism. The science of hair curvature is still catching up to the science of hair color. Geneticists have mapped the major loci involved, but how those genetic instructions translate into the precise cross-sectional shape, twist rate, and mechanical properties of a single strand remains only partly understood. Red hair, because it is rare and biochemically unusual, has been underrepresented in the fiber-property studies that do exist. Until someone runs a well-powered study comparing curvature distributions in redheads and non-redheads from the same population, the precise magnitude of the association remains an open question.
Follicle Biology and Why Hair Traits Are Hard to Separate
The deeper reason hair color and hair shape resist clean separation is that both traits emerge from the same small structure: the hair follicle. Within a follicle, melanocytes pump pigment into growing keratinocytes while the inner root sheath simultaneously shapes the fiber. The genes expressed in the inner root sheath, like TCHH and keratin 74, operate in close proximity to the melanocyte niche. When genetic variants alter the development or protein composition of the inner root sheath, they can indirectly affect how melanocytes deposit pigment, and vice versa. Melanocyte precursors proliferate and migrate down the developing follicle before settling in their final position above the dermal papilla.2Journal of Investigative Dermatology Symposium Proceedings. Fate of melanocytes during development of the hair follicle pigmentary unit Any disruption to follicle architecture during that migration can have cascading effects on both pigmentation and fiber shape.
This biological reality means that trying to ask “does the red-hair gene cause curl” is the wrong question. MC1R itself does not shape the follicle’s curvature. But the follicle is a tightly integrated organ, and the variants that influence one output of that organ frequently influence others. Red hair is one visible output. Curl is another. They share a factory, and some of the factory’s blueprints affect both production lines at once. That shared factory, more than any single genetic switch, is the most honest answer to why your redheaded friend probably has at least a wave in their hair.