Is curly hair dominant or recessive? What genetics research shows

Curly hair is neither dominant nor recessive in the simple sense, because it is shaped by many genes that each add a small effect, not by one gene with a dominant and a recessive version. Curl is still strongly inherited: twin studies find that genes account for most of the differences in curliness between the people they studied. The familiar rule that “curly hair is dominant” goes back to a family study published in 1908, and genetic research since then has not confirmed it for ordinary hair.

Dominant and recessive: what the words mean

A gene is a stretch of DNA, and many genes carry the instructions for one of the body’s building blocks. You inherit two copies of most genes, one from each parent, and the copies can be slightly different versions. A version is dominant when one copy is enough for its effect to show, and recessive when its effect shows only if both copies are that version.

Hair shape follows this pattern only in a few rare conditions. One form of woolly hair, a rare disorder of very tightly curled hair, was traced in a 2010 genetics study of one family from Pakistan to a single gene, KRT74, where one altered copy was enough to cause it. Uncombable hair syndrome is recessive in most known cases: a 2016 genetics study found both copies of one of three genes altered in each of 11 affected children, and a 2022 study noted that only about 100 cases have been reported. The inheritance of such rare conditions does not show how ordinary curls are passed on.

Is curly hair genetic?

Curly hair is genetic in the sense that genes explain most of the differences in curliness between people in twin studies, although no single gene decides it. MedlinePlus Genetics, a service of the US National Library of Medicine, says genetic factors appear to play a major role in hair texture.

Researchers measure this with heritability: the share of the differences between people in a particular population that comes from differences in their genes. It describes a group, not a percentage of one person’s hair. In a 2009 study of twins of mostly European ancestry, who rated their own hair as straight, wavy or curly, the heritability of curliness was 85 to 95 per cent. When a 2016 study measured hair curvature with an optical instrument in 2,332 Australian twins and siblings, the heritability came out lower: 53 per cent in men and 61 per cent in women.

Is curly hair dominant or recessive? Where the idea comes from

The idea that curly hair is dominant goes back to a study of hair form in families, published in 1908 in the journal The American Naturalist. As a 2010 genetics study by the DNA-testing company 23andMe summarised it, the 1908 authors concluded that straight hair was recessive to curly hair.

The single-gene idea later spread into teaching as incomplete dominance: two “curly” copies give curly hair, one of each gives wavy hair and two “straight” copies give straight hair. The Tech Interactive’s Ask a Geneticist service used this model in a 2004 answer, and in 2021 added an editor’s note describing hair texture as a complex trait, one “affected by many different genes plus environmental factors”.

Which genes are linked to curly hair?

Genome-wide studies scan the DNA of thousands of people for differences that are more common in those with a given trait. A 2018 analysis of almost 29,000 people from nine study groups identified 12 regions of DNA linked to hair shape in people of European ancestry, eight of them new; the authors noted that the genes found before their study explain only a small part of the differences in hair shape.

In Europeans, the best-known of these genes is TCHH, a gene active in the hair follicle, the small pocket in the skin from which each hair grows. In a 2009 genome-wide study of Australians of European descent, TCHH explained about 6 per cent of the variation in hair shape.

Other populations point to other genes. A 2016 study of more than 6,000 Latin Americans found a hair-shape region in a gene called PRSS53, also active in the follicle. In East Asian populations, a common version of a gene called EDAR has been linked mainly to thicker hair in 2008 studies of Thai, Indonesian and Japanese people, and to straighter hair in later genetic studies. Thicker here means wider strands, which our guide to coarse hair explains.

Even together, known markers predict hair shape only modestly: a 2018 forensic study that used 32 DNA markers to sort people into straight or non-straight hair got it right more often than chance but far from always, and somewhat more often in people of non-European ancestry than in Europeans.

What makes hair curly

Curl begins in the follicle. In a 2005 study of scalp samples from African volunteers, follicles that grew curly hair were curved, and their base, called the bulb, was bent like a golf club, while the follicles of straight-haired volunteers of European descent were straight. Curly follicles grown in the laboratory kept producing curved hair, which points to the follicle setting the shape.

The two sides of a curly hair are also built differently. As a hair forms, its cells go through keratinisation: they fill with keratin, the hair’s main protein, and harden. One proposed explanation is that this process runs unevenly across the two sides of a curly hair. A 2009 microscope study of Japanese hair found the different cell types spread evenly in straight hairs but split between the two sides in curved ones.

Can straight-haired parents have a curly-haired child?

Two straight-haired parents can have a child with curlier hair than their own, because curl depends on many genes with small effects and each child inherits a new mix of versions from both parents. That is why a child’s curl cannot be predicted reliably from the parents’ hair.

Curly hair can seem to skip a generation for the same reason: parents with straight hair can still pass on gene versions that push hair towards curl, and in their children these can add up differently. You inherit hair-shape genes from both your mother and your father, and genetic studies of hair shape give no sign that one parent’s hair decides a child’s curl.

Can your curl pattern change?

Your curl pattern can change over time even though the follicle largely sets it, because hormones, some medicines, age and chemical treatments such as perms or relaxers can all alter hair texture, according to the US National Library of Medicine. Lab studies show that bleaching also changes how readily hair takes up water, which is what a hair porosity test tries to gauge.

A 2009 study of 230 Japanese girls and women aged 10 to 70 found that hair became more irregularly curved with age, and the 2009 twin study saw hints that curliness increased slightly with age. Changes after some medicines, such as retinoids (a group of skin medicines) and certain cancer drugs, are known mainly from reports of single patients or small groups. Many people describe their curls changing after pregnancy or around puberty, but this has been little studied, so how often it happens and why is not known.

Some change may only be apparent: length, weight and styling can alter how the same pattern looks, as our guide to curly hair types describes. Curls you create with heat or rollers are a different matter: they are temporary, and our guide on why hair won’t hold a curl explains why they drop.

Caring for the curls you were born with

Genes and follicles set the curl pattern; everyday care cannot rewrite it, only help keep the curls you have well defined, and the night is part of that. Our guide on how to sleep with curly hair sets out how to gather curls loosely before bed, and our guide to curl types has a section on night care by curl type for waves, curls and coils. If your hair is wavy rather than curly, our guide on how to sleep with wavy hair covers the looser patterns.

Frequently asked questions

Is curly hair dominant or recessive?

Curly hair is neither simply dominant nor simply recessive: it is shaped by many genes, each with a small effect, and the single-gene rule traces back to a 1908 family study that later research has not confirmed for ordinary hair.

Is curly hair genetic?

Curly hair is largely genetic: a 2009 study of twins of mostly European ancestry put the share of curl differences explained by genes at 85 to 95 per cent for self-rated curliness, and a 2016 study of Australian twins and siblings put it at 53 to 61 per cent for measured curvature.

Can curly hair skip a generation?

Curly hair can seem to skip a generation, because parents with straight hair can pass on genes that push hair towards curl, and these can add up to curlier hair in their children.

Can your hair become curly later in life?

Hair can become curlier later in life, since hormones, some medicines and age can all change hair texture, although most specific triggers are known only from case reports and small studies.

Sources

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