IBeyond Dominant and Recessive

Many people learned hair color the way they learned eye color at school: a dominant dark version and a recessive light one, sorted in a neat square of possibilities. It is a useful introduction to inheritance. As a description of hair color, it is mostly wrong.

Hair color is what geneticists call a polygenic trait. Rather than one gene with two versions, many genes each make a small contribution, and their effects add up. The result is not a set of separate categories but a continuum, from the deepest black through every shade of brown to the palest blond, with each person sitting at some point along it.

Underneath that continuum sit only two pigments. Eumelanin, brown to black, sets how dark a strand reads. Pheomelanin, red to yellow, adds warmth. How the follicle makes both is set out in the plain guide to hair pigment. Genetics determines how much of each a person's follicles are inclined to produce.

There is one partial exception to the polygenic picture, and it is the shade people most often ask about. Red hair behaves much more like the simple school model than any other color, for reasons that come down largely to a single gene.

IIHundreds of Small Effects

The scale of the genetics became clear in 2018, when two large studies appeared within the same year. One, led from King's College London and drawing on data from close to 300,000 people of European ancestry, identified 124 genetic regions associated with hair color, most of them newly reported. The other, from the University of Edinburgh, analyzed hundreds of thousands of UK Biobank participants and identified around 200.

The Edinburgh team also asked how much of the inheritance their findings accounted for. Their answer was that the regions identified captured most of the variation in hair color that common genetic variants are estimated to explain, a notable result for a trait this complex.

Some of the genes involved sit directly in the pigment pathway: TYR, which encodes the enzyme that begins melanin production, along with TYRP1, OCA2, SLC45A2 and others. Many more have no obvious connection to pigment at all, and may act on the development of pigment cells, the structure of the follicle or the signals passing between them. Two of the better known, KITLG and TYRP1, appear in hair color around the world, where each has its own regional story.

Each of these variants nudges the result a little. A person's hair color is the sum of many such nudges, inherited half from each parent, which is why it varies so finely and why families can produce such a range of shades.

IIIRed, the Clearest Case

Red hair is where the school model comes closest to the truth. Most red hair traces to variants of MC1R, a gene encoding a receptor on pigment cells that tilts production between eumelanin and pheomelanin. When both copies of the gene carry variants that dampen the receptor, production leans toward pheomelanin, and hair tends to grow in red.

That has a familiar consequence. A person can carry one variant copy without having red hair, and pass it on without knowing. When two such carriers have a child, the simplified recessive picture gives roughly a one in four chance with each child of inheriting two variant copies. This is why red hair can appear in a family after skipping a generation or more, and why two dark-haired parents can have a red-haired child.

Carriers are not always invisible. People with one variant copy are more likely to freckle, and some have auburn or coppery tints in an otherwise brown head of hair. Variants also differ in effect, some dampening the receptor more than others. The Edinburgh work added that genes elsewhere in the genome also influence whether a person with MC1R variants ends up with red hair, so even this clearest case is not entirely simple.

The arithmetic also settles a persistent myth. Headlines periodically predict that red hair is disappearing. Because the variants are carried silently by many more people than show the trait, they persist from generation to generation, and there is no evidence that red hair is dying out.

IVWhy Children Differ From Their Parents

Every child receives half of each parent's genetic material, and which half is reshuffled for every child. With a single gene, that reshuffling produces a small set of outcomes. With well over a hundred regions, it produces a spread. Brothers and sisters can land at noticeably different points on the continuum while sharing the same parents.

That spread explains most family surprises. Two parents with medium brown hair can carry, between them, variants that pull lighter and variants that pull darker. A child who happens to inherit more of the lighter ones may be noticeably fairer than either parent; a sibling who inherits more of the darker ones may be noticeably darker. Two blond parents usually have light-haired children, but a darker child is uncommon rather than impossible.

Timing adds a second layer. Many children who are blond in their early years darken through childhood and adolescence, as eumelanin production in the follicle rises, so a child's hair color at five is not a reliable guide to it at fifteen. That pattern is covered in at what age hair usually changes.

Hair and eye color add a third. Because genes such as OCA2 and HERC2 influence both, they tend to travel together, which is why light eyes are more common with light hair. But they are not fully linked, and combinations such as dark hair with blue eyes are common.

VReading Hair Color From DNA

If hair color is inherited, it should be possible to estimate it from DNA, and researchers have done so with increasing precision. In 2013, a team at Erasmus MC in Rotterdam, led by Manfred Kayser, published a system that estimates both hair and eye color from a small panel of DNA markers, building on the genes described above.

The system performs unevenly across shades, and its developers have been open about it. Red is estimated most reliably, because a single gene carries much of it, and black, at the dark end of the continuum, is also estimated relatively well. Blond and brown are harder, because they sit in the crowded middle of the continuum, where many small effects overlap.

Methods of this kind are now used in forensic investigations in some countries where the law permits, and in ancient DNA research. The 2018 reconstruction of a roughly 10,000 year old individual from Cheddar Gorge in England, described in the world map of hair color, drew on this kind of prediction to estimate dark hair and light eyes.

The estimates describe a likely baseline, not a portrait. DNA cannot say whether a person's hair darkened in childhood, has begun to gray or has been colored, and every prediction is expressed as a probability rather than a certainty. Studies described here were conducted independently and did not involve any Codeage product.

VIWhat Genetics Does Not Settle

Genes set the range of shades a person's follicles are inclined to produce. They do not fix a single color for life. The follicle's output shifts across the decades, and the same inherited baseline can look quite different at five, twenty-five and sixty-five.

Graying is the clearest example. Its timing is itself partly inherited, and a 2016 study led from University College London reported the first gene variant associated with it, in IRF4, a gene also involved in pigment. But graying draws on a separate set of processes in the follicle, set out in why hair turns gray, which unfold over time rather than being fixed at birth.

The strand also changes after it leaves the follicle. Sunlight oxidizes melanin and lightens the oldest lengths of hair, and chemical coloring alters what is already there. None of this touches the genes, which is why new growth at the root returns to what the follicle is currently producing.

Genetics, in short, describes where a person starts and the range they move within. What the follicle does with that inheritance unfolds across a lifetime.

VIIWhere This Fits

Hair is a structural material, built from keratin and anchored in tissue that renews in cycles across a lifetime, which is why its genetics belong in the Codeage library beside the biology of keratin. It sits within Structural Integrity, the second pillar of The Longevity Code. A pillar names a location in a framework. It describes a category, not an outcome for any reader.

One distinction bears stating plainly. The genetics described here concern inheritance: how variants are passed down and how they shape pigment production. They do not describe any dietary ingredient changing the color of hair, and Codeage does not present any formula as acting on it.

What genetics offers instead is a way to read a family resemblance more precisely. A child's hair color is not a copy of either parent's. It is a new draw from both, and in that sense one of the most personal things a person inherits.