Why do some people have blond hair?

Most blond Europeans owe their hair colour to a single DNA letter change in a regulatory enhancer near the KITLG (KIT Ligand) gene. The substitution lowers KITLG activity specifically in hair follicles, which reduces melanin production and produces light hair without affecting eye colour, skin pigmentation, or other traits driven by the same gene elsewhere in the body. Guenther, Kingsley, and colleagues identified this exact variant in 2014.

Is blond hair a recessive trait?

In the classical Mendelian sense, blond hair behaves largely as a recessive trait: two copies of the lighter-hair variant tend to produce a clearly blond phenotype, while a single copy plus a darker-hair allele usually yields an intermediate brown. Hair colour is polygenic, though, so the recessive/dominant framing is a simplification of what is really a quantitative trait with multiple contributing loci.

Is blond hair a genetic mutation?

Yes. The classic light-hair variant in Europeans is a single-nucleotide substitution (an A-to-G change) in a regulatory enhancer about 350,000 base pairs from the KITLG gene. Because the change sits in an enhancer rather than the gene itself, it only affects expression in specific tissues (mainly hair follicles), which is why the lighter pigmentation is largely confined to hair.

Where did blond hair originate?

The KITLG light-hair variant probably arose in northern Europe during the late Pleistocene, perhaps as recently as 11,000-18,000 years ago, after the last glacial maximum. The same gene region has independent light-pigmentation variants in some Pacific populations (notably parts of Melanesia, where blond hair is also seen in dark-skinned populations), indicating convergent evolution rather than a single global blond mutation.

Does the blond gene also affect eye colour?

Not directly. Eye colour is controlled mainly by genes such as OCA2 and HERC2 on chromosome 15. The KITLG enhancer variant linked to blond hair changes KITLGs activity in hair follicles specifically, so it does not pull eye colour along with it. That is why blond hair pairs perfectly well with brown, hazel, green, or blue eyes.

Why Do People Have Blond Hair?

Table of Contents (click to expand)

Of Mice And Men
The ‘Color’ Gene

Most blond Europeans owe their hair colour to a single DNA-letter change in a regulatory enhancer near the KITLG (KIT Ligand) gene, identified by Guenther, Kingsley and colleagues (Nature Genetics, 2014). The variant turns down melanin production specifically in hair follicles, producing lighter hair without affecting eye colour or causing other side effects. It behaves as a near-recessive trait, which is why blond children often appear in families of dark-haired parents.

In June of 2014, an evolutionary geneticist and his team discovered that if a single DNA base pair was changed, if one rung was renamed from an A to a G, when just one letter among the billions that compose the book that is you is miswritten, the appearance of its cover is dramatically altered. This single mutation differentiates blonds from brunettes!

brunette hair and blond hair — (Photo Credit : Pixabay and Pexel)

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Of Mice And Men

Biologists already discovered several decades ago that color, whether of our eyes, hair or skin, is a function of the amount of melanin in our body. Melanin is the pigment with which our hair, skin and eyes (to some extent) are painted. A high level of melanin causes darkened skin and hair, whereas its scarcity causes them to lighten. Northern Europeans have blond hair and fair skin because they contain less melanin than the dark-haired and dark-skinned Africans.

However, David Kingsley, an evolutionary geneticist at Stanford University in California, wanted to discover the underlying fundamental genetic mutations that led to the production or curtailment of melanin. Bear in mind how excruciatingly difficult this venture was. David had to search for a single gene, for the tiniest variations of base pairs among the 20,000 genes or the innumerable base pairs that constitute the human genome.

DNA replication. — A strand of DNA. (Photo Credit: Pixabay)

However, David did successfully find it. After six years of arduously examining the genetic variation in thousands of people from Iceland and the Netherlands, David and his colleague Catherine Guenther zeroed in on a single DNA letter change about 350,000 base pairs downstream of a gene called KITLG (KIT Ligand), in a regulatory enhancer region. That single base swap (an adenine for a guanine) is what flips light-haired Europeans away from their dark-haired ancestors. Unsurprisingly, a number of variations were exhibited by genes that were directly or indirectly involved in the production of melanin.

To test his discovery’s veracity, David turned to mice, of course. David and his team found the region of DNA in mice that caused them to appear brown. In mice of lighter colors, some even white, the code was written backwards! Following this splendid discovery, the researchers created two variations of the human version of that DNA: while one variant was left unchanged, the other was changed and written just as it appears in a brunette. The researchers then inserted each variant into two different mice.

brown mice and white mice — (Photo Credit: PxHere)

As one would expect, the mice into whom the changed DNA was planted grew to be darker than the mice into whom the unchanged DNA was planted. Guenther, Kingsley, and colleagues published their findings in Nature Genetics on June 1, 2014.

The ‘Color’ Gene

Every discipline under the umbrella of evolutionary biology is certain that a trait is never the result of one mutation of a single gene. They are certain that a tweak of this magnitude cannot have such a profound effect. A trait is instead an inscrutable combination of several genes activating and deactivating, seemingly indiscriminately. For this reason, the color gene might not even exist. However, David discovered the signaling gene — a single gene that is pivotal to the combinatorial play of genes that produces melanin. It puts the play in motion. In a way, one can call it the color gene, but it only paints our hair and skin, not our eyes.

Gwyneth Paltrow (Photo Credit: Wikimedia Commons)

This is because the activation and deactivation of the signaling gene affects other genes as well. It affects genes responsible for the formation of blood, egg, sperm and stem cells. If the signaling gene was to switch on or off to kick-start the production of blond hair, the consequences would have been devastating. However, this isn’t the case.

David explains that “this isn’t ‘a turn the switch off’, it’s a ‘turn the switch down.'” That is, it’s not binary, but can be modulated like a thermostat. Consequently, the effects of the DNA mutation, David and his team discovered, don’t seep too deep, but alter only the properties of a single organ – our skin. It is the only way no fatal harm can be incurred. This discovery demolishes the archaic stereotype that whatever is responsible for hair color also determines the color of our eyes. It’s “literally skin deep,” David concludes.

References (click to expand)