In the most simplistic sense, hair can be divided up into dark and light, with dark hair being dominant and appearing more frequently than lighter shades. However, differences in shading can come from different sources and some colors, like red, have completely different genes involved in their appearance. Scientists are still working out many of the details of the genetics of hair color.
Dominant Dark Hair Genetics
Dark hair is the most common kind in the world, and many people presume it is a pure matter of dark hair being dominant while light hair is recessive. Of course, the truth is far more complex, as anyone can tell simply by noting the different shades of dark hair, from brown to black, found around the globe.
In reality, dark hair comes from the pigment eumelanin, and the amount of eumelanin in the hair determines exactly how dark the hair will be. So hair color is more a case of either codominance or incomplete dominance, but scientists don't know which it is yet.
To make things even more complicated, eumelanin comes in two different types, black and brown, which give a range from grey to black and from blonde to dark brown respectively.
Recessive Blonde Hair Color the Result of Many Genes
Blonde hair is the result of low levels of eumelanin, which is why it is less common than darker shades of hair color. Since any amount of eumelanin will show up in hair as brown or black, the only way to get truly blonde hair is to inherit only genes that don't produce eumelanin. The less eumelanin, the lighter the hair.
The Red Hair Gene and Mutations in the Mcr1 Gene
Red hair operates a little differently than the other colors. Pheomelanin is the important pigment in operation here and Mcr1 is the gene that controls it.
For people with two normal copies of Mcr1, pheomelanin production is stifled and hair is whatever color the other genes dictate: black, brown or blonde.
When an abnormal Mcr1 gene is there, pheomelanin runs free, giving a red tint to everything. There are actually nine known variants of abnormal Mcr1, any of which can combine together to influence hair color.
When this comes into play, the red color interacts with the other hair colors to give different shades of red. Two altered Mcr1 genes often brings out a strong red color, except for when high levels of black eumelanin cover it up. A single altered Mcr1 gives enough pheomelanin to tint blonde hair into strawberry blonde, but usually not enough to overcome black or brown color visually.
The red hair variants of Mcr1 are also linked to traits such as freckles, pale skin, an inability to tan and even a resistance to anesthesia.
Red Hair and Blonde Hair Aren't Disappearing
There have been many myths and rumors about the imminent disappearance of red and blonde hair, but the truth is, these colors will remain part of the human range of hair color for a while yet.
Because both of these colors require a certain threshold of pigment expression to be inherited, they exist at low levels in the population. However, many people carry the genes for blonde or red. They're just hidden or masked by black or brown hair color, so no one knows that these genes are secretly there and waiting to come out in some future generation.
For more information on genetic inheritance and how it works, see the articles Understanding Dominant and Recessive Traits and Incomplete Dominance and Codominance.
Further Resources:
Graeme O'Neill. Genetic interactions determine hair and skin colour. Life Scientist. April 23, 2007.
Edwin B. Liem, M.D, et al.
Anesthetic Requirement is Increased in Redheads. Anesthesiology. 2004 August; 101(2): 279–283.
Sulem P et al. (2007) . “Genetic determinants of hair, eye and skin pigmentation in Europeans.” Nat Genet 39(12):1443-52.
Bridget Coila - I'm a cell and molecular biologist, freelance writer and photographer currently living in Beijing, China. I'm fascinated by science, ...
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