How come blue became the most prevalent eye color/gene in northern Europe, despite being a recessive gene and one that was introduced only about 10,000 years ago?
-A curious adult from California
May 14, 2009
You're right, blue eyes are a recessive trait. But traits aren't called recessive based on how common they are. Recessive vs. dominant has to do with inheritance (click here to read more).
Some recessive traits are common, like blue eyes in Northern Europe. And some dominant ones like dimples are more rare.
So how does a trait become the norm? One way is if it gives some advantage.
Having blue eyes most likely helped ancient people in Europe stay healthy. This meant they could live longer and have more children. Through the generations, blue eyes became more and more common. That's called natural selection.
On the other hand, if a trait doesn't have an effect on survival, it will probably stay rare. That's why most people don't have dimples, even though it's a dominant trait.
Blue Eyes, Light skin, Vitamin D, and Health
Ok, so blue eyes helped ancient Europeans survive better. But how? By being associated with light skin. (We'll come back to how they are linked in a second.)
Light skin meant getting the most sun possible in dark, northern Europe. And sun was the only way to get vitamin D 10,000 years ago.
Vitamin D is important for good health. It helps the body use calcium to make strong bones. Without enough vitamin D, people can also get a nasty disease called rickets.
Way up north where there wasn't much sun, light-skinned people could get enough sunlight so their bodies could make vitamin D. The darker-skinned people got less sun and were less healthy.
Because the lighter-skinned people were healthier, they had more children than those with darker skin. The next generation, then, had more light-skinned people in it. These people were healthier, had more kids, and so on and so on.
Every generation afterwards had more and more light-skinned people. Because light skin is linked to light eyes, blue eyes became more and more common, too.
But just how are eye color and skin color associated? Through genes that control the pigment melanin.
Skin and Eye Colors Depend on Genes for Making Melanin
Pigments are like dyes: the less of them you have, the lighter the color. This means that the amount of the pigment melanin in your eyes, hair, and skin determine how dark each will be.
There are lots of genes that determine how much melanin someone makes. Some genes affect melanin all over the body and some affect it only in the skin, eyes, and/or hair.
Everyone has all of these same genes. But what makes a native Australian on average darker than a native Swede is they each have different versions of the same genes.
Usually people with lighter color have versions of melanin genes that do not work well. This makes sense, as a poorly working melanin gene would make less melanin, which would lead to a lighter color.
OK, now let's see how light skin and light eyes might be linked through the same gene. To simplify things, let's think of the melanin in your body as water flowing through your house. Genes control how much melanin ends up in which places, like pipes and faucets do for water.
You can control all of the water to your house with the house's shut off valve. If you turn this valve to the off position, no water gets in.
There are genes like this, too. Some people get versions of these genes that shut off all melanin in the body. These people end up with albinism.
Houses have lots of different pipes that lead to different rooms. For example, there is a pipe that leads to the kitchen. And one that leads to the bathroom.
The bathroom pipe then leads to separate pipes that go to the bathroom sink and the bathtub. There are genes that mimic all of these kinds of pipes in the body too.
There are genes that control just skin or eye color. These are kind of like pipes that lead to just the sink or bathtub.
There are also genes that control both skin and eye color. This would be more like the pipe that leads to the bathroom.
Turns out most Europeans have blue eyes because of a difference in their HERC2 gene. HERC2 is like the bathroom pipe. It controls melanin to both the skin and eyes.
Europeans with light colored skin and eyes have a version of HERC2 that doesn't make much melanin. Their HERC2 gene is sort of like a clogged pipe to the bathroom. Just like you only get drops of water from both the bathtub and the bathroom sink faucets with a clogged pipe, these folks only get drips of melanin to their skin and eyes.
Because in this case skin and eye color both depend on the same gene, they are linked. Those ancient people born with blue eyes also had light skin.
Light skin helped them make the vitamin D they needed to stay healthy. They had more kids than the weaker people, so the number of blue-eyed people in the next generation increased. This cycle repeated until the majority of people in northern Europe had blue eyes.