It seems to me that light skin and light eyes go together. Do scientists know why?
-A curious adult from California
September 17, 2010
For people of European descent, lighter skin and eyes do often seem to go hand in hand. But it doesn't have to be this way. For example, many people from Asia have light skin and brown eyes.
Part of the reason for this difference is that Europeans and Asians have fair skin because of changes in different genes. The key genetic change in Asians doesn't affect their eye color. The same is not always true for Europeans.
One of the main reasons Europeans have fair skin is because of a single change in the SLC24A5 or golden gene. Recent work has shown that this genetic change can also affect eye color. So if you have the difference in golden, then both your eyes and skin may be lighter.
Another reason lighter eyes and skin don't go together in Asians is that Asians rarely have an eye color other than brown. This is because they are missing a second key change in the HERC2* gene that many Europeans have.
Now this explains why Europeans can have blue eyes and why Asians usually don't. But it doesn't explain how this gene is linked to fair skin.
The reason has to do with where the genes are on a chromosome (see below for more information). Genes that are close together tend to travel as a group. Which means that the traits they control can travel as a group as well.
The HERC2 and golden genes probably aren't close enough together on chromosome 15 to always travel together. But golden isn't the only skin color gene.
Scientists recently identified a second key genetic change in Europeans with fair skin. This change is in the APBA2 gene which happens to be right next door to HERC2. This means these two genes almost always travel together which may partly explain why skin and eye color tend to be linked in Europeans.
If all of this is right, we would predict that even if Asians had the genetic change that can give blue eyes, it wouldn't have to be linked to fair skin. This is because the change in the kit ligand gene that leads to their fair skin is found on chromosome 12 and HERC2 is on chromosome 15. The genes are on separate chromosomes and so would not be forced to travel together.
To understand why two genes being close together matters for traits being linked, we need to go over a bit about how genes are stored in the body. And how they are passed down to our kids.
Location, Location, Location
Genes are linked together, one after the other, in long pieces of DNA called chromosomes. Humans have 23 pairs of these chromosomes that contain over 20,000 genes. This has a couple of implications for what happens when our genes are passed to our kids.
First off, genes (and their associated traits) aren't passed down individually. They are passed down as part of the chromosome they are on.
For example, if you get the version of the golden gene that leads to fair skin from your mom, you get the other 700-900 genes on that copy of chromosome 15 too. So it makes sense that genes from the same chromosome tend to travel together. But it isn't that simple...
Let's take a closer look at that chromosome 15 you got from your mom. The truth is that it is different from either of her two copies of chromosome 15.
See, we have two copies of chromosomes 1-22. We get one chromosome of each of our pairs from mom and one from dad.
Before mom or dad pass down their chromosome, something called recombination happens. Basically, the two chromosomes in a pair swap DNA with each other.
For the rest of this discussion, we'll use the image on the right. In the image, we'll say that HERC2 is A, APBA2 is B and the golden gene is C. Notice how much closer A and B are to each other compared to C.
So let's say mom has the pale versions of the golden gene and APBA2 and the blue version of HERC2 on the white chromosome and the not-pale versions and the not-blue version on the green one. With no recombination, her kids would either get the white or the green chromosome. They would either get all the lighter versions or none of them.
This means her kids would either get pale skin and blue eye versions of these three genes or the not-pale skin and not-blue eye versions. None of them would get one of each.
With recombination, it is possible for her to pass along any combination of the six different gene versions. In our example, you can see that recombination has moved the golden gene (C) from the white chromosome onto the green chromosome. Now the pale version of the golden gene is on the same chromosome as the not-blue version of HERC2 and the not-pale version of APBA2. So this child might end up lighter than he or she would without recombination.
It is technically possible that APBA2 and HERC2 could be separated as well; it just isn't as likely as the two of them being separated from the golden gene. Click here to learn why.
So if a chromosome has a blue HERC2 and a pale APBA2, kids will almost always get both traits linked together. And so these traits will be linked. Combine this with the fact that the golden gene can affect both eye and skin color, and you have a pretty good idea about why light eyes and skin tend to travel together.
What I don't have time to go into is why the blue version of OCA2 happens to be next to the pale version of APBA2 in most people. For now, just know that it is and that is why the traits go together.
*This change ultimately affects how a second gene, OCA2, works in the eye. OCA2 is responsible for a lot of a person's coloration.
By Dr. Barry Starr