One fish, two fish, red fish, blue fish
What the colors of fish have taught us about human skin color
When we look at a family, it is pretty obvious that skin color is passed on. Dark-skinned parents tend to have dark-skinned babies, light-skinned parents, light-skinned babies. Not always, but usually.
You would think that the genetics of something as obvious as this would have been explained years ago. And yet, until just last year, not much was known.
What we did know was that skin color, like hair and eye color, is the result of more than one gene. Lots of genes work together to determine our coloring.
We also knew of a few changes in genes that could make people have albinism (where they don't really have any pigment). But we didn't really have any idea what genes caused the more garden-variety skin color differences between humans.
OK. So by now you are wondering from the title of this article what fish coloring has to do with all of this. Well, you probably have heard that scientists often study other animals to learn things about ourselves. For example, mice and rats are often used to study things from cancer to learning and memory.
Just last year, a group of scientists were using fish to study cancer, when they stumbled onto a possible human skin color gene. The fish they were studying are called zebrafish, named after their dark stripes. This dark color is caused by a pigment called melanin. This is the same pigment responsible for dark skin and eye color in humans.
A few zebrafish, however, are lighter in color. Scientists named these zebrafish "golden". They knew that the golden coloring was genetic since golden fish usually had golden babies. They figured out that the golden color was caused by a single change in one gene (called a mutation).
It turns out that humans have a gene that is similar to the one changed in golden fish. To test whether this gene does the same thing in humans and fish, scientists put the human gene into golden fish. And guess what? The fish had dark stripes and were no longer golden!
OK, but does the gene have anything to do with color in people? Actually it looks like it might.
When the scientists looked closely at this gene in many different humans, they found a surprising result. Nearly all Africans, Native Americans, and East Asians had the original version of the gene. 93%, to be exact. But in European-American people, 98% had a single change in the middle of the gene.
Remember, we each have two copies of most of our genes (one from your mom and one from your dad). The Africans, Native Americans, and East Asians that were studied had two copies of the original version of the gene. The European-Americans looked at had two copies of the mutant gene.
What happens when people get only one copy of the gene with this small change? To answer this, researchers looked at people with a mix of African and European ancestors. They found that people with only one copy of the mutant gene had medium-colored skin.
So it seems that the same gene that controls the color of zebrafish also plays a role in determining skin color in humans. Of course, skin color genetics are not entirely this simple.
For example, most Africans are darker than East Asians, even though both usually have two copies of the original gene without the mutation. But the researchers think that this one change in one gene is responsible for 25-38% of the skin-color difference between Europeans and Africans.
What is this gene doing?
Remember, genes make proteins, and these proteins come together to make each of us who we are (or each fish who he or she is). In the case of the golden fish, this single change in one gene makes it so that the gene only makes the first part of its normal protein. Over half the protein is missing!
In people, the mutation wasn't so severe. The full protein is made, but the change in the gene just makes one small change in the protein.
How does this protein determine whether fish are dark or golden, or whether people have dark or light skin? We don't know the answer to this yet, but comparing this gene to other known genes gives us a clue.
This gene looks like other genes that make proteins that move sodium, potassium, and calcium across cell membranes (cell membranes act like a wall around the cell, and specialized proteins act like doors to let certain things into and out of the cell).
It is likely that the role of this protein is to move calcium into the cell. Somehow, having more calcium in cells makes more melanin get into the cell. In the golden fish, who are missing most of this protein, not as much calcium gets into the cells. Less calcium means less melanin, and golden-colored fish.
The same idea goes for people- mutant protein means less calcium in the cells, less calcium means less melanin, and less melanin means lighter skin. Of course, this is just a guess based on comparing this gene to other known genes. More work still needs to be done to figure out how all of this really works.
Does this gene have anything to do with hair color or eye color?
We know that Europeans have many different colors of eyes and hair, whereas most Africans and East Asians have dark hair and dark eyes. Does this gene discovered in zebrafish have anything to do with hair color or eye color?
Interestingly, eye color and hair color are also controlled by the amount of different melanin pigments. But in the case of red hair, it seems that changes in a different gene are responsible (see /ask/ask44). And in the case of blue eyes, changes in at least two other genes are necessary (see http://www.thetech.org/exhibits/online/ugenetics/ask.php?id=162).
But it is still possible that this gene discovered in zebrafish does play a role in eye color and hair color. One idea is that you might need the mutation for lighter skin in order to see the effects of these changes in the genes for red hair and blue eyes.
Why do so many Europeans have this mutation?
So why is it that Europeans nearly all have this mutation, whereas most other people do not? The answer to this is not known.
One possibility is that our need for vitamins played a role. We know that Vitamin D is very important for avoiding bone diseases like rickets. In sunny places, we can make plenty of Vitamin D. But in dark places, such as northern Europe, we can't make Vitamin D as well. Lighter skin helps people make more Vitamin D. For more details and some other ideas, see /ask/ask160.
Whatever the reason, this discovery means that much of the prejudice that has haunted our world for centuries has been due to a single change in one gene. For comparison, siblings have hundreds to thousands of changes between their genes. The truth is that despite our different looks, all humans are >99.9% identical.
Sadly, for years, some people have tried to see whether skin color was related to intelligence. Intelligence is something determined by a combination of MANY genes (and environment). It is extremely unlikely that changes in this gene for skin color have anything to do with intelligence. So in conclusion, race, like beauty, is only skin deep.