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Eye Color

Why are blue eyes so uncommon any more?

-A curious visitor to The Tech

April 12, 2010

Blue eyes are indeed becoming less common in the world. One study showed that about 100 years ago, half of U.S. residents had blue eyes. Nowadays only 1 in 6 does.

What is happening in the U.S. will undoubtedly happen throughout the world as well. Especially as Europe opens itself up to more immigration. This is one of the reasons blue eyes are becoming less common in the U.S. -- immigration.

In the recent past, lots of brown-eyed people have moved here. Even with no other explanation you'd expect a smaller percentage of blue eyed people because of this.

But this is only part of the story. Another reason blue eyes are declining is because they were at an artificially high level before. In the past, blue-eyed people tended to have kids with other blue-eyed people. When this happens you get blue eyed kids.

Today people use ethnicity less often as a way to pick a partner. This means that blue eyed people have kids with brown eyed people more often than before. And often that means mostly brown eyed kids.

These two facts -- more brown eyed people and more mixed eye color marriages -- will undoubtedly continue into the future. Which means that blue eyed people will continue to decline in numbers.

But they won't go away completely. The version of the eye color gene that leads to blue eyes doesn't disappear from the human race when someone with blue eyes has a brown eyed child. This gene version instead goes into hiding, waiting for the right opportunity to cause blue eyes again.

What I'll do for the rest of the answer is go over the history of blue eyes according to our DNA. From that you'll get a good feel for how blue eyes work. And why they'll become even rarer.

 

History of Blue Eyes

Back in the day, everyone had brown eyes. Basically, like brown eyed people today, our ancestors had brown eyes because of a gene called OCA2.

This gene tells our cells to make lots of pigment in the front part of our eye. Lots of pigment there means brown eyes.

Then sometime a few thousand years ago, someone developed a small glitch in their DNA. This glitch was caused either by damage from the outside or from some mistake made by this person's own cells.

Whatever the cause, this change made the OCA2 gene stop working in the eye. When OCA2 isn't working, it doesn't make any pigment. And when no pigment gets made in the front part of the eye, you end up with blue eyes*. Except this person undoubtedly had brown eyes.

Why? Because this person still had one working OCA2 gene.

Remember, we have two copies of most of our genes -- one from mom and one from dad. Since it is very unlikely that two glitches will happen in the same gene at the same time, this person probably had one working and one nonworking OCA2 gene.

The working copy could still do the job of making pigment on its own. So this person had brown eyes.

But the blue version of OCA2 was now released into human DNA through this person's children. At some point two people who each carried a blue OCA2 met and had a child. This child happened to get a blue OCA2 from each parent and so had no working OCA2 genes. The end result was no pigment and so blue eyes*.

This blue-eyed person must have been special somehow because the blue OCA2 quickly swept through the European population. Soon there were places where blue eyed people outnumbered brown eyed ones.

We don't know what made these blue eyed people special. It may have been that they were irresistible to the opposite sex and so had lots more babies than brown eyed people. Or blue eyes may have been tied to some useful Northern European trait like pale skin.

Whatever the reason for their eye color, these blue eyed people established themselves and spread. And, for various reasons, they tended to have children with only each other.

So there were pockets of blue eyed people in a world of brown eyes. When populations started to move around more easily, there started to be a blending of blue and brown eyed people. And then brown eyed people started having kids with blue eyed people.

Since brown beats out blue (brown is dominant, blue is recessive), the blue eyes started to fade into the background. The same number of blue OCA2 genes were there it was just that they were now hidden more of the time.

*This is a simplified version of the story. There are other genes that make pigment too but not as well as OCA2. So some people can have two nonworking OCA2 genes and have green or hazel eyes and not necessarily blue.

Blue Eyes are Down but not Out

To try to make this clearer, I've created a couple of hypothetical examples. In these examples, a little b stands for the blue OCA2 and a big B stands for brown.

Since we have two copies of the OCA2 gene, there are three possibilities: BBBbbbBB and Bb will give brown eyes (since B is dominant) and bb will give blue.

The first example on the right shows what happens with no mixing. What you see is that the population increases over time but you end up with the same proportion of blue and brown eyed people.

This is because the brown eyed people can only pass down a B so all their kids will have brown eyes. Same thing with the blue eyed people except they only pass down a b. And so have only blue eyed kids.

In the second example (see below), I have shown what happens in two generations when there has been an influx of brown eyed people and completely random mixing.

The end result is more brown eyed people but with the same number of blue OCA2 gene versions. This is because many more people have a copy of each version. These folks have brown eyes but they have a hidden blue OCA2 version. If they meet someone else who carries a blue OCA2, then they have a shot at a blue eyed child.

Eventually assuming no new immigration and no differences in the number of babies each group has, the populations will settle down to some new number and stay constant. Figuring out that number isn't easy because eye color is so complicated.

If it were due to a single gene, we might be able to make some sort of guess. When I try to apply it, I get that the number of blue eyed people in the world will fall from its approximate number of about 350 million today to 49 million in some distant future.

This assumes random mixing, that there are 350 million blue eyed people in the world, that eye color is due to a single gene and that there are essentially no carriers of the blue eye gene in predominantly brown eyed populations (think China, India, most of Africa, etc.). Not sure how useful this exercise actually was but it was fun! (At the end I'll show how I got the numbers for those of you interested.)

Even if the exact numbers aren't right, what we can conclude is that blue eyes will continue to become rarer and more exotic. But that they won't disappear completely.

Video Answer explaining how eye color works.

Hardy Weinberg

To try to figure this out I applied the Hardy-Weinberg (HW) equation. Basically by knowing the number of blue eyed people in the world, you can figure out how many carriers there are too.

I did one thing a bit differently in that I assumed there were two populations in the world. One population of 6 billion people had essentially no blue OCA2. The other billion people had a history of blue eyes and so the blue OCA2 gene could be found there.

It is pretty easy to figure out the OCA2 genes of the first population. It has 12 billion B's (two for each person).

The second group was a little trickier. If we assume about 350 million people with blue eyes out of a billion, then we can figure out how many B's and b's are in that population. The Hardy Weinberg equation is:

p2 +2pq+q2=1

In this equation p=B and q=b. Also, p2=BB people, 2pq=Bb people and q2=bb people. (Remember, BB and Bb have brown eyes and bb have blue.)

From 350 million blue eyed people, we come up with q=0.59 and p=0.41. Punching these numbers into the equation, we get 170 million BB, 480 million Bbb and 350 million bb.

Combining these numbers with the numbers from the first population, we get the following:

12.82 billion B

1.18 billion b

Punching those numbers back into the HW equation we end up with a q2 of .007. If we assume 7 billion people in the world, that translates to 49 million blue eyed people. Down but not out...

By Dr. Barry Starr


There will be many fewer blue eyes in the future.


Thousands of years ago, a DNA change caused blue eyes.

 

 

 

 

 

 

 

 

 

 


Something special about this first blue-eyed person caused blue eyes to quickly spread through Europe.