Eye Color

We have green eyed identical twin girls. At birth their eyes were brilliant blue and they slowly changed to brilliant green. At 14 their eyes are still green. My husband and I have dark brown eyes. There are blue on both sides of the family. The girl's maternal great-grandmother had blue eyes and their paternal grandmother had grey/blue eyes. But there are no green eyes to our knowledge going back three generations. Where does the green come from?

-A curious adult from Connecticut

June 19, 2009

Many people are surprised when some new eye color seemingly pops up out of nowhere. Suddenly their daughter has green eyes. And no one can recall any green-eyed relative.

This can seem especially weird when you have a perfectly good explanation for a different eye color. From your description, you and your husband wouldn't have blinked an eye if your daughters' eyes had stayed blue. But they didn't.

The most obvious explanation is that you and your husband carry the green eye version of an eye color gene along with your blue and brown versions. This is possible because green eyes are due to a different gene than brown eyes.

So you and your husband have a brown version of one eye color gene. And a green eye color version of another gene. And undoubtedly some blue eye versions of these genes too! What a mess!

Don't worry, though, I can take you through this. I'll first talk about how eye color is thought to work. Then I'll talk about how green eyes may have stayed hidden in your family for many years before being seen in your daughters.

At Least Two Genes Determine Eye Color

Before getting into eye color genes, let's first discuss genes in general. Genes are like the individual recipes that make up a cookbook. Each recipe is for a certain dish. And each gene is for a certain part of us.

All of us have recipes for the same dishes in our cookbooks. What makes each cookbook different is that each recipe for the same dish is slightly different.

So all of our cookbooks have cookie recipes. It is just that some of us have chocolate chip recipes. And others of us have recipes for peanut butter crisps.

Our genes are very similar to this. For example, we all have the same two eye color genes. What gives us different eye colors are which variations of these genes we have.

One of these genes is called HERC2. It comes in two variations, brown and blue. The other gene, called gey, also comes in two versions -- green and blue.

Your eye color depends on which combination of these versions you have as shown below:

If you have a brown version of HERC2, you'll have brown eyes no matter what the gey gene is. If you have only blue versions of HERC2, then which version of gey you have determines your eye color.

If your HERC2 is blue and your gey is green, you have green eyes. And if you only have blue versions of both genes, then you have blue eyes*.

We Have Two Copies of each of our Genes

As you have probably noticed, this isn't enough information yet to explain what happened in your family. If you have the brown version of HERC2, then you would pass it onto your children. And they would have brown eyes no matter what their gey gene was.

The other bit of information we need to explain all of this is the fact that we have two copies of most of our genes -- one from mom and one from dad. So we have two copies of HERC2 and two copies of gey for a total of four possible gene versions.

Next I'll show you a table that gives all the possible combinations of these versions and the eye color you would have. To make things simpler, I'll use genetic shorthand. In this shorthand, the brown version of HERC2 is B and the blue version is b. And the green version of gey is G and the blue version b. Here is the table:

The simplest explanation for your situation is that you and your husband are either BbGG or BbGb (one of you could also be Bbbb). Doesn't seem very simple does it! 

The key is that each of you must carry a blue version of the HERC2 gene. If each of you does, then you each have a 50-50 shot of passing that blue version of HERC2 to your kids. This means that each of your kids has a 1 in 4 chance of getting a b from each of you and having an eye color other than brown. They'll have green eyes if they also get at least one G version of the gey gene. (Click here to see where the 1 in 4 came from.)

OK, so if you both carry a b version of HERC2, you can theoretically have green-eyed kids. But where did the G version of gey come from? And why hasn't anyone seen green eyes in your family before?

Recessive Genes Can Stay Hidden for a Very Long Time

By their very nature, recessive genes can be hidden. For example, take a look at the eye color table. There are nine different possible combinations of genes. Six of these combinations lead to brown eyes. And four of these brown eyed combinations carry a green version of gey.

Let's show how this can lead to green eyes staying hidden for many generations. Imagine this family tree:

Here you can see that green eyes stay hidden for at least three generations. And this sort of thing could have gone on for many, many more generations so that green eyes stay hidden for hundreds of years. Or even longer!

What I've done is numbered a couple of steps in the chart so you can see why green eyes stayed hidden. These are just a couple of possible reasons and there are undoubtedly others.

  1. One parent was BB. If one of the parents is BB, then that parent can only pass a B to his or her kids. The end result is all brown eyed children. So in this case, the green would stay hidden.
  2. By chance, no green eyes were seen. Just because two parents could have a green eyed child, that doesn't mean they will. For example, the children of two parents who are Bb have a 1 in 4 chance of an eye color other than brown. But that doesn't mean these parents will for sure have a green or blue eyed kid. Even if the parents had 10 kids, they still might only have blue or brown eyed children. (Click here for why this is.)

So there you have it. Green eyes can seemingly pop up out of nowhere because they are recessive to brown eyes. And because green eyes are determined by a separate gene.

*In genetics lingo, we say that brown is dominant over blue and green. And that green is recessive to brown but dominant over blue. Blue is recessive to both. Zhaoqing (Ching) Ding