Eye Color

I have blue eyes and my mom has blue and my dad green. My husband is Vietnamese and no one in his ancestry has ever mixed racially. His entire family has brown eyes. I used your eye color calculator on the Understanding Genetics website and it said that our next child could have about a 16% chance of blue, 16% chance of green and 66% of brown. My friend said the site is wrong and that we could never have a blue or green eyed child. I don't care either way, but I'd love to know if she's right as I have mixed friends with green and blue eyes with no known blue or green eyed family.

-A curious adult from California

March 26, 2009

This is a great question. Given your husband's background, the odds for a blue or green eyed child are probably much lower than the eye calculator is telling you. And luckily we've built in a way for you to get a better prediction.

When I plug in your numbers, I get that each of your kids has about a 14% for blue eyes, a 14% for green and a 72% chance for brown. When I adjust the calculator for your husband's ethnic background, the numbers become much more reasonable. Each of your kids has a 0.7% chance for blue, a 0.7% chance for green and around a 98.5% chance for brown.

Now this is higher than your friend's claim of a 0% chance for blue or green. What the 0.7% represents is the possible if unlikely chance that there is an ancestor in your husband's family tree that carries a blue or green eye color gene version. A Dutchman in the woodpile, so to speak.

Let's spend the rest of the answer going over how I adjusted things to get the new numbers. And why it works.

A Genetic Model for How Eye Color Works

Humans pretty much share the same set of genes. This is a big part of what makes us human. What makes us each unique are small differences in this same set of genes.

Eye color is no different. We all share the same eye color genes. We get different eye colors from having different gene versions.

The best system we have for easily predicting eye color uses two different genes. One gene comes in two versions, brown (B) and blue (b). And the other gene comes in two versions, green (G) and blue (b).

Before going any further, we need to remember one more thing. We have two copies of most of our genes -- one from mom and one from dad. So we have two copies of each eye color gene too.

Here are all nine possible combinations of gene versions and the eye color you get with each:

As you can see, there are six ways to have brown eyes, two ways to have green and just one way to end up with blue eyes. Click here for why this is.

Eye Color Calculators Have Trouble with Brown Eyes

Any eye color calculator struggles with the six different brown eye combinations. When someone punches in brown, which of the six are they?

Each brown gives very different chances for their possible children's eye color. For example, a BB bb will almost always have brown eyed kids no matter who the other parent is. A Bb bb, however, has a chance for kids with blue or green eyes.

This is why there are places to put in your parents' eye color and your siblings' eye colors in our calculator. This is an attempt to figure out the chance that the brown-eyed person is BB or Bb.

But even if everyone on one side of a family has brown eyes, there still could be a b lurking in their genes. To try to account for this, we added a way to factor in someone's ethnic background to the eye color calculator.

The idea is that different ethnic backgrounds have differing amounts of BB and Bb amongst their brown-eyed folks. For example, a brown eyed person from Germany will have a higher chance of carrying a b than will a brown eyed person from Vietnam. 

Figuring Out the b in a Population

Up to now I have been using pretty simple terms like gene versions. Scientists, being scientists, have of course come up with a more complicated term. A gene version to them is an allele.

When a scientist figures out how much of a certain gene version there is in a group, that scientist is calculating something called allelic frequency. In this case that just means how many B's and how many b's there are in a population.

Let's take your husband as an example. In Vietnam, nearly everyone has brown eyes and they are mostly BB instead of Bb. What this means is that there are mostly B's in this population. So the allelic frequency of B is very close to 100% (or 1.0 for mathematical purposes). Almost no large populations will have 100% of any allele so we tend to give 99% or 0.99 as the upper limit. 

Figuring Out Eye Color Using our Calculator

Now we are ready to see how I used the eye color calculator to figure out your kids' chances for brown, blue, or green eyes. Let's do this step by step to help anyone who hasn't visited the eye color calculator before.

  1. Go to Understanding Genetics and click on the What Color Eyes Will Your Children Have?  Then hit Click to Begin.
  2. Now put in you and your husband's eye color and the eye colors of your and his parents too. Don't add the siblings' eye colors as a bug has been found that makes the allelic frequency part not work if sibling data is included.

If this is all you do, you'll end up with a page like this: 


This is pretty close to the results you got which are probably not correct. To account for your husband's background, we need to add one more step. Notice where I put the big red arrow. This is where we can add the information about the allelic frequency of B for Vietnamese folks like your husband.

What you want to do next is go to the brown allele in this Allelic Frequency box. Push the little toggle up to 0.99. This means that 99% of the gene copies in this population are the B version.

Then hit calculate again.  Now you should get the following:

As I said at the beginning, this is a much more reasonable answer. This is how people should use the calculator if one of the parents comes from a background where eye colors other than brown are pretty rare. Think most Asians, Africans, Native Americans, and Native Australians.

So there you have it. Your kids will most likely all have brown eyes but will be carriers for blue (and possibly green) eye color gene versions. Which means that one day, you may have blue or green-eyed grandkids!

By Dr. Barry Starr

Some ethnic groups have little or no blue eye gene versions within their population.

Blue eyes are easy for eye color calculators. Brown eyes are much trickier.