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Eye Color

I hope that your explanation can clear up some confusion in my family. I have brown eyes and my husband has blue eyes. We have 3 daughters: one has blue eyes, one green and one brown. Everybody says husband,s genes are stronger so that explains the blue and green, but what about the brown?

-A curious adult from California

October 14, 2004

Actually, the eye colors of your kids fits perfectly well with current genetic theories. In fact, in your case, your genes for eye color are "stronger" than your husband's because brown is dominant over blue (we'll go more into this later).

We usually get the opposite question where people wonder how someone with brown eyes could have blue-eyed kids. We answered this question previously and I encourage you to give it a once over after reading the rest of your answer.

To explain why you can have brown, green, and blue-eyed kids, we need to go into a bit of detail about eye color genetics. In the simplest models of eye color, there are two genes involved. (This model is too simple to explain a lot of things, but it will explain your situation.) Remember that for each gene, we inherit two copies, one from our mom and one from our dad.

For gene 1, there are two possibilities, brown or blue. The brown version of gene 1 is dominant over the blue one. What dominant means is if at least 1 of your two copies is brown, then you will have brown eyes. The way geneticists represent these two different versions of this eye color gene are B for brown and b for blue (the capital letter is always the dominant, the lowercase, the recessive). So you will have brown eyes with either BB or Bb and blue eyes with bb.

For gene 2, there are two possibilities, green or blue. Green is dominant over blue and so G usually represents green and b, blue. Green eyes, then, can be GG or Gb while blue eyes are bb.

With me so far? One other thing you need to know is that brown is dominant over green -- if you have a B version of gene 1 and a G version of gene 2, you will have brown eyes. Given all this, here are the possible gene combinations that can give you brown, green, or blue eyes:

So how did I come up with your genotype (genetic makeup) as Bb Gb? As I'll try to show below, it is the only possibility where you could have green or blue-eyed children. To help make this clearer, I'll introduce you to Punnett squares. Punnett squares are a relatively easy way to figure out if something is possible genetically and how likely it is.

OK, what does this mean for you? Well, your husband can only be bb bb as he has blue eyes. Since you have brown eyes, you could have any of six different possibilities. But since you have brown-eyed, green-eyed and blue-eyed children, the most likely possibility for you is Bb Gb. You have brown eyes but are a carrier* for both blue and green eyes.

The way a Punnett square works is you make a table. We'll do an easy one first with just gene 1 (the brown or blue eye gene). The first step is to put your two possible gene versions on the top like this:

Since we are saying you are a brown-eyed carrier of blue eyes, you have a B (brown) and a b (blue) version of gene 1. The next step is to put your husband's gene versions on the side of the table like this:

As you can see, in this example, I put your brown eyes on top (Bb) and your husband's blue eyes (bb) on the side. Remember, you and your husband only contribute one version of gene 1 each. You can give either a B or a b to your kids, not both. The Punnett square gives you all four possibilities of you and your husband's combinations.

The next step is to fill in each square with the letters from the top or side to figure out what is possible. For example, in the first square, since there is a B from you and a b from your husband, a Bb goes in like this:

This represents a child like you who has brown eyes (B) but carries a blue eye version of gene 1 (b). You then fill in the rest of the square like this:

From this, you can figure out that you have a 50-50 shot of having either blue eyed or brown-eyed kids. In the table, there are 2 Bb and 2 bb squares meaning you have a 2 in 4 chance of brown-eyed kids (Bb) and a 2 in 4 chance of blue-eyed kids (bb). Note that all your brown eyed kids will be carriers for the blue eyed version of the gene, b.

Now, to add the green gene, it gets more complicated. Each of the two genes is independent of each other so you need to figure out all of the possibilities you could have. Your husband is easy as all his possibilities will be bb. For you, it is a little trickier.

Remember, you are most likely Bb Gb. So what should we put on the top of the square for you? You figure this out by starting with B. If one of your children gets B, then they can get either G or b from gene 2. The same is true for b. So the possibilities are BG, Bb, bG, and bb. The Punnett square for you and your husband would look like this:

You do the same thing as before and combine the boxes. The first box would be Bb Gb, a brown-eyed carrier of green and blue eyes. If we fill in all of the possibilities, we get:

From this, we can figure that you and your husband have a 50% chance of a brown-eyed child (anything with a B), a 25% chance of a green-eyed child (anything with a G and no B), and a 25% chance of a blue-eyed child (anything with just b).

So as you can see, if you really are a Bb Gb, then this genetic model explains your kids perfectly. You can try out other possible brown eye genetic combinations and see whether or not you can get green or blue-eyed children.

*A carrier is someone who can pass on a trait without having that trait -- the trait is hidden. In our example, if you are Bb, that means you have brown eyes but could pass blue eyes to your children.

 

Video Answer about why everyone in a family has blue eyes except mom.

By Dr. Barry Starr


Brown is dominant over blue no matter which parent it comes from.