Can two parents with blue eyes have a child with brown eyes?
A curious adult from California asks:
“Can two parents with blue eyes have a child with brown eyes?”
Yes, blue-eyed parents can definitely have a child with brown eyes. Or green or hazel eyes for that matter.
If you stayed awake during high school biology, you might find this answer surprising. We were all taught that parents with blue eyes have kids with blue eyes. Every time.
This has to do with the fact that blue eyes are supposed to be recessive to brown eyes. This means that if a parent has a brown eye gene, then that parent will have brown eyes. Which makes it impossible for two blue-eyed parents to have a brown-eyed child -- they don't have a brown eye gene to pass on!
In fact, this is the model we used for our eye color calculator.* And that we talk about extensively here at Ask a Geneticist.
Now we aren't being dishonest or trying to hide anything by presenting this model. It works great most of the time. But as with anything genetic, there are always exceptions.
For example, DNA can and does change between generations. So if a change happened that turned a blue eye color gene into a brown one, then blue-eyed parents could have a brown-eyed child.
As you might guess, this sort of thing is pretty rare. Too rare to explain all the exceptions we see with eye color. So something else must be going on. That something is most likely other genes involved in eye color that we don't know about.
And You Thought a Two Gene Model was Complicated...
Eye color used to be presented as a fairly simple trait. A big part of the model was the idea that we had an eye color gene that came in two varieties -- brown and blue. Geneticists represented the brown version as “B” and the blue version as “b”.
The model also said that blue (b) was recessive to brown (B). This matters because it is an explanation for how brown-eyed parents can have a blue-eyed child.
See, we have two copies of each of our genes -- one from each biological parent. This means there are three possible combinations for this eye color gene: BB, Bb, and bb.
BB is of course brown and in this model, bb would be blue. Since blue is recessive to brown, Bb people have brown eyes. But they can pass a “b” down to their kids, who might end up with blue eyes.
Now eye color is obviously more complicated than this. This model doesn't explain green eyes for example. Scientists added a second gene to try to explain green eyes but we don't need to go into that here (click here to learn more about the two-gene model).
All we need to know is that with this expanded model, if you have a B, you have brown eyes no matter what this green eye gene says. So if this were the case, then we'd expect the following possibilities:
What it Means
Not brown eyes
Again, bb people should not be able to pass on brown eyes to their kids. But we know they can. Which means that this model is incomplete (or wrong).
The results I just put into the previous table are theoretical and based on the model I talked about. Here are some actual results I adapted from 23andMe's website:
What it Means in Europeans
85% chance of brown eyes
14% chance of green eyes
1% chance of blue eyes
56% chance of brown eyes
37% chance of green eyes
7% chance of blue eyes
1% chance of brown eyes
27% chance of green eyes
72% chance of blue eyes
As you can see, the original model holds up pretty well for BB and bb people. Most BB people have brown eyes and most bb people don't. But the model clearly doesn't explain the following:
- 1% of bb people have brown eyes
- 1% of BB people have blue eyes (and 14% have green)
- 44% of Bb people do not have brown eyes
The biggest disconnect is with Bb people. Only 56% have brown eyes. If this holds up, I am not sure we can even call blue and green recessive to brown. Whatever the reason, these data give some clues about how two blue-eyed parents might have a brown-eyed child.
For example, imagine two parents are Bb and have blue eyes. They each pass a B down to one of their children. That child will be BB and most likely have brown eyes.
This example uses known data to show how blue-eyed parents might have a child with brown eyes. But it doesn't explain why a Bb person has blue eyes in the first place.
To do this, we need to guess what other genes may be doing. And how they might be affecting the original eye color gene.
Going into detail about these possibilities would need more space than I have here! And in the end, the truth is that eye color is a complex trait that we don't fully understand yet.
* Editor’s note (7/27/21): The eye color calculator is no longer available.
Author: Dr. Barry Starr
Barry served as The Tech Geneticist from 2002-2018. He founded Ask-a-Geneticist, answered thousands of questions submitted by people from all around the world, and oversaw and edited all articles published during his tenure. AAG is part of the Stanford at The Tech program, which brings Stanford scientists to The Tech to answer questions for this site, as well as to run science activities with visitors at The Tech Interactive in downtown San Jose.