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Three most recent questions: 2010-01-22 Although we can't publish all of the questions we receive, we do our best to share our insights on these issues. If you'd like us to consider a question you have about genetics, please fill out the question form.
Ask a Geneticist by Dr. Barry Starr, Stanford University Can two parents with blue eyes have a child with brown eyes? -A curious adult from California
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, they’ll have brown eyes. Which makes it impossible for a blue-eyed parent to have a brown-eyed child—they don’t have a brown eye gene to pass on! In fact, this is the model we use for our eye color calculator. And that we talk about extensively 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 was 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 mom and one from dad. This means we all have 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 do not have blue eyes. But they can pass a b down to their kids so that these kids 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 this green eye gene). 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:  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:  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:
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.
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! What I’ll do is provide some links to ways that other genes or the environment can affect how a gene works:
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