How Do Two Brown Eyes Make Blue?
- Pieter Maas
Is it possible for two brown eyed people to have a child with blue eyes? Editor’s Note (4/14/2021): The following article and diagrams present an over-simplified, outdated version of eye color genetics. Eye color is influenced by at least 50 genes, not all of which are well understood.
Yes. The short answer is that brown-eyed parents can have kids with brown, blue or virtually any other color eyes. Eye color is very complicated and involves many genes. To begin to understand how parents with brown eyes could have blue-eyed children, let’s imagine that eye color is due to a single gene, EYCL3, which comes in two versions or alleles, brown ( B ) and blue ( b ).
Explanation On How Two Brown Eyed People Can Have a Blue Eyed Child
Remember that for most genes (including eye color), you have two copies of each gene, and that you inherited one from your mother and one from your father. The brown version of the eye color gene ( B ) is dominant over the blue version ( b ). Dominant means that if either of your genes is the B version, then you will have brown eyes.
Genetically speaking, then, people with brown eyes could be either BB or Bb while people with blue eyes could only be bb, Example of a one-gene model for eye color. For two parents with brown eyes to have a blue-eyed child, both parents must genetically be Bb, When this happens, there is a 1 in 4 chance that these parents will have a bb child with blue eyes.
Unfortunately, eye color is not as simple as this. Besides the EYCL3 gene described above, at least two other genes, EYCL1 and EYCL2, are also involved. Although this set of genes explains how people can have green eyes, it does a poor job of explaining how blue-eyed parents could have brown-eyed children or how anyone can have hazel or gray eyes at all.
- To understand green eyes in all of this, we only need to review EYCL1 and EYCL3 (EYCL2 is a poorly understood brown eye color gene).
- Remember, EYCL3 has two versions, brown ( B ) and blue ( b ).
- EYCL1 also comes in two versions, green ( G ) and blue ( b ).
- The way these genes work is that if you have a B allele, you will have brown eyes ( B is dominant over b and G ), if you have a G allele and no B allele, you will have green eyes ( G is dominant over b ) and if you have all b genes, then you will have blue eyes.
Example of a two-gene model for eye color. I hope this helps to answer your question. As you can tell, while some progress has been made, eye color is a very complex, polygenic trait that is not yet fully understood. : Is it possible for two brown eyed people to have a child with blue eyes?
Can 2 brown eyes make a blue-eyed baby?
Myth: Two blue-eyed parents can’t produce a child with brown eyes. Fact: Two blue-eyed parents can have a child with brown eyes, although it’s very rare. Likewise, two brown-eyed parents can have a child with blue eyes, although this is also uncommon.
Are brown eyes dominant over blue?
Why are our kids’ eyes different colours? – Let’s look at why a blue-eyed parent (dad) and a brown-eyed parent (mum) and can have brown, green, and blue-eyed children. For gene 1, OCA2, there are two possibilities: brown or blue. The brown version of gene 1 is dominant over the blue one. Dominant means that if at least 1 of your two copies is brown (Bb), then you will have brown eyes. Geneticists represent the different versions of the eye colour gene as B for brown and b for blue (the capital letter is the dominant, the lowercase, recessive).
So brown eyes are either Bb or BB and blue eyes are bb. For gene 2, there are two possibilities, green or blue. Green is dominant over blue. Green eyes can be GG, or Gb, while blue eyes are bb. Brown is dominant over green, so if you have a B version of gene 1 and a G version of gene 2, you will have brown eyes.
The possible gene combinations that can give you brown, green, or blue eyes are shown in the chart. Back to the green or blue-eyed children. Dad can only be bb bb as he has blue eyes. Since mum has brown eyes, she could have any of six different possibilities.
Is blue eyes more rare than brown?
– The vast majority of people in the world have brown eyes. The second most common color is blue, but people can also have green, gray, amber, or red eyes. Some people have eyes that are different colors than each other.
Can two parents with blue eyes have a child with any other eye color?
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.
Blue-eyed parents can have kids with brown eyes. (Image via Shutterstock) 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.
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 ( to learn more about the two-gene model).
|Genes||What it Means|
|B b||Brown eyes|
|bb||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 ‘s website:
|Genes||What it Means in Europeans|
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.