Can Two Hazel Eyes Make Blue Eye Baby

Two of the most important genes in eye color are OCA2 and HERC2.  Both come in versions that can cause blue eyes.  And they need each other to work. (In genetics, a relationship like this is called epistasis.)

These two facts provide one explanation for how blue eyed parents can have a brown eyed child.  To understand how, a little genetics refresher course is in order.

People have two copies of most of their genes.  They get one copy from mom and one copy from dad.

These genes can come in different versions (or alleles).  For the sake of this discussion, we will say that OCA2 comes in brown (O) and blue (o) versions.  Reality is actually a bit more complicated and this will be dealt with in the second article of this series.

HERC2 also comes in two different versions, brown (H) and blue (h).  Since people have two copies of each gene, there are nine different possible genetic combinations.  They are:

These different combinations give the following eye colors:

Looking at the table, it quickly becomes obvious that any time there are two lower case h's or two lower case o's, a person has blue eyes.  As will be discussed in the next section, this is because HERC2 and OCA2 need each other to have an effect and the blue versions of each gene are broken.

But this part of the article will focus on how two blue eyed parents might have a brown eyed child.  Imagine the following blue eyed parents:

The easiest way to figure out what possible eye colors their kids might have is with a Punnett square.  The first thing to do in a Punnett square is to array dad's possible sperm cells across the top of the table and mom's eggs down the left hand side like this:

Shown below is a simplified version of the table in which duplicate sperm and eggs have been eliminated:

The next step is to fill in the squares with the possible genetic combinations.  Filling in the first one immediately gives a brown eyed child:

Here are the rest of the possibilities:

Each child has a 25% chance of having brown eyes and a 75% chance of having blue eyes.  This is simplified since the other eye color genes (like those that influence green eyes) are being ignored.  But it does mean that brown (or green or hazel or…) eyes are a definite possibility for these blue eyed parents.

Also note that the child in the lower right hand corner no longer carries a brown eye gene.  They will not have a brown eyed child as easily as their parents!

And the chances could be even higher with oo HH and OO hh parents.  This is what their Punnett square would look like:

All of their kids would have brown eyes!  (The table has been simplified to eliminate duplicate gene combinations.)

(Also, things are actually a bit more complicated than what I have shown here because HERC2 and OCA2 are so close together on the same chromosome.  Go to the end of the article if you'd like to learn what effect this has.)

Again this is understandably a bit confusing since these parents are technically carrying a dominant brown trait.  To understand this, we need to get into the nitty gritty of how eye color and these two genes work.

Can Two Hazel Eyes Make Blue Eye Baby

Source: https://genetics.thetech.org/how-blue-eyed-parents-can-have-brown-eyed-children

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