Eye color is much more complicated than is usually taught in high school (or presented in The Tech’s eye color calculator). There we learn that two genes influence eye color. One gene comes in two versions, brown (B) and blue (b). The other gene comes in green (G) and blue (b). All eye color and inheritance was thought to be explained by this simple model. Except of course for the fact that it is obviously incomplete.
The model cannot, for example, explain how blue eyed parents can have a brown eyed child. Yet this can and does happen (although it isn’t common).
New research shows that the first gene is actually two separate genes, OCA2 and HERC2. In other words, there are two ways to end up with blue eyes.
Normally this wouldn’t be enough to explain how blue eyed parents can have a brown eyed child. Because of how eye color works (see below), if one gene can cause brown eyes, it would dominate over another that causes blue. In fact, that is what happens with green eyes in the older model. The brown gene dominates over the green one resulting in brown eyes.
The key is that if someone makes a lot of pigment in the front part of their eye, they have brown eyes. And if they make none there, they have blue.
Part of the pigment making process involves OCA2 and HERC2. A working HERC2 is needed to turn on OCA2 and OCA2 helps to actually get the pigment made. They need each other to make pigment.
So someone with only broken HERC2 genes will have blue eyes no matter what OCA2 says. This is because the working OCA2 can't be turned on so no pigment gets made.
And the opposite is true as well. Someone with broken OCA2 genes will have blue eyes no matter what the HERC2 genes are. Turning on a broken pigment making gene still gives you no pigment. You need a working HERC2 and a working OCA2 to have brown eyes.
Because the two genes depend on each other, it is possible for someone to actually be a carrier of a dominant trait like brown eyes. And if two blue eyed parents are carriers, then they can have a brown eyed child.