-A curious adult from California
August 30, 2017
As your case shows, it can definitely happen. And it is surprisingly common too.
But while we know it can and does happen, figuring out the how has proved to be tricky.
It turns out that eye color genetics is not simple. Not by a long shot.
So HERC2 can come in either a “T” or a “C” version, OCA2 in either an “A” or a “G”, and so on. (Scientists call each version an allele.)
OK, getting tricky but still just manageable. We are only dealing with something like 64 different possibilities.
Except that we aren’t.
It turns out that like most of the rest of our genes, we have two copies of each of these eye color genes too. This means that each of us has three possibilities for each gene instead of just two.
Green + Green = Brown Example
Let’s imagine the following parent:
So he has a T copy of HERC2 and a C copy of HERC2, an A copy of OCA2 and a G copy of OCA2 and so on through all six genes.
IrisPlex predicts that this person has a pretty good chance for green eyes (24.4%). Or as they put it, intermediate colored eyes which is pretty much everything except brown and blue.