Partial Heterochromia - E's Awesome EyeConversation on our way to my kid’s school:

T: Dad, I heard people used to marry their cousins and sisters and stuff.  Is that true?

Me: Well yes.  People used to do that for various reasons.

T: I also heard that people who did that have kids with no arms.

After chuckling a bit to myself I started to explain, as simply as I could, the concepts of genetic diversity and how kids have a better chance of not having certain problems if the parents are not related.  This quickly led to a quick discussion about dominant and recessive genes in the over simplified context of eye color.

Me: So Mom has brown eyes and I have blue eyes.  What color are your eyes?

T: Blue. So blue eyes always win?

Me: Not usually.  Mom and I both have two genes for eye color.  But we each only give you one of those.  If you get a brown gene from either of us, then no matter what the other gene is, you get brown eyes; brown is dominant.  I have blue eyes so I must have two blue genes.  I gave you one of those.  Since you have blue eyes, you must have two blue genes.  That means Mom must have both a brown and a blue gene and gave you her blue gene.  If she had given you her brown gene, then your eyes would have been brown, even though you got a blue gene from me.

T: Ok.  But then why does E (his little bother) have an eye that is both brown and blue.

I looked at him and laughed a little more.  I had to admit that I wasn’t 100% sure.  Of course that meant, for the rest of the day, that lack of knowledge was nagging me at the back of my mind until I had time to look into it a little.

So after some research my best guess is that E has Partial Heterochromia Iridum[1].  This condition is associated with a variety of  syndromes.  Fortunately our son exhibits no other signs associated with any of these syndromes.  His appears to be “Simple Heterochromia” that is a congenital hypoplasia of the iris.  In other words, it’s simply a case where parts of his iris are either underdeveloped or just missing melanin (blue eyes are caused by low concentrations of melanin in the iris or ocular fluid[2]).

So while it is somewhat rare, it’s nothing to be concerned about (in our case).  Both he and his brothers think  it is pretty cool and have nicknamed it the “awesome” eye.