Expert: Dana Krempels, Ph.D. - 7/14/2007

Question
My husband and I are the parents of six children. Both my husband and myself have blue eyes as do 5 of our children. Our 6th child (a twin by the way) to our surprise has hazel eyes. My question is how did this happen, how do two blue-eyed parents produce a hazel-eyed child? I know genetics play a huge role in determining eye color but am very confused as both my parents have blue eyes (as do my 6 siblings). Now my husbands father is blue-eyed and his mother brown-eyed (all 9 of his siblings are either blue or brown eyed...no hazel).Thank you.

Answer
Dear Susan,

It is unusual for two blue-eyed parents to produce a hazel-eyed child, but not impossible.  There are at least five  genes controlling the color of the human iris.  One of them codes for the deposition (or lack of deposition) of the dark brown pigment, melanin.  Other genes code for the *amount* of melanin deposited, so some people will have very dark brown eyes, others light brown, and others a golden color (not common, but it happens).  

Typically, hazel, green, and grey eyes are produced when a blue iris is overlaid by other pigments, such as very small amounts of melanin or by the reddish-orange pigments known as carotenoids.  

In blue-eyed people, these genes are generally inactive.  However, it's possible for a child to inherit a reversion mutation, in which the gene that's inactive in the parent changes when the parent makes gametes, and the child inherits the newly activated gene.  This isn't common, but it's not impossible.

Gene expression also can be affected by the conditions the embryo/fetus experiences in utero.  Certain medications and other chemical can result in unexpected gene expression.  It's complicated, not well understood, and pretty unpredictable.

In short, it's not common for two blue-eyed parents to produce a hazel-eyed child, but it's not impossible.

I hope that helps!

Dana