Expert: Dana Krempels, Ph.D. - 5/19/2009

Question
I saw the preview of a movie where a doctor commented that he knew the girl had a twin by the color of one of her eyes. That got me wondering if this were a true possible trait.  I only ask because I have boy/girl twins and 1/4 of my daughters left eye is a light brown while the rest and her other eye is blue. Is this a true twin trait or a freaky coincidence  and how does that happen?
Thank you.

Answer
Dear Tonya,

I didn't see the movie preview, so don't know exactly what the doctor was talking about.  But the condition you describe in your daughter's eye (i.e., having a portion of the iris one color, and the rest a different color) isn't going to be the same in two identical twins.  

This "piebalding" of the eye is most often caused by a somatic mutation--one that occurs while one (or sometimes more) of the cells in the the tissues that will become the eye undergo a non-lethal mutation in the gene that controls the color of the iris.  Usually a mutation inactivates a gene, so this is most commonly expressed as a blue patch in a brown iris.  But it can happen either way.

Because this happens while the embryo is developing, and is essentially a unique accident in a single cell that's not encoded in the DNA, two identical twins will not likely both share iris piebalding caused by somatic mutation.

Hope that helps.

Dana