Expert: Christopher Rosch - 1/4/2006

Question
Is it possible for identical twins to have different eye color?   what is the precentage?  Parents have different eye color and grandparents.

Answer
Hello Robin,

the eye color is determined by about five to six genes, which makes it difficult to predict a child's eye color. In contrast to, e.g, body height, environmental factors play almost no role in eye color; scientists say the heritability of eye color is 98% [Bito et al. (1997). Arch. Ophthalmol. 115(5):659-63].

In identical twins the genes determining eye color are of course identical, too. So, both twins SHOULD have identical eye color. BUT, in rare cases (<2%), identical twins have indeed different eye color, which has no genetic cause.  However, I have no idea which factor(s) are responsible for this.

By the way, be aware that a child's eye color often changes during the first year (usually becomes darker); sometimes the eye color even changes until adulthood (Bito et al. 1997). Identical twins may therefore have slightly different eye colors during infanthood but the same eye color as adults.

Best regards,
Christopher