Expert: Dana Krempels, Ph.D. - 5/10/2010

Question
my parents both have hazel eyes, as do I, but my younger sibling has dark brown eyes.  Possible?

Answer
FOLLOWUP:

Dear Martha-Jean,

I am sorry to hear that the uncertainty about your brother's parentage has caused a rift in your family.  Please be assured that while it is unusual for two light-eyed parents to have a dark eyed child, it is not impossible.

I would not base suspicion of unrelatedness on a single character like this.  If your brother bears resemblance to other members of the family in other traits, then this is evidence that he is a blood relative, too.  But if you really are concerned, DNA tests are now available that will reveal paternity and maternity of a child.

In any case, I am sorry that the issue of genetic relatedness has caused a problem, since people can certainly love each other without being genetically related.

Take care,

Dana

*****


Dear Martha-Jean,

It is unusual, but not impossible.  It could be that one of your parents is passing on a reversion mutation to your younger sibling, meaning that the allele coding for a reduction of melanin in the iris (resulting in light colored iris) is reversed, and now coding for normal melanin production.

Also note that eye color is polygenic, with several genes contributing to the trait.  Different combinations of carotenoids might result in more darkly pigmented eyes.  But it is definitely uncommon for two light-eyed parents to produce a child with very dark eyes.

Hope that helps.

Dana