Expert: Dana Krempels, Ph.D. - 7/14/2008

Question
QUESTION: My son who is now 4 years old has always had a patch of hair on the back of his head that is very light blond.  The rest of his hair is a sandy blond.  It was somewhat lighter before, but never as light as that one patch.  A beautician once told me it was due to a birth mark.  Is this true?

ANSWER: Dear Paula,

The definition of a "birth mark" is just a mark (usually considered a blemish) on the skin that forms before birth.  Which doesn't tell us much.  Yes, it's a birth mark.  But *why* is the hair pale?

It's possible that your son was born with a patch of skin that was destined to grow pale hair because of a somatic mutation.  

Hair color is controlled by several different genes, each of which can  contribute not only to the color of the pigment (eumelanin is brown, phaeomelanin is reddish, and different people have genes for one or the other or both), but also its concentration in the hair shaft.  The lower the concentration of melanin, the lighter the hair.

It's possible that when your son was an embryo, the cell or cells that would some day give rise to the patch of cells that now form that lighter patch on his scalp underwent one or more somatic (body cell) mutations that inactivated one or more genes that control pigment deposition in the scalp.  That means that the patch will always remain pale, throughout his life.  (The same type of somatic mutation is responsible for patches of a brown iris (the colored part of the eye) being blue.)

So it's nothing to worry about.  Just something that makes him distinctive.  :)

Hope that helps explain.

Dana

---------- FOLLOW-UP ----------

QUESTION: What causes a somatic mutation in an embryo?

Answer
Dear Paula,

In many cases, it's a nondisjunction mutation, where two chromosomes migrate to a single cell, leaving the other daughter cell with a "blank" .  In many cases, this is not lethal if it happens in only a tiny area of the body.

In other cases, a random point mutation can simply cause an inactive form of an enzyme to be manufactured by the cells in the mutant area.  There are many different possibilities, but often they are just random errors in DNA replication or chromosome segregation.

Dana