Expert: Dana Krempels, Ph.D. - 12/3/2008

Question
I have hazel eyes and my husband has blue eyes. Is it possible for us to have a
brown eyed child?

Answer
Dear Jennifer,

In general, the genes that code for a light-colored iris (e.g., blue or hazel) are recessive to those that code for dark-colored iris (e.g., brown).  However, several different genes contribute to eye color.  

Your husband has a blue iris overlain by yellow/brown carotenoid pigments.  If the baby were to inherit genes that encoded both those carotenoid pigments and a strong deposition of those pigments in the iris, then the eyes could look more on the brown side of hazel than on the green side.  But it is very unlikely that the baby would have dark brown eyes, since neither of you carry the forms of the eye color gene that code for that.  If you did, you would have brown eyes, not blue or hazel.

Human hair, eye, and skin color are very complex and difficult to predict, because each of these traits is controlled by more than one gene.  What determines the physical appearance (expression) of each trait is the dominant versions of the various genes that affect the traits in question, because these are the ones most likely to be expressed by the child--though not always.  I'll try to explain.

Every human carries two copies of every gene.  Scientists now estimate that a human has about 30,000 genes in the genome, and every human has two copies of that genome:  one from mom, and one from dad.  The  versions of each gene (called *alleles) that a child gets from each parent may be the same in a single person, or they may be different.  For example, if there's a gene we'll call X, and the father's alleles are XX and the mother's are xx, then the child will get one from each parent, and have the genotype Xx for that particular green.

How does this affect the physical appearance of a trait?  Here's a brief example.  Let's say that there's a human gene that codes for the shape of the forehead hairline.  There are two versions of the gene.  One, which we'll call "W" codes for a small "V" of hair to point down onto the forehead (Widow's Peak).  The other version, which we'll call "w", codes for a straight hairline.  In this case, the W allele of the gene masks the expression of the w allele.  The W is *dominant*, and the w is *recessive*.  So if every person has two copies of this gene, then the possible combinations are:

WW - Widow's peak
Ww - Widow's peak
ww - straight

Human hair, skin and eye color are not that simple.  Instead of being controlled by only one gene, these traits are each controlled by *several* different genes, each with two or more versions (alleles).
This means that the different versions can combine in unpredictable ways to produce a wide range of phenotypes (physical appearance).

A trait that is controlled by several genes is called a POLYGENIC TRAIT. A polygenic trait is the expression of a single phenotypic trait that is affected by the action of more than one gene.

There are too many examples to list, since most traits are, at least to some degree, polygenic.  But human hair color, eye color, and skin color are among them.

One cute, easy-to-see example of a polygenic trait is the inheritance of fruit color in bell peppers, and it is a bit analogous to the human traits just named. There are at least three genes involved here, which we'll abbreviate as:

  * Y - timing of chlorophyll (green pigment) elimination
        (Y - early; y - normal)
  * R - color of carotenoid pigments
        (R - red; r - yellow)
  * C - regulation of carotenoid deposition
        (C - normal; c1, c2 - two alleles for lower concentration)

(The capital letters indicate the dominant alleles; the lower case indicate various versions of recessive alleles.)

This leads to a few possible genotypes producing interesting phenotypes:
        o Y- rr c1c2 - pale yellow
        o Y- rr Cc2 - darker yellow
        o yy rr CC - green
        o Y- R- CC - red
        o yy Rr CC - purple
        o Y- Rr Cc2 - pale yellow

You can see what these look like here:

http://www.bio.miami.edu/dana/pix/bellpeppers.jpg

See?  It is a little bit like human color, but in this case there are only *three* genes involved.  Imagine how complicated things get when there are more than three genes, as there are in human hair, eye, and skin color!

The more genes involved in the expression/appearance of a trait, the more possible variations there are, and the more impossible it becomes to guess what a baby will look like, especially if you don't know the exact genetics of the parents.  (Knowing the grandparents' phenotypes can help, but usually not very much.)

Similarly, light colored eyes (blue, green, hazel, grey, etc.) are usually considered recessive to dark-colored eyes.  But this trait, too, is controlled by at least five different genes.  

We can, to some degree, predict eye color based only on the gene that controls whether the iris is dark (brown) or light (blue), and not consider the other genes that may make the iris different *shades* of brown, or--with the addition of carotenoid pigments--make a "blue" eye into a hazel, grey or green eye.  The dark iris gene(s) (which we'll abbreviate as B) are generally considered to be dominant to the blue-eye genes (which we'll abbreviate as b).  A person has two alleles for this gene--one from mom and one from dad.  So you can be either BB, Bb or bb.  

A person with BB or Bb will have brown eyes, and a person with bb will have blue eyes as the base color (other genes can affect the actual *shade* of brown or blue, as I already said, and it's far too complex an interaction to even begin to predict what this might be.)  And to make things more complicated, a baby can be born with light eyes, and then develop pigment as she gets older, and end up with brown eyes!

Hope that helps.

Dana