Expert: Dana Krempels, Ph.D. - 2/18/2009

Question
Both of my parents have green eyes, yet all of their children (4) have brown eyes, not light brown either quite dark brown.
I've been trying to understand how this is possible since in order for them to give us brown eyes, shouldn't one of them have brown eyes seeing as brown is a dominant gene?

Answer
Dear Sarah,

Eye color, like hair color and skin color, are controlled not by one, but by several genes each.  This means that there can be a great deal of variation in eye color, even in the offspring of two parents who have what's usually considered a recessive version of the eye color gene.

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.  Bear with me for a short genetics review, or you can just skip the next part.  :)

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

This is a little bit like human color, but in the case of the peppers, there are only *three* genes involved.  Things get very complicated when there are more than three genes, as there may be 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!

All that said, it's still unusual for two light-eyed parents to produce children with dark brown eyes.  One possible explanation is that one of your parents inherited a germline cell (i.e,. the cells that give rise to sperm or eggs) reversion mutation, which would cause the mutant light-eyed allele to revert to its original brown-eyed form, and the children inheriting that reversion mutation would have brown eyes of various shades.

Have you asked your parents about this?  That might be a good place to start.  I doubt there's anything they're not telling you about your parentage.  But for now, assume that the reversion mutation is a good possibility in your family.

Hope this helps.

Dana
It's not impossible, though fairly unusual.