Expert: Dana Krempels, Ph.D. - 4/11/2008

Question
I have hazel eyes and brown curly hair. My mother has brown curly hair and brown eyes. My father has brown curly hair and blue eyes. One of my brothers has hazel eyes and brown curly hair, the other has brown eyes and brown curly hair.
My husband, his parents, and 2 of his brothers are blonde and blue eyed. His sister and one brother have brown hair and blue eyes.  
Can you predict what our child might have?

Answer
Dear Christina,

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.)

Hair color is a result of interaction between several genes that not only control the *color* of the hair pigmentation (brown eumelanin pigment or red phaeomelanin pigment), but also *how much* pigment is deposited in the hair shaft.  The darker the hair, the greater the melanin deposition, but one can't really predict how dark a baby's hair will be, since s/he may inherit a wide variety of "darkness level" genes from both parents, and they can recombine in various ways.

Since  genes coding for darker pigmented hair tends to mask the expression of those coding for lighter colored hair, a baby will have darker hair *if* s/he inherits the darker pigmentation genes.  But that's not a given, as either parent may pass along recessive genes for lighter hair, and they can recombine in the child to produce light hair even from two parents with medium brown hair.  Since both your parents have brown hair like yours, it's a bit more likely that you have an array of "medium pigment" genes to pass on.  But you may find out something different once your baby is born.

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.  

Since both you and your husband have light-colored eyes, it is very unlikely that the child will have dark brown eyes.  They will likely be blue, green, hazel or grey.  But because other genes are involved in the various shades, it's not possible to predict the exact color.  And babies born with blue eyes sometimes have darker eyes or eye color that changes as they get older.  So don't get too attached to those blue eyes if s/he comes with them:  they could change!  You'll probably know that by the age of six months.  If they're still blue by then, there's a better chance that they'll stay blue.

In general, the gene coding for curly hair (C) is considered dominant to that coding for straight hair (c).  But this trait, too, may be affected by the action of more than one gene.  You don't say whether your husband has curly or straight hair.  But if he has straight hair, then there's probably a 50% chance that the child will have curly hair, if you have one allele for each trait (Cc) (which is possible), or a 100% chance if you have two alleles for curly hair (CC).  But since either or both of your parents might also be Cc, there's no way to know for sure in advance.

I know this doesn't give you the answer you were seeking, but I hope it at least gives you an idea of how truly complicated (if not impossible) it is to predict something like this.  

Whatever color the baby, I hope s/he will be happy and healthy!  Good luck!

Dana