Expert: Dana Krempels, Ph.D. - 8/25/2008

Question
Hi.  I'm curious about something.  First of all, I am a fan of red hair.  I just
think it is so pretty.  My own hair is medium brown but turns bright red when
I bleach it, no matter what product I use-- even professionals are unable to
counter this effect.  Does this mean I have a recessive red gene?  My hair was
blonde as a young child; my mom's hair was reddish-brown when she was
younger, now is dark brown, and my dad's hair is almost black.  Both were
also blonde as children.  Another question:  My husband's hair is light brown
and was also blonde as a child.  I don't think he has any red in his family.  My
2-year-old daughter's hair is a golden blonde with ever so subtle coppery
highlights.   Does she carry the recessive red gene?   Is there a chance I could
ever have a red-headed child?  I guess I'm confused about the number of
genes involved, and what are my chances of having offspring (children or
grandchildren) with some form of beautiful red hair!

Answer
Dear Lara,

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.  It's not really a matter of a child taking after the father or mother's side.  Genes don't work that way.  What matters is which parent has 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.

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 two versions of each gene (called *alleles) may be the same in a single person, or they may be different.

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 elimination (Y - early; y - normal)
  * R - color of carotenoids (R - red; r - yellow)
  * C - regulation of carotenoid deposition
   (C - normal; c1, c2 - lowered 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--the one that's your personal favorite!  :) ), 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.

The darker the hair is pigmented with eumelanin (brown), the less any phaeomelanin (red) will show through, but sometimes even a very dark-haired person will have coppery highlights when the sun shines on the hair just so.

That said, pure red hair is usually considered to be recessive to the other hair colors, even though hair color is controlled by several genes.  Unless your husband has red hair in his family tree, it's more likely that your baby will have, at best, reddish-brown hair, but not bright red.  Again, it's nearly impossible to predict what the combination of genes in your baby will produce, and hair color can also *change* with age, as you already know from experience with your own family members.

I know this doesn't give you the exact 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.  My educated guess would be that your children could have hair ranging anywhere from golden red (like your daughter) to reddish-brown or even dark brown with red highlights in the sun.  But as you can see from all of the above, that is just what it is:  an educated (if somewhat wild) guess.

Hope that helps.

Dana