Expert: Dana Krempels, Ph.D. - 1/4/2009

Question
Why is it possible for some people with dark brown hair to have light haired children but not others with almost the same dark brown or black hair colour when paired with a light haired person?

Dont these people have almost the same amount of pigments to pass on?

I read somewhere that haircolour is determined in HHHHHHHH for black hair and hhhhhhhh for blonde. A dark haired person would be HHHHHHHH or HHHHHHHh. There would technically not be a big difference in the genes being passed on, but it looks different in real life?

Mary

Answer
Dear Mary,

Hair color, like eye and skin color, is controlled by more than one gene.  And this makes its inheritance quite complex. To completely understand this, and if you've got some time, here's a quick overview...

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.

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!

Skin color is probably the most complex of all the traits.  Freckles are apparently controlled by only one gene, and freckles are considered dominant to non-freckles.  But various factors during development can affect this, and exposure to sunlight can also determine the level of freckling that is expressed.  Genetics of overall skin tone is much more complex, and nearly impossible to predict.  For example, two people of African American descent (which means they usually will have some Caucasoid skin pigmentation genes that are recessive to the darker pigmentation genes) and medium-brown skin can produce children with skin tones ranging from very pale to extremely dark, depending on the combination of skin color genes each child inherits.

So when you cite HHHHHHH as the genotype for black hair and hhhhh for blonde, that's not quite all that's going on.  There are several different *alleles* for each hair color/pigment deposition gene.  Let's say there are only five different genes for hair color.  A person with very dark brown hair might have a genotype something like this:

Xx Xx XX xx Xx  etc.

Since a X will mask the expression of the x at any given gene locus, this person can have dark hair despite *carrying* alleles for light hair.

Since only one of the paired alleles is passed on to the child in each gamete, it's possible for the dark-haired person above to produce a gamete carrying:

x  x  X  X x

or

X  X  X  x X

or

x  X  X  x  x

etc.  Any possible number of combinations, given the starting material.

If the person above has kids with a blonde person, or even with a dark-haired person of mixed heritage, then it's possible for the alleles to combine in numerous different ways, and for the children to have any number of shades of hair, from blonde to brown to very dark.

Hope that helps!

Dana