Introduction
It is common knowledge these days that we inherit physical traits from our parents who pass them to us in the form of genes.
The genes function as a look-up table, called genotype, which acts as the main, though not only, determining factor of what characteristics we develop (which is the phenotype).
Because we have two parents, we inherit the genes in pairs called alleles. To follow our look-up table analogy, we can think of each gene of either having the value of 0 or 1, a binary code. For example, if the gene for freckling the nose is 1, we are to develop freckles on our nose. If it is 0, we aren't.
But what if we inherit 1 from one parent, 0 from the other? Simple, the two values are run through the logical or operation. Hence, if we have at least one 1, we grow freckles. If we have two 0's, we don't. The 1-gene is known as dominant, the 0-gene as recessive.
Incomplete Dominance
What most people don't realize is that not all genes use the logical or method for determining which trait we inherit.
That is the cause of many a myth, such as that brown hair is recessive vs. black hair but dominant vs. blond hair, and that red hair is somehow recessive to all other hair colors.
That is not true. For that matter, while we like to classify hair colors into just four (black, brown, blond, and red), there are in reality many different shades of hair color. From the genotype perspective there are as many as 27 possible shades of hair color. On the phenotype level, there are even more as a result of environmental influences.
This wonderful variety is made possible due to the method of incomplete dominance (other names are used as well, but I will stick to this one). In this case, the ones and zeros are not ored, they are added up. So, a single allele can be 0+0=0, 0+1=1, 1+0=1, 1+1=2. (Note that statistically, this makes 1 more likely to occur than either 0 or 2.)
Polygenic traits
Alas, that still gives us only three possibilities. Not enough for 27 shades of hair color, is it.
Polygenic method to the rescue: Some traits use more than one gene pair to determine what to do. For example, skin color (totally independent of hair color) is determined by three gene pairs, which makes it 6 genes altogether, just added up to anything between 0 (very light color) to 6 (very dark color), for a total of 7 possible skin colors.
Hair color
If you think we have now discussed enough to know how hair color is inherited, well, no, we did not.
There are two types of pigments that determine one's hair color, and they are "managed" independent of each other. One is brown, the other is red. You can have both, you can have neither, or you can have some of one, or any combination of both.
Brown hair
The amount of brown melanin in the hair is determined by the polygenic method. Four different gene pairs determine how brown the hair is: Four from daddy, four from mommy.
That's eight genes altogether which, through the incomplete dominance method can add up to a value between 0 and 8, for nine possible shades of brown. If you have 0, your hair is of the lightest blond color possible. If you have 8, your hair is the darkest brown, i.e., "black". Statistically, again, the middle values are more likely than the extremes of 0 and 8.
So, all things being equal, medium brown should be the most common hair color. Of course, it isn't, because all things are not equal. Due to various conditions in different parts of the world, dark-haired (and dark-skinned) people survived around the equator, while the others did not make it. On the other hand, of the people living way up North, the light-haired and light-skinned variety survived.
Red hair
Red hair is not polygenic, but it does use incomplete dominance. That means there is only one gene pair responsible for red hair, and it can add up to 0, 1, or 2.
If you have 0, there is no red in your hair. If you have 1, there is red in your hair, if you have 2, there's lotsa red in your hair. Again, statistically, 75% of people should be redheads.
So, why aren't they? Well, the funny thing is that if your hair is dark, you may be a redhead and not know it!
The red and the brown colors in our hair combine visually, just as the red, green, and blue pixels of your computer screen do!
Suppose your brown genes are few (so, you should be blond), but you have one red gene. You will be perceived as having light red hair and considered a redhead. Or you may result in strawberry blond which some people consider red, others do not.
If you have blond hair and both red genes, you may end up with flaming red hair, and no one will ever question your redheadness.
If you have brown and one red, you will have "reddish brown" hair. If two red genes, you will have auburn hair. If you have very dark brown (black) hair, it may completely cover up the redness visually, so you (and others) may have no idea that you actually are a redhead! Though it may show in direct sunlight.
Summary
There are nine possible shades between blond and black hair. There are three possible states of red hair (no, yes, very much so). That combines to 9*3=27 possible hair colors on the genotype level. Of those, 18 are various shades of red, 9 are shades that are not red.
Yep, redheads come in a wider variety than everyone else! What can I say to that? By nature's own design, we (redheads) are more original than anyone else!
Think about it next time you feel like making fun of redheads. Besides, never forget, for all you know, you may be a redhead and not even know it!
For a more in-depth discussion of this topic, as well as for additional links, visit my write-up at http://www.redprince.net/genlab/.