|MadSci Network: Genetics|
Good question Michelle,
The answer to your question is 'genetic linkage', the phenomenon observed when two genetic markers (loci) are transmitted from a parent to the same child more often than 50% of the time. The rules of simple Mendelian genetics suggest that any two markers will assort independently of one another, so that each functional version (or allele) of each gene has a 50% chance of being transmitted to the next generation. When alleles are transmitted together more often than 50%, it usually means that they are on the same chromosome, and when they are transmitted together a much higher %age of the time than 50%, it usually means that they are very close to each other on that chromosome. This close proximity of linked genetic loci means that recombination (the creation of new combinations of alleles on a chromosome) between these loci will be rare. The closer a pair of genetic loci are the lower the chance of recombination.
Hair color and eye color are traits which depend on the presence or absence of particular alleles of genes that produce color pigments. There are a number of different genes that are responsible for eye and hair color, but they seem to be found primarily on chromosomes 15 and 19. Almost all of these genes have multiple alleles which produce different levels of color pigments. Most members of our species have brown hair and brown eyes because most people have pigment genes that produce brown pigment (eumelanin). However, some human populations (mostly European populations, although some groups in other parts of the world have blonde hair) have high frequencies of blue or green eyes, or blond or red hair. These colors are produced by pigment genes producing yellow-red pigment (phomelanin) or other pigments. People with brown hair or brown eyes might be producing these other pigments, but the are covered up by the darker melanin.
It seems likely that traits like blonde hair or green eyes arose independently in isolated populations, and since the genes for eye color and hair color tend to be linked, the traits that arose in these small populations tend to be transmitted together. The genes for red hair and green eyes seem to have arisen in the ancient Celtic populations of the British Isles, and blond hair and blue eyes likely arose in northern Europe. These traits are transmitted together because they are closely linked on their respective chromosomes, and because they are closely linked, that means that the chance that an allele for brown pigment (either hair or eyes) will recombine onto one of those chromosomes. So, if one of your 15th chromosomes has a gene for brown hair, it is most likely that the gene for eye color on that same chromosome will produce brown eye pigment as well, and if the other chromosome 15 has a gene for blue eye color, it is likely that the gene for hair color on that chromosome will produce blonde hair pigment.
Now, there is one additional issue confounding this whole matter. Everyone has two copies of each chromosome. This raises the question, what happens in the example I gave above, where you have one copy of a gene for brown eyes and one for blue, and one copy of a gene for brown hair and one for blonde? Well, if you remember above in the discussion of different types of melanin, darker eumelanin tends to overpower lighter phomelanin, so if you have a situation where both types are being produced, only the darker pigment will be visible. Another way to say this is that traits like red or blonde hair or like blue or green eyes are recessive traits, while traits like brown hair and brown eyes are dominant traits.
So to summarize, genes for hair and eye color are found on several chromosomes, and they tend to be very close to each other on their respective chromosomes. Because traits like red hair and green eyes arose in the same population, they are found on the same chromosome. In addition, traits like brown hair and brown eyes are dominant, while traits like red hair and green eyes are recessive. So, red hair and brown eyes are uncommon because there is a low chance that the gene for brown eyes will recombine onto a chromosome with the gene for red hair, and in order for someone to have red hair and brown eyes, they would have to have two copies of the gene for red hair and at least one copy of a gene for brown hair.
Try the links in the MadSci Library for more information on Genetics.