Sorry to have taken so long answering your question Sam. The answer to the first two parts of your question, is not as easy as you think. I have been trying to get more information for you on the genes for height. As you will see below, that is a difficult issue to address in a simple manner.
First off, you should know that there is more than one gene for eye color. As you will see, all of these issues of color are more complicated than they would be if they were controlled with a single gene. The genes for brown and blue eye color, called Bey1 and Bey2 are found on chromosome 15, but the gene for green and blue eye color, called Gey1, is found on chromosome 19. There are probably several other genes that control eye color as well, but which have not been identified yet. In addition, there is a condition known as Occular Albinism (OA) where the iris us unpigmented. OA occurs when there is some defect in the enzymatic pathways which result in pigment being produced in the iris. Two of the genes responsible for OA, called OA1 and OA2, are found on the X chromosome.
Hair color is probably even more complicated than eye color. Two genes for hair color are very close to the genes for eye color. There are at least two genes that control brown hair color, HCL1 on chromosome 19 and HCL3 on chromosome 15. Each of these is very close to the corresponding eye color gene on each chromosome, so that these genes are usually inherited together, in what we call linkage. The linkage of these genes explains why you frequently see people with either brown hair and brown eyes or blond hair and blue eyes, but see people with blond hair and brown eyes or brown hair and blue eyes much less frequently.
In addition, there seems to be at least one gene responsible for red hair. This gene, HCL2, is found on chromosome 4. In addition, red hair color seems to be associated with a genetic disease called Brittle Cornea Syndrome. The location of the gene responsible for this gene is still not known, but it seems likely that there are still other genes which control hair color.
Like the hair color genes, there are likely multiple genes that control skin color. All of these hair and skin color genes are involved in the production of various types of melanin, which comes in either a red-blond or a brown- black color. In addition to all of these melanin producing genes, there is a gene which controls the cells that produce the various types of melanin, causing them to produce more or less. This gene, called MC1R directs the synthesis of the melanocortin-stimulating hormone receptor, and is found on chromosome 16. MC1R is therefore a major contributant to skin and hair color, and works by regulating the activity of the other hair and skin color genes.
On top of all of this, there is the melanin is the product of a long chain of enzymatic reactions that begin with the amino acid tyrosine. If there is a break in this chain, you will not be able to make melanin, and albinism will result. The various genes in this chain are found on many different chromosomes. One such albinism gene, the Occulocutaneous Albinism gene or OCA1, is found on chromosome 11, and another, OCA2, is found on chromosome 15. There is no tyrosinase (the enzyme that produced tyrosine) activity in the type of albinism associated with OCA1, but there is in the type of albinism associated with OCA2.
If you think that the genetics of eye, hair and skin color is complicated, try to figure out the genetics of height and growth regulation. There are a large number of traits that contribute to height. I have just collected a few examples to demonstrate.
Laron syndrome is a genetic disorder that results in short stature, among other complications. This syndrome is controlled by a gene on chromosome 5, which is responsible for the synthesis of the insulin-like growth factor in response to the presence of growth hormone. If you cannot respond to the signal to grow indicated by growth hormone, you will be short.
Marfan syndrome is a genetic disorder that results in excessive height, long limbs, and a long face (also along with other problems). President Abraham Lincoln is thought to have suffered from this syndrome. This syndrome is controlled by the fibrillin-1 gene on chromosome 15, which produces the connective molecule fibrillin. Something about abnormal fibrillin results in excessive growth in length, which results in an overall increase in height.
The gene for growth hormone is found on chromosome 17, and the gene for growth hormone receptor is found on chromosome 5. Variation in these genes can result in higher or lower levels of growth hormone production, or stronger or weaker responses to the presence of growth hormone, all of which can result in height variation.
In addition, a gene has been identified for a condition called Short Stature (SS). The SS gene is found in what is called the pseudoautosomal region of the sex chromosomes X and Y. This is the region of the sex chromosomes that acts like a regular non-sex chromosome, in that the two chromosomes can pair up and recombine. People who have only one copy of the SS gene are often only 5 feet in height.
Obviously, I could go on and on. The best thing to do would be for you to take a look on the Online Mendlian Inheritance in Man database. I have included numerous links to the database below. I hope this answers at least some of your questions!
Here are some general links on the various genes and associated issues.
Eye Color
Occular Albinism
Hair Color
Skin Color
Occulocutaneous Albinism
Growth and Height
Try the links in the MadSci Library for more information on Genetics.
Created on ... décembre 22, 2003