The Genetics of Eye Color
Linda Claire Guttery
How do children inherit eye color? Can a child's eye color be predicted? Why are an albino's eyes pink? How can two brown eyed parents produce a blue eyed child? Why are my eyes a darker blue than my sibling's? How are the colors in the iris formed? These are questions one may have wondered from time to time. The answer to all of these question lies in the genes inherited from a one's parents.
Different eye colors are produced because of the different amounts and patterns of pigment in the iris. The amount of pigment and the pattern of the pigment is determined by a person's genetic makeup. The DNA received from one's parents determines what color eyes they will have.
Each human has 46 chromosomes located in the nucleus of the cell. These are divided into 23 pairs of chromosomes. A baby inherits one chromosome from each parent in each pair of chromosomes. A piece of DNA on a chromosome is called a gene. Genes are the basic unit of heredity, they determine many characteristics about a baby. Genes also come in pairs. Alleles are found in genes and determine the appearance of any characteristic. There are two alleles for each trait inherited. If the two alleles are the same then they are homozygous for that gene. If the alleles are different, then they are called heterozygous. One allele is expressed over the other allele. This is called the dominant allele, the unexpressed allele is called recessive. For example, if there was a brown allele and a blue allele, the brown is dominant, so the person would have brown eyes. But not just one pair of genes can control a single trait. Right now there are three known gene pairs that control eye color. The bey 2 gene on chromosome 15 contains a brown and blue allele. Also on chromosome 15, the bey 1 gene is the central brown gene. On chromosome pair 19 the gey gene contains a green allele and a blue allele.
A green allele is dominant over a blue allele, and a brown allele is dominant over both green and blue alleles. For the bey 2 gene if a person has a brown allele then they will have brown eyes. In the gey gene the green allele is dominant over the blue allele, but it is still recessive next to a brown allele. For example if a person has a brown allele on chromosome 15, but all the other alleles are blue or green, they will have brown eyes. A green eyed person would have a green allele on chromosome 19 and all or some other blue alleles. Blue eyes are produced only with two blue eye genes. All four alleles must be blue to produce a blue eyed person.
Another way of predicting the color of a child's eyes is to use the parent's eye color genes. If both parents have a blue and brown gene, their eyes are brown, but if the child inherits the blue gene from each parent then the child will have blue eyes. If the child only inherits one blue gene then they will have brown eyes. The genetics determine what color a child will have, but how exactly does this color form in the eye?
Melanin, a pigment also found in the skin, is the substance that produces the eye colors specified by the genes. The amount and placement of the melanin produces the different eye colors that we see. Melanin is a dark brown pigment that is placed in the iris. The more melanin used in the iris means the darker the eye color will appear, the less melanin used means that the eye color will be lighter. The genes tell the enzymes how much melanin to deposit in the iris. A newborn's eyes appear blue, but may darken over the next few years. Melanin production has not begun at the time of birth. A child's true eye color cannot be determined until the age of three.
There are two layers to the iris, the anterior and the external, or front and back layers. To produce blue eyes, there is no pigment found in the front layer. The brown pigment melanin is deposited in the back layer only. It appears blue because of reflection and diffraction of light. In green eyes, a small amount of melanin is deposited in the front layer of the iris along with the melanin found in the back layer. The additional pigment to the amount needed for blue eyes, causes the eye to appear green. To produce gray eyes, the dark pigment is distributed in the front layer of the iris and over the blue background it appears gray. In brown eyes there is so much pigment in the front layer, that the blue behind is completely covered up. Some people have so much pigment in the front layer that their eyes appear very dark brown or black. Hazel, blue-green, gray-blue eye colors are produced by different amounts of pigmentation and the pattern in which the pigment is placed. Albino eyes are have no pigment at all in either layer of the iris. The iris appears pink or red because of the reflection of blood vessels in the back of the eye. The pattern in which the pigment is deposited is also determined by genetics. The pigment may be deposited in rings, clouds, radial stripes, or spread over the entire iris.
If a person has two different eye colors this is called heterochromia. This is considered abnormal and may be pathological. If a person has always had two different colored eyes, then this is called congenital heterochromia and is cause by a difference in the early stages of development of the iris. This is rare, but nothing to have medical concern over. If the heterochromia is acquired after time then there is reason to believe that something has happened within the body to create changes in melanin synthesis or degradation. This can be caused by diseases in the eye or by certain types of medicines.
A person's eye color is determined by the genes inherited from their parents. The types of alleles received from the parents are assigned to certain chromosomes. The dominant genes are expressed and the recessive genes are hidden. In the development of the iris those genes tell enzymes to produce and place a certain amount of melanin in the iris to form the eye color.
Eye Color Calculator
The Genetics of Human Eye Color. Oregon State University. 6 Dec. 2003 <http://www.seps.org/cvoracle/faq/eyecolor.html>.
Inheriting Eye Color. Athro, Limited. 8 Dec. 2003 <http://www.athro.com/evo/gen/genefr2.html>.
Oyster, Clyde. The Human Eye: Structure and Function. Massachusetts: Sinauer Associates, Inc., 1999.
Scheinfeld, Amram. You and Heredity. New York: Frederick A. Stokes Company, 1939.