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 questions lies in the genes inherited from a one's parents. Blue eyes are found only in people with European ancestry. The widespread nature of blue eyes in these populations suggests that it was a trait that was somehow selected for after it arose. But there is no known advantage to having blue eyes. In fact, some eye diseases, such as uveal melanoma (the most common eye cancer in adults) and Age-related Macular Degeneration, are more common in people with lighter eyes. The lower rate of such diseases in people with darker eye colors may be due to the protective effects of eumelanin in darker eyes. Just as melanin helps protect skin against UV damage from the sun, melanin may also protect cells in the back of the eye that are important for vision. According to (Frudakis, 2004)Because blue eyes don't offer an advantage when it comes to dealing with the environment, some scientists have posited that blue eyes stuck around simply because people with blue eyes were considered more desirable mates. This may be an example of what scientists call "frequency dependent selection" where individuals with rare traits are considered more attractive 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 are 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. 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 genes 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. According to (Amram., 1939) 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. There are actually two types of melanin. Eumelanin is a blackish-brown pigment, while pheomelanin is reddish-yellow. Both types of melanin are contained in tiny granules within skin, hair, and eye cells. 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 having 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. (Clyde, 1999) Say’s 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. You may have noticed that on certain occasions, your eye color looks different. Is it changing? Not really. The eye color you observe in yourself or another person is due to the interaction of light with the eyes. That means that the eye color you perceive can change depending on the lighting conditions. But the melanin content of adult eyes doesn't change. One exception is that some glaucoma medications can actually cause eyes to become darker. Babies' eyes can change color. Many infants are born with blue eyes that then darken during their first year of life. This happens because the melanin in the outer layers of their irises takes a while to build up. 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.
Amram., S. (1939). You and Heredity. New York: Frederick A. Stokes Company, . Clyde, O. (1999). The Human Eye: Structure and Function. Sinauer Associate Inc. Frudakis, S. a. (2004). Eye Colour: Portals into Pigmentation genes and ancestry. Trends Genet.