When someone asks, "What color are your eyes?" he should really be asking "What color are your irises?" because it is the iris that contains the pigment that determines the color of your eyes. Despite the fascination eye color holds for us, the genes responsible for it in humans are not well-known and are more complex than most people think.
The iris is the most visible portion of the uveal tract, which is the middle compartment of the eye. The iris is made up of blood vessels and connective tissue, in addition to melanocytes and other pigmented cells that are responsible for its distinctive color.
The iris contains muscles that control its movement and allow for changes in the size of the pupil, controlling the amount of light that enters the eye and that ultimately reaches the retina. Though its function is easily observed and obvious to any who look at it under varying lighting conditions, it is the appearance of the iris—specifically, its color—that is most striking and apparent. The structure itself takes its name from Iris, the Greek goddess of the rainbow and messenger of the gods.
The iris consists of two layers of different embryological origin. The anterior border of the iris consists of the stroma, a loose and interrupted layer of connective tissue. It is composed of melanocytes and nonpigmented cells, as well as other types of cells and tissues. Melanocytes contain melanin, a brown or black pigment. The overall structure of the stroma is similar in irises of all colors. The iris pigment epithelium forms the densely pigmented posterior layer of the iris. It consists of two layers of tightly fused, pigmented cells.
Differences in iris color depends on the amount of pigmentation in the deep stroma, especially the anterior border layer, and on the density of the stroma, both of which influence how much light, and what wavelengths, are absorbed and reflected. As with other objects, the color we see is the result of reflected light. The stroma of brown irises is densely pigmented with melanin and absorbs much of the light that enters it. In many human populations, brown is the only eye color. Blue irises have lightly pigmented stroma, and light of longer wavelengths (red to yellow) readily penetrates the iris and is absorbed, while some light of shorter wavelength (blue) is reflected back and scattered by the iris stroma; hence the blue color.
The inability to make melanin, as in albinism, leaves the iris without any pigment. The iris appears pink from the color of the blood flowing through it. Albinism is a recessive condition, requiring two defective alleles for melanin production, one inherited from each parent. Albinism also prevents pigment production in the hair and skin.
Genetics of Eye Color
Differences in iris color have been attributed to such causes as the temperature of the brain and eyes. Some people have stressed differences between dark-eyed and light-eyed populations and have suggested that eye color is related to general traits such as temperament or intellect. But, toward the middle of the nineteenth century, it had become clear that iris color was due to iris pigment, that this pigment developed soon after birth, and that the final quantity and distribution of the pigment was a hereditary trait.
Originally, iris color was thought to be a simple trait—one governed by a single gene with multiple forms, or alleles, corresponding to each color. In this scheme, blue was thought to be recessive, requiring two copies of the blue allele in order to be displayed. Therefore, two blue-eyed parents could have only blue-eyed children, since each parent had only blue alleles. However, repeated observation of brown-eyed offspring from two blue-eyed parents showed this view to be wrong. Iris color is likely to be a polygenic trait—one governed by at least two genes and possibly more.
Brown versus blue eye color is believed to be controlled by two genes on chromosome 15, called BEY1 and BEY2. Green versus blue eye color is believed to be controlled by a gene on chromosome 15, called GEY. In this system, blue is believed to be recessive to both brown and green. The protein products of these genes are unknown, however, as is the number of alleles possible for each. Furthermore, these three do not fully explain inheritance of all eye colors. More genes, which likely modify the action of these three, are probably involved.
Traditionally, iris color was felt to be stable throughout adulthood. However, iris color may change in response to disease. For example, there is a gradual unilateral (one-sided) loss of pigmentation in Horner's syndrome and in Fuchs' heterochromic iridocyclitis. There is also evidence for pigment loss in the iris as a result of aging, and changes in iris color may also occur spontaneously in normal people after adolescence. In addition, some commonly used drugs such as latanoprost (which lowers intraocular pressure) have caused hyperpigmentation in some irises.
see also Complex Traits; Inheritance Patterns.
Eric A. Postel
American Academy of Ophthalmology. Fundamentals and Principles of Ophthalmology: Basic and Clinical Science Course. San Francisco: American Academy of Ophthalmology, 1995.
Imesch, Pascal D., et al. "The Color of the Human Eye: A Review of MorphologicCorrelates and of Some Conditions That Affect Iridial Pigmentation." Survey of Ophthalmology 41, supplement 2 (1997): s117-s123.