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.