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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

Bibliography

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 Morphologic Correlates and of Some Conditions That Affect Iridial Pigmentation." Survey of Ophthalmology 41, supplement 2 (1997): s117-s123.

McKusick, Victor A. Mendelian Inheritance in Man: A Catalog of Human Genes and Genetic Disorders, 12th ed. Baltimore: Johns Hopkins University Press, 1998.

Internet Resource

Online Mendelian Inheritance in Man. Johns Hopkins University and National Center for Biotechnology Information. <http://www.ncbi.nlm.nih.gov/Omim>.

Fertilization

Fertilization is the fusion of a female's egg cell (oocyte) and a male's sperm cell (spermatozoa) to form the first cell of a new and unique being. While on the surface this sounds like a simple process, there are many factors that make this possible.

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