Color of the iris
The iris is usually strongly pigmented, with colors ranging from brown to green, blue, gray, and hazel. Occasionally its color is due to lack of pigmentation, as in the pinkish-white of oculo-cutaneous albinism, or to obscuration of its pigment by blood vessels, as in the red of an abnormally vascularised iris (although human albinos generally have very light blue eyes, as the un-pigmented color of the human iris is a pale blue).
Despite the wide range of colors, there is only one pigment that contributes substantially to normal human iris color, the dark pigment called melanin. Structurally, this huge molecule is only slightly different from its equivalent found in skin and hair.
Genetic and physical factors determining iris color
Iris color is a highly complex phenomenon consisting of the combined effects of texture, pigmentation, fibrous tissue and blood vessels within the iris stroma, which together make up an individual’s epigenetic constitution. A person’s “eye color” is actually the color of one’s iris, the cornea being transparent and the white sclera entirely outside the area of interest. It is a common misconception that the iris color is entirely due to its melanin pigment, but this varies only from brown to black.
Melanin is yellowish-brown to dark brown in the stromal pigment cells, and black in the iris pigment epithelium, which lies in a thin but very opaque layer across the back of the iris. Most human irises also show a condensation of the brownish stromal melanin in the thin anterior border layer, which by its position has an overt influence on the overall color.
The degree of dispersion of the melanin, which is in subcellular bundles called melanosomes, has some influence on the observed color, but melanosomes in the iris of man and other vertebrates are not mobile, and the degree of pigment dispersion cannot be reversed.
Abnormal clumping of melanosomes does occur in disease and may lead to irreversible changes in iris color (see heterochromia, below). Colors other than brown or black are due to selective reflection and absorption from the other stromal components. Sometimes lipofuscin, a yellow “wear and tear” pigment also enters into the visible eye color, especially in aged or diseased green eyes (but not in healthy green human eyes).
Blue is one of the possible eye colors in humans. The “blue” allele, existing in the Bey2 and Gey genes of chromosome 15, is recessive. This means that both genes must have both blue alleles i.e. “blue-blue”, in a person with blue eyes. If one of the alleles were not “blue” (“green” for Gey or “brown” for Bey2) then the person would have those colored eyes respectively.
As either allele (though not both) can be passed on to offspring it is perfectly possible for someone who does not have blue eyes to have blue-eyed children. Because of its recessive nature, this is a certainty if both parents have blue eyes. Though this explanation gives an idea of eye color delineation, it is incomplete, and all the contributing factors towards eye color and its variation are not fully understood.
Faking the iris color
Certain eye colors are sometimes seen as being especially attractive and motif-expressing contact lenses can be worn to mask one’s natural eye color with another. They are rarely needed and almost never recommended by serious medical doctors, unless the patient’s retina needs extra protection, as in aniridia.
Since the introduction of machines which can automatically analyse iris patterns, and their use at some airports as a security measure, it is reported that some people have resorted to colored contact lenses, or deliberate iris injury with lasers, to prevent personal identification.
Iris color as paternity test
As stated above, although there has been much fuss about finding the genes for eye color, there is no simple genetic determinism for such a complex trait, as there is more to iris color than pigmentation. Overall, there is no simple Mendelian inheritance in iris color. Consequently no serious test of paternity can be based on observations or even measurements of iris color, except to note that blue eyes are normally phenotypically recessive, so that a brown-eyed child of two blue-eyed parents may create some doubt about paternity.
Different colors in the two eyes
Heterochromia (also known as a heterochromia iridis or heterochromia iridium) is an ocular condition in which one iris is a different color from the other iris (complete heterochromia), or where the part of one iris is a different color from the remainder (partial heterochromia or sectoral heterochromia).
Uncommon in humans, it is often an indicator of ocular disease, such as chronic iritis or diffuse iris melanoma, but may also occur as a normal variant. Sectors or patches of strikingly different colors in the same iris are less common. Alexander the Great and Anastasios the First were dubbed (dikoros, “with two pupils”) for their patent heterochromias. In their case, this was not a true dicoria (two pupils in the same iris). Real polycoria can be due to disease but is most often due to previous trauma or surgery.
In contrast, heterochromia and variegated iris patterns are common in veterinary practice. Siberian Huskies show heterochromia due to interbreeding, possibly analogous to the genetically-determined Waardenburg syndrome of humans.
Some white cat fancies (e.g., white Persians) may show striking heterochromia, with the commonest pattern being one uniformly blue, the other green. Striking variegation within the same iris is also common in some animals, and is the norm in some species.
Several herding breeds, particularly those with a blue merle coat color (such as Australian Shepherds and Border Collies) may show well-defined blue areas within a brown iris as well as separate blue and darker eyes. Some horses (usually within the white, spotted, palomino or cremello groups of breeds) may show amber, brown, white, and blue all within the same eye, without any sign of eye disease.
One eye with a white or bluish-white iris is also known as a walleye.