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