question: Why do albinos specifically have red eyes but not *"regular" pigmented people/animals?
comment: Some people with albinism have red eyes because of a lack of pigment in their iris. The red color is just the blood vessels in their eyes.
passages: Normally, there is a thick layer of melanin on the back of the iris. Even people with the lightest blue eyes, with no melanin on the front of the iris at all, have dark brown coloration on the back of it, to prevent light from scattering around inside the eye. In those with milder forms of albinism, the color of the iris is typically blue but can vary from blue to brown. In severe forms of albinism, there is no pigment on the back of the iris, and light from inside the eye can pass through the iris to the front. In these cases, the only color seen is the red from the hemoglobin of the blood in the capillaries of the iris. Such albinos have pink eyes, as do albino rabbits, mice, or any other animal with a total lack of melanin. Transillumination defects can almost always be observed during an eye examination due to lack of iridial pigmentation. The ocular albino also lacks normal amounts of melanin in the retina as well, which allows more light than normal to reflect off the retina and out of the eye. Because of this, the pupillary reflex is much more pronounced in albino individuals, and this can emphasize the red eye effect in photographs.
   A further difference between albinism and leucism is in eye colour. Due to the lack of melanin production in both the retinal pigmented epithelium (RPE) and iris, those affected by albinism typically have red eyes due to the underlying blood vessels showing through. In contrast, most leucistic animals have normally coloured eyes. This is because the melanocytes of the RPE are not derived from the neural crest, instead an outpouching of the neural tube generates the optic cup which, in turn, forms the retina. As these cells are from an independent developmental origin, they are typically unaffected by the genetic cause of leucism.
   Some rabbits that appear to be pointed white lack the red eyes, which indicates they are of different breeding. The following such coat colors are examples of those created, not with the ch gene, but with the cchl, cchd, or cchm gene in conjunction with the e gene:
  
  BULLET::::- Blue point
  
  BULLET::::- Chocolate point
  
  BULLET::::- Lilac point
  
  BULLET::::- Pearl
  
  BULLET::::- Sable point
  
  BULLET::::- Sallander
  
  BULLET::::- Seal point
  
  BULLET::::- Siamese
   Section::::In mammals.
  
  The only pigments that mammals produce are melanins. For a mammal to be unable to chemically manufacture melanin renders it completely pigmentless. This condition is more commonly called albinism. Amelanistic mammals have white hair, pink skin, and eyes that have a pink, red, or violet appearance. Reddish eyes are due to the lack of pigment in the iris pigment epithelium. When the stroma is unpigmented but the iris pigment epithelium is not, mammalian eyes appear blue. Melanin in the pigment epithelium is critical for visual acuity and contrast.
   Melanin functions in the normal development of various parts of the eye, including the iris, retina, eye muscles, and optic nerve. The absence of melanin results in abnormal development of eyes and leads to problems with focusing, and depth perception. The eyes of albino animals appear red because the colour of the red blood cells in the retina can be seen through the iris, which has no pigment to obscure this. Some albino animals may have pale-blue eyes due to other colour generating processes. Albino vertebrates exposed to intense light typically lose photoreceptors due to apoptosis.
   Section::::In fish.:Naturally occurring.:In Sarcopterygii.
  
  Albinism in the African lungfish has been reported on at least two occasions.
  
  Section::::In reptiles.
  
  Many reptiles labeled as albino are, in fact, not completely lacking in all colour pigments. They are actually amelanistic, not albino. Reptiles often possess at least two pigments. Among the most common are xanthin (yellow) and etherin (red). An amelanistic reptile therefore, may still have pale yellow, orange, or red pigmentation.
   While mammals derive their pigments only from melanins, fish, reptiles and birds rely on a number of pigments apart from melanins: carotenoids, porphyrins, psittacofulvins, pterins, etc. Most commonly, reptiles with a condition homologous to human OCA1A retain their reddish and orangish hues. As a result, birds and reptiles without the ability to manufacture tyrosinase are more accurately described as "amelanistic." Horses do not have non-melanin pigments and so if they were albino, would have no pigmentation. The retained pigment of dilute horses, like cremellos, is not comparable to the retained pigment of amelanistic "albino" birds and reptiles.
   Humans exhibit a wide range of pigmentation levels as a species. However, the diagnosis of albinism in humans is based on visual impairment, which has not been described in white horses. Vision problems are not associated with gray, dilute, or white coat colors in horses, and blue eyes in horses do not indicate poor vision. Eyes are pigmented at the front of the iris called the stroma, and in a thin layer at the back of the iris in tissue called the iris pigment epithelium. The iris pigment epithelium prevents damaging light scattering within the eye. Blue-eyed humans and mammals have little or no pigment in the stroma, but retain pigment in the iris pigment epithelium. If pigment is missing from both the stroma and the iris pigment epithelium, the only pigment in the eye is the hemoglobin in blood vessels. This accounts for the reddish appearance of eyes in some types of albinism.
   BULLET::::- "Whiteface," a condition that affects some parrot species, is caused by a lack of psittacins.
  
  BULLET::::- Axanthism is a condition common in reptiles and amphibians, in which xanthophore metabolism is affected rather than synthesis of melanin, resulting in reduction or absence of red and yellow pteridine pigments.
  
  BULLET::::- Leucism differs from albinism in that the melanin is, at least, partially absent but the eyes retain their usual color. Some leucistic animals are white or pale because of chromatophore (pigment cell) defects, and do not lack melanin.
   Other vertebrates, such as fishes, amphibians, reptiles and birds, produce a variety of non-melanin pigments. Disruption of melanin production does not affect the production of these pigments. Non-melanin pigments in other vertebrates are produced by cells called chromatophores. Within this categorization, xanthophores are cells that contain primarily yellowish pteridines, while erythrophores contain primarily orangish carotenoids. Some species also possess iridophores or leucophores, which do not contain true pigments, but light-reflective structures that give iridescence. An extremely uncommon type of chromatophore, the cyanophore, produces a very vivid blue pigment. Amelanism in fishes, amphibians, reptiles and birds has the same genetic etiology as in mammals: loss of tyrosinase function. However, due to the presence of other pigments, other amelanistic vertebrates are seldom white and red-eyed like amelanistic mammals.
   In all albino mammals studied, the centre of the retina is under-developed and there is a deficit of rod cells; the central ganglion cell density is approximately 25% below normal (except for the gray squirrel). In nearly all mammals, the overwhelming majority of photoreceptors are rods rather than cones. Albinism specifically affects the rod cells, but the number and distribution of the cones is unaffected. In contrast, the retina of birds is cone rich meaning that the vision of albino birds is affected less than albino mammals.
  
  Section::::Consequences.:Reduced protection from sunlight.
   Cnidaria, which are an outgroup to the taxa mentioned above, express c-opsins—but r-opsins are yet to be found in this group. Incidentally, the melanin produced in the cnidaria is produced in the same fashion as that in vertebrates, suggesting the common descent of this pigment.
  
  Section::::See also.
  
  BULLET::::- Adaptation (eye) (Night vision)
  
  BULLET::::- Emission theory (vision)
  
  BULLET::::- Eye colour
  
  BULLET::::- Eye development
  
  BULLET::::- Eye disease
  
  BULLET::::- Eye injury
  
  BULLET::::- Eye movement
  
  BULLET::::- Eyelid
  
  BULLET::::- Nictitating membrane
  
  BULLET::::- Orbital x-ray
  
  BULLET::::- Simple eye in invertebrates
  
  BULLET::::- "Tapetum lucidum"
  
  BULLET::::- Tears
  
  Section::::External links.
  
  BULLET::::- Evolution of the eye
   The color of the fundus varies both between and within species. In one study of primates the retina is blue, green, yellow, orange, and red; only the human fundus (from a lightly pigmented blond person) is red. The major differences noted among the "higher" primate species were size and regularity of the border of macular area, size and shape of the optic disc, apparent 'texturing' of retina, and pigmentation of retina.
  
  Section::::Clinical significance.
   Zebrafish have three types of chromatophores—iridophores, melanophores, and xanthophores—which produce silver, black, and yellow pigmentation respectively. Zebrafish that lack iridophores are known as "roy mutants", those that lack melanophores as "albino mutants", and those which lack both melanophores and iridophores are "ruby mutants". The gross eye morphology, feeding and swimming behaviours between wild-type and albino zebrafish were indistinguishable, except under dim or bright light or low contrast.
   Knowing this, the geneticist may perform a complementation test on two separately obtained strains of pure-breeding white-eyed flies. The test is performed by crossing two flies, one from each strain. If the resulting progeny have red eyes, the two strains are said to complement; if the progeny have white eyes, they do not.
   Amelanism
  
  Amelanism (also known as amelanosis) is a pigmentation abnormality characterized by the lack of pigments called melanins, commonly associated with a genetic loss of tyrosinase function. Amelanism can affect fish, amphibians, reptiles, birds, and mammals including humans. The appearance of an amelanistic animal depends on the remaining non-melanin pigments. The opposite of amelanism is melanism, a higher percentage of melanin.
  
  A similar condition, albinism, is a hereditary condition characterised in animals by the absence of pigment in the eyes, skin, hair, scales, feathers or cuticle. This results in an all white animal, usually with pink or red eyes.
   BULLET::::- Turacin, a red poryphrin pigment found almost exclusively in turacos
  
  BULLET::::- Psittacofulvin, a brightly colored pigment unique to parrots
  
  BULLET::::- Melanin, a class of pigments responsible for a large range of coloration in many animal groups, and includes:
  
  BULLET::::- Phaeomelanin, a melanin pigment responsible for many russet, brown and tan hues in feathers
  
  BULLET::::- Eumelanin, a melanin pigment responsible for the deep black common to flight feathers
  
  BULLET::::- Carotenoids, a class of pigments responsible for many of the yellow, orange and red hues found in birds, and includes:
  
  BULLET::::- Astaxanthin, a red carotenoid pigment
   An albino anole has been reported and photographed. In 2007, it was reported that an albino stumpy-tail lizard (possibly a shingleback lizard), approximately 12 cm long and about one-year-old, had been found in Victoria, Australia. (Also see )
  
  There are three known "albino" strains of leopard gecko, however, breeders state that albinos are generally recognized by their lack of black pigment and having red eyes is not a requirement to be considered an albino. These three strains are called "tremper albinos", "rainwater albinos" and "bell albinos".
  
  Section::::In amphibians.
   Eye color is considered a subset of coloring, and coat color definitions often include standards for the eyes, as many genes which control eye color will also affect the coat color or "vice versa". The American Fancy Rat and Mouse Association (AFRMA) lists black, pink, ruby, and odd-eyed (two different-colored eyes) as possible eye colors, depending on the variety of rat shown. "Ruby" refers to eyes which at a glance appear black but which are on closer observation a deep, dark red.
   Dominant and recessive major solid white plumage varieties are not albinos, but just white chickens in which melanin is limited to eyes. In the ornithological literature there are many reports about totally or partially albino birds, but most of them are not studied genetically.
   Other definitions of albinism encompass most of the major animal taxa, but ignore the several other pigments that non-mammalian animals have and also structural colouration. For example, "Absence of the pigment melanin in the eyes, skin, hair, scales, or feathers." refers only to the pigment melanin.
  
  Because of the various uses of different terms applied to colouration, some authors have indicated that the colour of the eyes is the defining characteristic of albinism, e.g. "This leads to a good diagnostic feature with which to distinguish leucistic and albino individuals – the colour of the eye."
   Leucism is a rare genetic pattern that causes a reduction in the pigment of an animal's hair and skin. The natural colour of the red deer ranges from dark red to brown. They are often thought to be albinos; however, unlike albinos, who have characteristically red eyes, deer with leucism have normal colouring in their eyes. It is distinguished biologically from albinism in that it causes a reduced pigmentation in all skin types, and not just melanin.
  
  Section::::Symbolism.
  
  Section::::Symbolism.:Myth and legend.
   All so-called "albino" horses have pigmented eyes, generally brown or blue. In contrast, many albino mammals, such as mice or rabbits, typically have a white hair coat, unpigmented skin and reddish eyes. The definition of "albinism" varies depending on whether humans, other mammals, or other vertebrates are being discussed.
   Most other mammals are currently thought to be dichromats, with only two types of cone (though limited trichromacy is possible at low light levels where the rods and cones are both active). Most studies of carnivores, as of other mammals, reveal dichromacy, examples including the domestic dog, the ferret, and the spotted hyena. Some species of insects (such as honeybees) are also trichromats, being sensitive to ultraviolet, blue and green instead of blue, green and red.
   Homozygous cream dilutes, sometimes called double-dilutes, always have light blue eyes to match their pale, cream-colored coats. Heterozygous or single-dilute creams, such as palominos and buckskins, often have light brown eyes. The eyes of horses with the Champagne gene are typically greenish shades: aqua at birth, darkening to hazel with maturity.
  
  As in humans, much of the genetics and etiology behind eye color are not yet fully understood.
  
  Section::::The anatomy of the equine eye.:The adnexa.
  
raw_presuppositions: Only albinos have red eyes.
raw_corrections: Everyone has red eyes but only albinos have no pigment to cover the red in their eyes.