Leucism or Leukism is a very unusual condition whereby the pigmentation cells in an animal fail to develop properly. Unlike in albinism, a leucistic animal is usually pure white with blue eyes, since there is a reduction of color in all pigmentation of the animal, not just melanin.
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Albinism (also called achromia or achromatosis) is a different condition due to the total absence of melanin pigment in the eyes, skin and hair. Albinism always affects the entire body (not just parts of it), but can be partial when the melanin pigment is not entirely missing (hypomelanism) or complete, when no melanin is present in the cells (amelanism). Albino animals usually have a yellow hue over their fur, as pigments, other than melanin can show through.
A second difference between leucism and albinism, is in eye color. The easiest way to tell the difference between the two is that in albinism the eyes are usually pink or red, due to the lack of melanin production in both the retinal pigmented epithelium (RPE) and iris. In fact, it are the underlying blood vessels showing through the uncolored eyes which cause the eyes to be red. In contrast, leucistic animals have normally colored eyes.
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Albino Pug 'Rudy'
Complete albinism in a cat
Most white housecats are leucistic, though.
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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.

Leucism can effect only certain parts of the body, resulting in animals known as "pied" or "piebald", with the most popular example being the paint horse. The pied pattern is the result of a localized or incomplete hypopigmentation. This is notable in dogs, cats and horses, but is also found in many other species.
The Bengal or Indian Tiger (P.t. tigris) is the only subspecies in which white individuals have been reported. Theoretically white tigers and white lions could be the result of albinism (entirely white), leucism (white with missing stripes) or a chinchilla mutation (white with pale stripes). However, there are no known instances of true albino tigers. Reports of albino tigers have always turned out to be white Bengal tigers with really pale stripes. Evidence for the two other explanations is still lacking.
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Leucism and Albinism in Dogs and other Animals
Albino Pug
Photo submitted by Snake Tattoo
Contrary to popular belief white tigers are not albinos. Two types of white tigers occur in captivity and probably also in the wild. The first one, commonly referred to as "white tigers" are also known as leucistic tigers or chinchilla tigers, the latter because some consider the white color to be caused by a recessive gene known as chinchilla or color inhibitor. The truth is that the gene responsible for the white color has not been identified yet. The second type, a white stripeless tiger, sometimes also referred to as snow tiger (not to be confused with the Siberian tiger) has a snow-white body and excessively faint striping that may darken as it nears the extremities. In some cases, the body of the feline lacks stripes entirely (see photo).
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Stripeless (snow) Bengal Tiger
Photo by Olaf Loose
The same goes for white lions, which are often described as leucistic, although they black features on the tip of their noses as well as "eye-lining" and, dark patches behind their ears ("follow-me signs").
White lion
More Albino Animals
Albinistic Peacock
Photo by M. Betley
Albinistic Alligator
Photo by Tsu Nellis
Albino Sea Turtle
Photo by Tim Hansen
Albino Squirrel
Photo by Yang Lee