at Tobu Zoo, in
Saitama,
Japan. This phenotype is due to a mutation in the same gene that results in Type IV
oculocutaneous albinism in humans.
Leucism is often used to describe the
phenotype that results from defects in
pigment cell differentiation and/or migration from the
neural crest to skin, hair, or feathers during
development. This results in either the entire surface (if all pigment cells fail to develop) or patches of body surface (if only a subset are defective) lacking cells that can make
pigment. Since all pigment cell-types differentiate from the same
multipotent precursor cell-type, leucism can cause a reduction in all types of pigment. This is in contrast to
albinism, for which leucism is often mistaken. Albinism results in the reduction of
melanin production only, though the
melanocyte (or
melanophore) is still present. Thus in species that have other pigment cell-types, for example
xanthophores, albinos are not entirely white, but instead display a pale yellow color. More common than a complete absence of pigment cells is localized or incomplete
hypopigmentation, resulting in irregular patches of white on an animal that otherwise has normal coloring and patterning. This partial leucism is known as a "pied" or "
piebald" effect; and the ratio of white to normal-colured skin can vary considerably not only between generations, but between different offspring from the same parents, and even between members of the same litter. This is notable in
horses,
cows,
cats,
dogs, the urban
crow, and the
ball python but is also found in many other species. Due to the lack of melanin production in both the
retinal pigmented epithelium (RPE) and
iris, those affected by
albinism sometimes have pink pupils due to the underlying blood vessels showing through. However, this is not always the case and many albino animals do not have pink pupils. The common belief that all albinos have pink pupils results in many albinos being incorrectly labeled as 'leucistic'. The neural crest disorders that cause leucism do not result in pink pupils and therefore most leucistic animals have normally colored eyes. This is because the melanocytes of the RPE do not derive from the
neural crest. Instead, an out-pouching of the
neural tube generates the
optic cup that, in turn, forms the
retina. As these cells are from an independent developmental origin, they are typically unaffected by the genetic cause of leucism.
Notable examples •
Platypus (
Ornithorhynchus anatinus) – in 2021, a leucistic example was found in the
Gwydir River, near
Armidale,
New South Wales,
Australia. • The
Anchorage White Raven – a leucistic
common raven known for its "trickster" behavior, spotted in 2023 and 2024 in
Anchorage, Alaska. == Genetics ==