White peacocks are not albinos, and neither is the ash-grey albino gaur
Plenty of pale animals get called albino without deserving it. White peacocks, swans and geese carry a gene for white feathers rather than lacking melanin, and the so-called albino gaur is simply grey instead of black. Scientists themselves struggle to agree what albinism means once you look beyond mammals.
At its core, albinism is an inborn lack of pigment. In mammals that pigment is melanin, which comes in two forms: eumelanin for dark browns and blacks, and pheomelanin for reddish tans and blondes. Humans have seven recognised types of oculocutaneous albinism, genetic mutations that reduce or block melanin production; the best-known, type 1a, removes melanin from skin, hair and eyes entirely. In plants the equivalent is losing chlorophyll, which hampers photosynthesis.
Defining it neatly is hard because most mammals rely on melanin alone, while many other animals mix several pigments with structural colour. Fish, amphibians, reptiles, crustaceans and cephalopods colour themselves with chromatophores holding yellow carotenoids, red pteridines, black melanins or, more rarely, blue pigment. Others shimmer thanks to guanine crystals that reflect light without any pigment at all. A definition built on melanin misses much of this, so some biologists use eye colour as the test, and some reject the phrase partial albino as a contradiction. One broad proposal calls it an inborn absence of pigmentation, partial to total, which covers every organism.
Inheritance is usually recessive, but not always. Palomino horses and several arctic mammals have dominant white colouring and still make the enzyme tyrosinase, so they are only pseudo-albinos. In people, albinism persists through a balance between a slight fitness cost and a low rate of new mutations in melanin genes.
Across the animal kingdom it shows up most often in birds, reptiles and amphibians. In mammals it occurs about once in every 10,000 births, and some cases are singular: one albino gorilla, one albino koala. Being pale carries costs. Without melanin's protection from ultraviolet light and without camouflage, albino alligators have been suggested to survive only 24 years on average.
Source: Albinism
