Amelanism

Melanin is a compound found in plants, animals, and protists, and is derived from the amino acid tyrosine.

A critical step in the production of melanins is the catalysis of tyrosine by an enzyme called tyrosinase, producing dopaquinone.

Phaeomelanin, meaning rufous-black, is characterized by the presence of sulfur-containing cysteine, and it appears reddish to yellowish as a result.

Melanocyte-stimulating hormone (MSH) binds to the Melanocortin 1 receptor (MC1R) and commits melanocytes to the production of eumelanin.

In many mammals, variation in the level of ASP switches melanocytes between eumelanin and phaeomelanin production, resulting in coloured patterns.

Melanocytes, and the parallel melanophores found in fishes, amphibians, and reptiles, are derived from a strip of tissue in the embryo called the neural crest.

Certain alleles of this gene, TYR, at the Color locus, cause oculocutaneous albinism type 1 in humans and the familiar red-eyed albino conditions in mice and other mammals.

Some species also possess iridophores or leucophores, which do not contain true pigments, but light-reflective structures that give iridescence.

[5] Amelanism in fishes, amphibians, reptiles and birds has the same genetic etiology as in mammals: loss of tyrosinase function.

Loss of melanocortin 1 receptor function or high activity of the MC1R-antagonist, Agouti signalling peptide, can cause the widespread absence of eumelanin.

[6] Aeumelanic hair coats, associated with mutations of the MC1R gene, have also been identified in mice,[7] cattle,[8] dogs,[9] and horses.

[10] These coat colors are called "yellow" in mice and dogs, "red" in cattle and chestnut in horses.

Loss of function of agouti signalling protein can permit unmediated eumelanin production, producing a uniformly black-to-brown coat color.

Often called " albino ", this amelanistic python owes its yellow color to unaffected carotenoid pigments.
The normal zebrafish embryo, above, shows the migration and maturation of melanocytes. The amelanistic embryo, below, has melanocytes but no melanin.
Amelanistic laboratory mice, such as these, have no pigment in their skin, hair, or eyes. Their eyes are reddish.
Without melanocortin 1 receptor to signal eumelanin production in melanocytes, this Labrador retriever has a yellow coat. His eyes and skin are normal.
Amelanistic ("lutino") cockatiels retain their carotenoid-based red and yellow pigments.
The bay horse, left, has both eumelanin and phaeomelanin in her coat; the agouti signaling peptide suppresses black color to the "points" v the mane, tail, ear tips, and legs. The horse at right lacks the agouti signalling protein, and has a uniformly black or aphaeomelanistic coat. In a chestnut horse , the solid red coat is created by a recessive aeumelanic mutation in MC1R and agouti, if present, is masked. In all cases, the eyes and skin are unaffected.