Aeumelanism
Melanocytes depend on the hormone Melanocortin 1 receptor (MC1R) to signal the production of eumelanin. Loss of melanocortin 1 receptor function or high activity of the MC1R-antagonist, Agouti signalling peptide, can cause the widespread absence of eumelanin. Loss of MC1R function, a recessive trait, has been observed in many species. In humans, various mutations of the MC1r gene result in red hair, blond hair, fair skin, and susceptibility to sundamaged skin and melanoma. Aeumelanic hair coats, associated with mutations of the MC1R gene, have also been identified in mice, cattle, dogs, and horses. These coat colors are called "yellow" in mice and dogs, "red" in cattle and chestnut in horses. The loss of eumelanin in the coat is, in these species, harmless. The distinction between aeumelanism and hyperphaeomelanism - over abundance of phaeomelanin - is semantic.
Read more about this topic: Amelanism