<unk>  is a variation in coloration. The term is most often used to describe color differences of the iris, but can also be applied to color variation of hair or skin.<unk>  is determined by the production, delivery, and concentration of melanin (a pigment). It may be inherited, or caused by genetic mosaicism, chimerism, disease, or injury. It occurs in humans and certain breeds of domesticated animals.<unk>  of the eye is called<unk>  or<unk>  iridis. It can be complete or sectoral. In complete<unk> , one iris is a different color from the other. In sectoral<unk> , part of one iris is a different color from its remainder. In central<unk> , there is a ring around the pupil or possibly spikes of different colors radiating from the pupil. Though multiple causes have been posited, the scientific consensus is that a lack of genetic diversity is the primary reason behind<unk> , at least in domestic animals. This is due to a mutation of the genes that determine melanin distribution at the 8-HTP pathway, which usually only become corrupted due to chromosomal homogeneity. Though common in some breeds of cats, dogs, cattle and horses, due to inbreeding,<unk>  is uncommon in humans, affecting fewer than 200,000 people in the United States, and is not associated with lack of genetic diversity. The affected eye may be hyperpigmented (hyperchromic) or hypopigmented (hypochromic). In humans, an increase of melanin production in the eyes indicates hyperplasia of the iris tissues, whereas a lack of melanin indicates hypoplasia. The term is from Ancient Greek: , "different" and , "color".
Heterochromia iridum
Spotted lunulae
Pili multigemini
Aniridia
Macrodontia 
Hippocratic facies
Cryptotia
Conduplicato corpore