White Tiger - II

A white tiger's pale coloration is due to the lack of the red and yellow pigments that normally produce the orange color.^[7] This had long been thought to be due to a mutation in the gene for the tyrosinase enzyme. A knockout mutation in this gene results in albinism, the inability to make either pheomelanin or eumelanin, while the consequence of a less severe mutation in the same gene is the cause of a selective loss of pheomelanin, the so-called Chinchilla trait. The white phenotype in tigers had been attributed to this Chinchilla mutation in tyrosinase, and some publications prior to the 1980s refer to it as an albino gene for this reason.^{[citation needed]} However, genomic analysis has demonstrated instead that a mutation in the SLC45A2 gene is responsible. The resultant single amino acid substitution in this transport protein, by a mechanism yet to be determined, causes the elimination of pheomelanin expression seen in the white tiger. This is a recessive trait, meaning that it is only seen in individuals that are homozygous for this mutation.^[7] Inbreeding promotes recessive traits and has been used as a strategy to produce white tigers in captivity.
The stripe color varies due to the influence and interaction of other genes. Another genetic characteristic makes the stripes of the tiger very pale; white tigers of this type are called snow-white or "pure white". White tigers, Siamese cats, and Himalayan rabbits have enzymes in their fur which react to temperature, causing them to grow darker in the cold. A white tiger named Mohini was whiter than her relatives in the Bristol Zoo, who showed more cream tones. This may have been because she spent less time outdoors in the winter.^[8] White tigers produce a mutated form of tyrosinase, an enzyme used in the production of melanin, which only functions at certain temperatures, below 37 °C (99 °F). This is why Siamese cats and Himalayan rabbits are darker on their faces, ears, legs, and tails (the color points), where the cold penetrates more easily. This is called acromelanism, and other cats breeds derived from the Siamese, such as the Himalayan and the snowshoe cat, also exhibit the condition.^[9] Kailash Sankhala observed that white tigers were always whiter in Rewa State, even when they were born in New Delhi and returned there. "In spite of living in a dusty courtyard, they were always snow white."^[10] A weakened immune system is directly linked to reduced pigmentation in white tigers.