page_title: White tiger
section_title: Genetics
hierarchical_section_title: White tiger / Genetics
context_page_description: The white tiger or bleached tiger is a pigmentation variant of the Bengal tiger, which is reported in the wild from time to time in the Indian states of Madhya Pradesh, Assam, West Bengal, Bihar and Odisha in the Sunderbans region and especially in the former State of Rewa. Such a tiger has the black stripes typical of the Bengal tiger, but carries a white or near-white coat.
context_section_description: A white tiger's pale coloration is due to the lack of the red and yellow pheomelanin pigments that normally produce the orange coloration.  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 ability to make neither pheomelanin (red and yellow pigments) nor eumelanin (black and brown pigments), while a less severe mutation in the same gene in other mammals results in selective loss of pheomelanin, the so-called Chinchilla trait. The white phenotype in tigers had been attributed to such a Chinchilla mutation in tyrosinase, and in the past white tigers were sometimes referred to as 'partial albinos'.  However, whole genome sequencing of normal and white tigers demonstrated instead that a naturally-occurring point mutation in the SLC45A2 gene is responsible.  The resultant single amino acid substitution introduces an alanine residue that protrudes into the transport protein's central passageway, apparently blocking it, and by a mechanism yet to be determined this prevents pheomelanin expression in the fur.  Mutations in the same gene are known to underlie the cream coloration of horses, and play a role in the paler skin of humans of European descent.  This is a recessive trait, meaning that it is only seen in individuals that are homozygous for this mutation, but that white tigers can be bred from any colored Bengal tiger pair each possessing the unique mutation.  Inbreeding promotes recessive traits and has been used as a strategy to produce white tigers in captivity, but this has also resulted in a range of other genetic defects.
  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. 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." A weakened immune system is directly linked to reduced pigmentation in white tigers.
caption: Pseudo-melanistic white tiger
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