The white tiger is a rare pigmentation variant of the Bengal tiger, which was reported in the wild from time to time in Assam, Bengal, Bihar and especially from the former State of Rewa.
White tigers are distinct for the normal coloration in that they lack the pheomelanin pigment that in normal tigers produces the orange color. They still produce the other color pigment, eumelanin, and hence are not considered albino. Compared to normal colored tigers without the white gene, white tigers tend to be somewhat bigger, both at birth and as fully grown adults.
The existence of white Siberian tigers has not been scientifically documented, despite occasional unsubstantiated reports of sightings of white tigers in the regions where wild Siberian tigers live. It may be that the white mutation does not exist in the wild Siberian tiger population: no white Siberian tigers have been born in captivity, despite the fact that the subspecies has been extensively bred during the last few decades (with much outbreeding between the different Siberian lineages for purposes of conservation genetics); a recessive allele should occasionally turn up in a homozygous state during such breeding, and in this particular case yield white tigers from normally-colored parents, but no such animals have been reported.
The famous white Siberian tigers found in captivity are actually not pure Siberian tigers. They are instead the result of Siberian tigers breeding with Bengal tigers. The gene for white coating is quite common among Bengal tigers, but the natural birth of a white Bengal tiger is still a very rare occasion in the wild, where white tigers are not bred selectively.
The white tiger is not considered a tiger subspecies, but rather a mutant variant of the existing tiger subspecies. If a pure white Siberian tiger were to be born, it would therefore not be selectively bred within the tiger conservation programs. It would, however, probably still be selectively bred outside the program in an effort to create more white Siberian tigers. Due to the popularity of white tigers, they are used to attract visitors to zoos.