White tigers, all known specimens of which belong to the Bengal tiger subspecies, owe their characteristically pallid pelage (a condition called leucism, genetically distinct from albinism) to a single amino acid change, A477V, in a particular transporter protein known as SLC45A2 (which mediates pigment production). This change inhibits the synthesis of red and yellow pigment (phæomelanin), but not black (eumelanin), thereby explaining why white tigers still possess dark stripes. Although never common, white tigers did occur naturally in the wild state for centuries (as confirmed by early artistic representations and hunting records), but their spectacular appearance marked them out as sought-after hunting trophies, and the strain was eventually wiped out.
The last known wild specimen, a magnificent male named Mohan, was captured alive on 27 May 1951, and was housed thereafter in the now-disused summer palace of the Maharajah of Rewa. Mohan was bred with normal tigers, thereby preserving and passing on the mutant recessive gene allele responsible for the above amino acid change causing leucism, and it is therefore from Mohan that many of the white tigers existing in captivity today ultimately descend (a few have arisen independently, due to spontaneous mutations, or to normal tigers carrying the recessive gene allele mating with one another and thereby engendering the white phenotype, i.e. outward appearance, in offspring carrying two copies of this mutant allele).
So it came as a huge surprise to zoologists lately when a white tiger was not only reported but also conclusively photographed in the wild, by wildlife photographer Nilanjan Ray while on a road trip through a reserve forest with a guide during a recent visit to the Nilgiri Biosphere Reserve, situated in southern India’s Nilgiri Hills and Western Ghats. To ensure the white tiger’s safety, the precise location of Ray’s sighting has not been made public, but camera traps are to be sited there in the hope of recording further sightings of this remarkable animal, whose existence demonstrates that, against all the odds, the mutant leucism allele does still survive within the wild tiger population. NB – many online reports of this incident (but not the report cited here) are extremely muddled, conflating leucism with albinism, so those should be read with caution. www.thehindu.com/news/national/ tamil-nadu/white-tiger-in-the-nilgirisis-a-first/article19217223.ece 5 July 2017.