Genetics - Ashok475/White-Tiger GitHub Wiki
A white tiger's pale coloration is due to the lack of the red and yellow pheomelanin pigments that normally produce the orange coloration.[6] This had long been attributed to a mutation in the gene for the tyrosinase (TYR) 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,[6][7][8] and in the past white tigers were sometimes referred to as 'partial albinos'.[7][9] While whole genome sequencing determined that such a TYR mutation is responsible for the white lion leucistic variant, a normal TYR gene was found in both white tigers and snow leopards.[10] Instead in white tigers a naturally-occurring point mutation in the SLC45A2 transport protein gene was found to underlie its pigmentation. 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.[6] Mutations in the same gene are known to result in 'cream' coloration in horses,[6] and play a role in the paler skin of humans of European descent.[11] This is a recessive trait, meaning that it is only seen in individuals that are homozygous for this mutation,[6] and that while the progeny of white tigers will all be white, white tigers can be also bred from colored Bengal tiger pairs in which each possesses a single copy of the unique mutation.[12][10] 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.[13] 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."[14] A weakened immune system is directly linked to reduced pigmentation in white tigers.
Outside of India, inbred white tigers have been prone to crossed eyes, a condition known as strabismus,[15] due to incorrectly routed visual pathways in the brains of white tigers. When stressed or confused, all white tigers cross their eyes.[16] Strabismus is associated with white tigers of mixed Bengal x Siberian ancestry. The only pure-Bengal white tiger reported to be cross-eyed was Mohini's daughter Rewati. Strabismus is directly linked to the white gene and is not a separate consequence of inbreeding.[17][18][19]
The orange litter-mates of white tigers are not prone to strabismus. Siamese cats and albinos of every species which have been studied all exhibit the same visual pathway abnormality found in white tigers. Siamese cats are also sometimes cross-eyed, as are some albino ferrets. The visual pathway abnormality was first documented in white tigers in the brain of a white tiger called Moni after he died, although his eyes were of normal alignment. The abnormality is that there is a disruption in the optic chiasm. The examination of Moni's brain suggested the disruption is less severe in white tigers than it is in Siamese cats. Because of the visual pathway abnormality, by which some optic nerves are routed to the wrong side of the brain, white tigers have a problem with spatial orientation, and bump into things until they learn to compensate. Some tigers compensate by crossing their eyes. When the neurons pass from the retina to the brain and reach the optic chiasma, some cross and some do not, so that visual images are projected to the wrong hemisphere of the brain. White tigers cannot see as well as normal tigers and suffer from photophobia, like albinos.[20]
Other genetic problems include shortened tendons of the forelegs, club foot, kidney problems, arched or crooked backbone and twisted neck. Reduced fertility and miscarriages, noted by "tiger man" Kailash Sankhala in pure-Bengal white tigers, were attributed to inbreeding depression.[14] A condition known as "star-gazing" (the head and neck are raised almost straight up, as if the affected animal is gazing at the stars), which is associated with inbreeding in big cats, has also been reported in white tigers.[20]
There was a 200 kg (450 lb) male cross-eyed white tiger at the Pana'ewa Rainforest Zoo in Hawaii, which was donated to the zoo by Las Vegas magician Dirk Arthur.[21] There is a picture of a white tiger which appears to be cross-eyed on just one side in the book Siegfried and Roy: Mastering the Impossible.[22] A white tiger, named Scarlett O'Hara, who was Tony's sister, was cross-eyed only on the right side.
A male tiger named 'Cheytan', a son of Bhim and Sumita who was born at the Cincinnati Zoo, died at the San Antonio Zoo in 1992, from anaesthesia complications during root canal therapy. It appears that white tigers also react strangely to anaesthesia. The best drug for immobilizing a tiger is CI 744, but a few tigers, white ones in particular, undergo a re-sedation effect 24–36 hours later.[23] This is due to their inability to produce normal tyrosinase, a trait they share with albinos, according to zoo veterinarian David Taylor. He treated a pair of white tigers from the Cincinnati Zoo at Fritz Wurm's safari park in Stukenbrock, Germany, for salmonella poisoning, which reacted strangely to the anaesthesia.[24]
Mohini was checked for Chédiak-Higashi syndrome in 1960, but the results were inconclusive.[25][26] This condition is similar to albino mutations and causes bluish lightening of the fur color, crossed eyes, and prolonged bleeding after surgery. Also, in the event of an injury, the blood is slow to coagulate. This condition has been observed in domestic cats, but there has never been a case of a white tiger having Chédiak-Higashi syndrome. There has been a single case of a white tiger having central retinal degeneration, reported from the Milwaukee County Zoo, which could be related to reduced pigmentation in the eye.[25][27] The white tiger in question was a male named Mota on loan from the Cincinnati Zoo.
There is a myth that white tigers have an 80% infant mortality rate. However, the infant mortality rate for white tigers is no higher than it is for normal orange tigers bred in captivity. Cincinnati Zoo director Ed Maruska said: "We have not experienced premature death among our white tigers. Forty-two animals born in our collection are still alive. Mohan, a large white tiger, died just short of his 20th birthday, an enviable age for a male of any subspecies, since most males live shorter captive lives. Premature deaths in other collections may be artifacts of captive environmental conditions...in 52 births we had four stillbirths, one of which was an unexplained loss. We lost two additional cubs from viral pneumonia, which is not excessive. Without data from non-inbred tiger lines, it is difficult to determine whether this number is high or low with any degree of accuracy."[25] Ed Maruska also addressed the issue of deformities: "Other than a case of hip dysplasia that occurred in a male white tiger, we have not encountered any other body deformities or any physiological or neurological disorders. Some of these reported maladies in mutant tigers in other collections may be a direct result of inbreeding or improper rearing management of tigers generally."[25]
