The white tiger is not a separate species or subspecies of big cat, but rather a color variation, or leucistic morph, primarily of the Bengal tiger. This striking appearance features a coat that is white or near-white, contrasted by the typical black or dark brown stripes. The specific genetic change that results in the white fur also influences eye pigmentation, creating a strong association between the two traits.
The Genetics Behind White Fur and Blue Eyes
The majority of white tigers have blue eyes, a trait that is directly connected to the same genetic mechanism responsible for their lack of orange pigment. The white coat is caused by a specific point mutation in the \(SLC45A2\) gene, which codes for a transport protein involved in melanin production. This protein is responsible for moving the building blocks of pigment within cells, and the mutation affects its function. The mutation specifically blocks the production of pheomelanin, the red and yellow pigment that gives a typical tiger its orange color. This allows the tiger to produce only eumelanin, the black and brown pigment, which is why the stripes remain dark against the white background.
The reduced overall pigment level resulting from this gene change extends to the iris of the eye. Normal tigers have irises rich in pigment, which results in the typical yellow or green eye color. In white tigers, the lower concentration of pigment in the iris causes light to scatter, making the eyes appear blue. In order for a tiger to display this white coat and blue eye combination, it must inherit two copies of this recessive gene, one from each parent.
Why White Tigers Are Rarely Seen in the Wild
The trait for white coloration is an autosomal recessive one, meaning it can only be expressed if both parents carry the gene and pass a copy down to the cub. The natural occurrence of a white tiger in the wild is extremely rare, with estimates suggesting it happens in only about one in every 10,000 tiger births. The last confirmed sighting of a wild white tiger was recorded in 1958, and all living white tigers today are descendants of captive individuals.
The white coat is a significant disadvantage for a predator in its natural habitat. A tiger’s orange and black coloring provides excellent camouflage within the shadows and tall grasses, allowing it to ambush prey successfully. The conspicuous white coat makes it difficult for the tiger to hide, severely reducing its ability to hunt and lowering its chances of survival. Nearly all white tigers currently in existence are the result of intentional breeding programs in captivity. These programs selectively breed tigers known to carry the recessive gene, often relying on inbreeding to reliably produce the desired white coat for display.
Common Physical Traits Beyond Coat and Eye Color
While the white coat and blue eyes are the most immediate physical differences, the way white tigers are bred has led to other common physical traits, many of which are detrimental to their health. The necessity of pairing genetically related individuals to ensure the recessive gene is expressed has resulted in a high degree of inbreeding within the captive population. This practice frequently amplifies other recessive traits, leading to various congenital defects.
One of the most commonly observed physical issues is strabismus, a condition where the eyes are crossed. This defect is a direct consequence of the same genetic pathway that affects pigment production also influencing the routing of the visual nerve pathways in the brain. Other health issues seen due to inbreeding depression include spinal deformities, such as scoliosis or kyphosis, neurological defects, immune system deficiencies, shortened limbs, and kidney problems. These associated health problems are a direct consequence of the intensive inbreeding required to perpetuate the white coloration, not the white gene itself, which is a naturally occurring variant.