The white coat/blue eyes trait of the White Tiger is caused by normal genetic inheritance from the parents, and follows a simple Mendelian recessive pattern.
- It is not caused by inbreeding.
- It is not caused by spontaneous mutation.
- “Recessive” does not mean faulty.
- The white gene does not cause defects, it affects only the pigmentation.
This is important, because it means that:
- White Tigers are not “mutant freaks”, but are a natural color variant.
- You do not have to breed white tiger with white tiger to produce a white cub, or to pass the white gene on to the next generation.
- The white gene can be spread throughout a population of orange tigers. This is called out-crossing or out-breeding, and the more it is done, the healthier and more diverse the gene-pool.
- It is not necessary to inbreed to produce white tigers. All that is necessary is to breed together 2 tigers each of which has at least one white gene. This can produce healthy white cubs without any inbreeding at all.
Identification of the Gene Responsible for the White Coat
It was not until 2013 that the actual gene responsible was identified – a change in a single amino acid (A477V) in one pigment-related gene, SLC45A2, that blocks the production of red/yellow pheomelanin while still allowing the black/brown melanin to be produced. This is the same gene that causes color variation in humans, horses, and chickens. The scientists concluded that it affected only pigmentation, and is a naturally occurring variant.
“They dismiss the notion that the white tiger trait is a genetic deformity. That matured white tiger adults have been sighted in the wild negates this notion. Despite its low frequency, they emphasise that the mutation is a naturally occurring one and should be considered as a “part of genetic diversity of tigers that is worth conserving.”
Before this, there were various theories that it was caused by albinism, or the chinchilla variant. The 2013 research proved these to be false.
Mendelian Recessive Inheritance
Physical traits are decided by genes. Genes come in pairs, one inherited from each parent. There can be different versions of the same gene, called alleles, causing different effects – these can be either dominant or recessive. Note that “recessive” does not mean inferior or defective, it merely indicates a trait that is masked (hidden) when paired with it’s dominant allele.
The tiger’s white coat/blue eyes allele is recessive – this can be written as a little “w” – with a capital “W” for the dominant orange coat. The possible gene pair combinations are WW, Ww, and ww.
In order for a tiger to be white, it must have ww, a full pair of the recessive gene, one w from each parent. An orange tiger may have WW or Ww – as long as one of the pair of genes is W, ie dominant, the tiger will be orange colored.
This gives several possible combinations for breeding, including:
- One ww (white) with WW (orange) – all the cubs will be Ww (orange, but carrying the white gene).
- Ww (orange carrier) with ww (white) gives a 50% chance of white cubs (2 in 4)
- Two Ww (orange carriers) gives a 25% chance of a white cub (1 in 4)
- Two ww (white) together – all their cubs must be ww (white)
The white gene is thought to have been caused by gene mutation at some time in the past – 100’s of years ago, or 1000’s or…. we don’t know when. This does not make the White Tiger a “mutant freak”. The genes do not spontaneously mutate every time a white tiger cub is conceived, it happens by normal genetic inheritance from the parents.
Mutation is normal and necessary for evolution and genetic diversity. Gene mutations can be good, bad, or indifferent. Harmful mutations tend not to survive, but the White Tiger did indeed survive and breed in the wild for 100’s of years so we really cannot call it a harmful trait.
“Mutations are changes in the genetic sequence, and they are a main cause of diversity among organisms …mutation is one of the fundamental forces of evolution” [from Loewe, L. (2008) Genetic mutation.]
William Conway, Director of the New York Zoological Society, made a comment back in the ’80’s referring to white tigers as “Freaks….two-headed calves and white tigers”. But Edward Maruska of Cincinnati Zoo, pointed out the error in that type of sentiment:
“Although the comment makes for interesting prose, his approach is not sound biology”
Our knowledge of the White Tiger has come a long way since the 1980’s.
Further reading “White Tiger numbers can be increased without inbreeding” R. Prasad and Shu-Jin Luo, May 24 2013, The Hindu Sci-Tech