White Tigers are NOT Genetically Defective
There is no evidence of a genetic defect inherent in the white color variant of the Royal White Bengal Tiger, notwithstanding the erroneous claims to the contrary by the Humane Society of the United States (HSUS) and the Association of Zoos and Aquariums (AZA). White tigers have a normally occurring, simple recessive genetic color variant known as leucism, much the same as the leucistic (white) deer common to the Carolinas. Leucism and albinism are not the same. White tigers are not albinos and do not carry the genetic weaknesses associated with albinism. According to a recent study published in Current Biology, the gene known as SLC45A2, is a naturally expressed color variant that was common in wild tiger populations prior to extirpation by poachers, hunters and habitat fragmentation in the 1950’s.
Leucism, as a simple recessive genetic trait, can be carried by normal (orange) tigers, even though the white color is not visibly exhibited. In humans, Type O blood is a simple recessive trait. This means that in order for a person to have the phenotype for Type O blood, he or she must inherit one Type O gene from each parent. A phenotype is an individual’s observable traits, such as height, eye color, and blood type. The genetic contribution to the phenotype is called the genotype (the genetic ingredients that an animal has, whether or not those traits are visible).
The white gene can be inherited undetected in orange tigers for generations, because a tiger only appears white when each parent in any particular breeding passes the gene for leucism. When two normal colored tigers are bred carrying the recessive leucistic gene, white offspring can occur. The spontaneous occurrence of white tigers in the U.S. population proves that the gene for leucism is widespread in the species.
An animal is heterozygous at a gene locus when it contains two different alleles of a gene. Many orange tigers are heterozygous for leucism, meaning that they carry a dominant gene for the orange color as well as a recessive gene for white. These cats have one of each gene to potentially contribute to offspring, with an equal chance of passing the orange gene or the white gene. When an orange tiger carrying the white gene mates with a white tiger, each of the offspring has a 50% chance of of being white and 100% of the offspring will carry the white gene. If two heterozygous tigers (orange tigers each carrying the recessive white gene and a dominant orange gene) mate, each offspring has a 25% chance of being white; each has a 50% chance of being orange but carrying the white gene, and each has only a 25% chance of being orange and not carrying the white gene. Two visually white tigers bred together will produce only white cubs.
Science over Superstition
The best available science recommends saving and strengthening the diversity in the white Bengal tiger, leaving the AZA “no breeding” policy in stark contradiction with the welfare of the species. The facts are simple. The recessive white gene SLC45A2, in and of itself, is not flawed. For example, the gene that makes black leopards black, is also a simple recessive, and AZA zoos do not ban black leopard breeding. The change in policy at AZA seems more correlated to what appears to be an informal alignment with the Humane Society of the United States (HSUS), to protect AZA market share from non-AZA zoos. The policy to ban the breeding of white tigers is an ideological choice, unrelated to science.
Much of the misinformation surrounding white tigers has been proliferated by Carole Baskins of Big Cat Rescue. She has posted her HSUS inspired interpretation of the AZA white paper, along with William Conway’s views, on her website, which in turn has been republished by The Dodo and One Green Planet, and punctuated with horrible photos of inbred tigers. It is worthy of note that Ms. Baskins uses no scientific citations to support her claims of genetic defect. She uses inflammatory rhetoric and shocking images in lieu of facts.
While the original confusion regarding white tiger genetics understandably stemmed from personal opinions derived from a poverty of scientific data, the current anti-white tiger sentiment of AZA/HSUS flies in the face of the best available science. The AZA appears to have aligned themselves with HSUS not only to protect themselves from being targeted by the animal rights industry, but to discredit other legitimate zoological institutions that don’t toe the HSUS/AZA line on captive wildlife policy. It should be interesting to see, as more science becomes available dispelling superstitions about white tiger genetics, if AZA will defy HSUS and adopt a science based policy?
According to Dr. Brian Davis of the Exotic Genome Repository, “There has been no genetic study that has demonstrated a negative biological effect connected to the white variant in tigers. When the white tiger existed in the wild prior to human eradication, adults were common, indicating no decrease in fitness.” Additionally, leucism exists in numerous other species. These traits, erroneously associated with white, are actually strongly linked with inbreeding, regardless of coat color. Contemporary American tiger populations do not need inbreeding to perpetuate the white variant, since the gene is prevalent within the orange population as well other genetic variation that humans have driven extinct in the wild.