Abstract

Introduction

Records of a breeding colony of Bengal tigers were analyzed, classifying tigers and their offspring according to color (standard, golden tabby, white, and snow white) and sex, where known. The “standard,” or wild type, Bengal tiger is a reddish orange with narrow black, gray, or brown stripes.⁴ The popular “white” tiger is usually a creamy to chalky white with darker stripes.¹ The “snow white” tiger appears to be a variation of the white tiger. Snow whites are also pale white, but with pale to almost nonexistent stripes. The last type will be referred to as a “golden tabby,” or “tabby.” The golden tabbies are orange like the standard but have darker orange to brown stripes, rather than black.

Hair samples from each of the color variations were analyzed to determine the type of pigment present. The presence of melanin deposits in the hair shaft determines the color of the tiger’s coat. There are two different kinds of melanin: eumelanin and phaeomelanin. Eumelanin absorbs almost all light, thus producing a black color. Phaeomelanin reflects light in the red-orange-yellow range.⁵ The standard tiger has phaeomelanin in the hair of the base coat, producing an orange ground color, and eumelanin in the hair of the black stripes.³

The coloration of the white tiger is due to a single autosomal recessive gene.⁶ This gene is c^ch symbolized as c^ch and appears to have an effect similar to the Chinchilla gene in other species. It acts by drastically reducing or eliminating phaeomelanin, causing the base coat color to change from standard orange to a cream or white color. The eumelanin present in the stripes is hardly affected, at most fading to a dark brown.³

Breeding Record Analysis

The breeding records of this Bengal tiger colony listed 15 litters produced by 5 female and 5 male tigers (Tables 1 and 2). These matings consisted of golden tabbies bred to each of the four colorations, and standards bred to white and snow white. Based on the hypothesis that there are two autosomal recessive genes involved in the production of these four colorations, the following results were expected assuming all individuals were either recessive or heterozygous at both loci (Fig. 1). Tabbies bred to standards should produce 3/8 standard, 3/8 tabby, 1/8 white, and 1/8 snow white. Tabbies bred to tabbies would produce 3/4 tabbies and 1/4 snow white. Tabbies bred to snow whites would produce equal numbers of tabbies and snow whites. Tabbies bred to whites would produce equal numbers of all four colorations, as would standards bred to snow whites. Finally, standards bred to white would produce 3/8 standard, 3/8 white, 1/8 tabby, and 1/8 snow white.

Table 1. Presumed genotype of breeding tigers: based on phenotype, pedigree and progeny information

Table 2. Mating types and resulting offspring phenotypes

^aStandard
^bGolden tabby
^cWhite

^dSnow white
m-male offspring
f-female offspring

Figure 1 Example of determining offspring ratios Tabby (tt C+cch) × Standard (T+t C+cch) cross.

The data were analyzed by locus using Chi-square analysis, with the expected ratios based on the stated hypothesis (Tables 3 and 4). None of the results of the crosses deviated from the expected ratios more than that due to chance (p>0.05). The gender ratios were also analyzed to confirm that both genes were autosomal rather than sex-linked. If the tabby gene were sex-linked, all male offspring of a tabby female would be tabbies or snow whites, which was not seen. However, as an autosomal gene, the ratios would be expected to be evenly divided between male and female. This was found to be the case, with only some chance deviation, as determined by Chi-square analysis (Table 5).

Table 3. Chi-square analysis of results of matings, segregating at the tabby locus

^aStandard or white
^bTabby or snow white

Table 4. Chi-square analysis of results of matings, segregating at the chinchilla-like locus

^aStandard or tabby
^bWhite or snow white

Table 5. Gender ratios of offspring with Chi-square analysis

^aDoes not include two offspring of unknown gender

Determination of Pigment Type

The hair samples were analyzed by high-performance liquid chromatography (HPLC) following chemical degradation. The chemical degradation consisted of permanganate oxidation to produce pyrrole-2,3,5-tricarboxylic acid (PTCA), a specific indicator of eumelanin, and hydriodic acid (HI) hydrolysis to produce amino hydroxyphenyl alanine (AHP), a specific indicator of phaeomelanin.²

Based on the results of chemical degradation and HPLC analysis, the standard tiger was found to have a phaeomelanic base coat with eumelanic stripes, as stated in previous literature. The white tiger has an amelanotic coat, having only a minimal level of pigment, with eumelanic stripes. This agrees with the hypothesis that a “chinchilla”-like gene is acting to dilute phaeomelanin in the coat.

The tabby has a phaeomelanic base coat with phaeomelanic stripes. This supports the inhibition of eumelanin production as the cause of the color variation. The snow white has an amelanotic base coat and stripe. This would be expected based on the hypothesis that the snow white coloration is due to the presence of both homozygous recessives. The tabby gene changes the stripe pigment from eumelanin to phaeomelanin, which is then diluted in both the stripe and the base coat due to the chinchilla gene, to produce the snow white.

Literature Cited

1.  Busch Gardens, Inc. Animal bytes: Bengal tiger. www.bev.net/education/SeaWorld/animal_bytes/tigerab.html. 1995. (VIN editor: Link not accessible).

2.  Ito S, Fujita K. Microanalysis of eumelanin and pheomelanin in hair and melanomas by chemical degradation and liquid chromatography. Analyt Bioch. 1985;144:527–536.

3.  Robinson R. The white tigers of Rewa and gene homology in the Felidae. Genetica. 1969;40:198–200.

4.  Roychoudhury AK. White tigers and their conservation. In: Tigers of the World. Park Ridge, NJ: Noyes Publications; 1987:380–387.

5.  Starbuck O, Thomas D. Cat colors FAO: Cat color genetics. www.ai.mit.edu/fanciers/other-faqs/color-genetics.html. 1994. (VIN editor: Link not accessible).

6.  Thornton IWB, Yeung KK, Sankhala KS. The genetics of the white tigers of Rewa. J Zool. 1967;152:127–135.