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When most people hear "albino", they probably picture either a very pale-skinned, gray-eyed human, or a white-furred, pink-eyed animal. The Siamese cat is probably the absolute last image that would come to mind if someone were asked to name albino animals. However-- surprisingly-- the Siamese cat actually has a form of albinism, giving it its distict markings.
Cats and other animals (such as rabbits) who have this genetic trait, called temperature-sensitive albinism, are considerd to have "point" coloration by breeders, handlers, and show organizations. The most well-known of point color is the seal-point pattern, seen in the Siamese cat breed, which is a deep, nearly-black shade of chocolate. However, other cat breeds such as the colorpoint shorthair, the Himalayan, and the Birman may have point-markings of any other color or pattern, including tabby and tortoiseshell points.
This uncommon form of albinism results from a mutation carried genetically on the C-locus-- the same locus that also houses the gene for complete albinism. Temperature-sensitive albino cats have a mutated form of tyrosinase, the enzyme resposible for producing melanin, the pigment involved in darkened fur.
Instead of tyrosinase and melanain completely absent, as in animals with true albinism, Siamese cats and other pointed cat breeds have a form of tyrosinas that is able to operate only at below-average body temperatures. Because of this, only the coolest parts of the body-- the feet, tail, and face-- receive properly-processed levels of melanin.
The mutated tyrosinase found in temperature-sensitive albino cats will not act to pigment the eyes, so the eyes of all cats with true point-coloration have blue eyes. This trait can assist with differentiating unusually dark temperature-sensitive albinos from unusually light cats who lack the tyrosinase mutation. The depigmentation, or albinism, of the eye also causes the cats' eyes to flash red when exposed to light, unlike the eyes of pigmented cats, which "glow" green.
The womb of a cat is uniformly warm, so cats with point coloration, including purebred Siamese cats, are born solid white. As specific parts of the body become cooler with exposure to air, the kitten's markings slowly darken in the body's coolest areas. This causes his tail, face, and paws to darken over time to the color he would have expressed had it not been for the mutation. The point-markings of cats can be any color or pattern, including red tabby, cream tabby, dilute tortoiseshell, standard tortoiseshell, blue tabby, blue, chocolate, or lilac.
The longer a cat or kitten with temperature-sensitive albinism is exposed to cold, the darker his markings will get. For this reason, many older colorpoint shorthairs appear to be standard tabbies, and very old Siamese cats may look solid blackish-brown. It was, in fact, a mutation in Siamese cats that overrode the parital albinism and led to the existence of chocolate cats.
Although the Siamese cat and other pointed cat breeds carry a form of albino gene, it is mostly harmless. While some of these cats have comprimised vision, there are very few overall dysfunctional traits for temperature-sensitive albinism. Because the tyrosinase mutation only partially causes the absence of melanin, they are not as sensitive to light as other albino animals.
Temperature-sensitive albinism is a common, beautiful, and generally benign mutation, which adds variety to the already glorious spectrum of cats' coat colors and genetics. While many cat breeds exhibit this trait, it maintains an exotic appeal for cat fanciers and breeders because of its unique beauty... more
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THE "ALBINO SIAMESE"
Feline geneticist Don Shaw, writing in Cats Magazine, September 1972 to February 1973, looked into the so-called Albino Siamese and wrote that not all so called Albino Siamese were true genetic albinos and that many are blue-eyed whites of Siamese type (what we now call Foreign Whites). However, some Albino Siamese had pinkish-blue eyes rather than the clear blue of other blue-eyed white breeds. His research found that early registrations of "Albino Siamese" mentioned "Chinese Whites" - reportedly cats of Asian origin with Malay type and apparently blue-eyed whites having the piebald (white spotting) gene. Unlike other albino animals, the cat's eye structure always gave bluish tone, even to the pink or ruby eyes normally associated with albino animals; this made true albinism in cats difficult to detect.
A mutation for "true albinism" was reported when a Chocolate Point female bred to her Chocolate Point son. Most of the early breedings to produce these reported "Albino Siamese" were within the same Chocolate Point line, meaining the cats were relatively inbred. It had always been suggested that the mother/son mating resulted in doubling up of a recessive allele for albinism, however Shaw challenged this theory.
A Chinese White, and thus white spotting, had been introduced into the Chocolate Point Siamese line and this Chinese White may have carried both white spotting and albinism. If so, the apparently true albinism in Albino Siamese would have been introduced from outside and not occurred as a mutation within the breed. Alternatively, he had noted that Chocolate Point and Lilac Point Siamese seemed to have a pink glow to their otherwise blue eye which wasn't seen in Seal Points or Blue Points.
Shaw believed the supposed Albino Siamese were blue-eyed whites resulting from the white spotting inherited from the Chinese Whites PLUS the pinkish tone due to being homozygous Chocolate Point Siamese (this would be masked by the white spotting) - this combination mimicked true albino. Since the alleged Albino Siamese resulted from inbreeding Chocolate Point Siamese, so Shaw reasonably expected the offspring to have the same pinkish-blue eyes he had observed in Chocolate Points along with a double dose of white spotting that produced a wholly white coat.
The possible alternatives presented by Shaw were that Albino Siamese might be true genetic albinos or they might simply be blue-eyed whites with pale blue, slightly pink-tinted, eye colour. He didn't have enough breeding data to determine which, but there was enough data to demonstrate the presence of white spotting genes in that line (white toes, lockets etc). Since Shaw’s time, true albinism has been seen in cats (the eyes being pink with only a slight blue tint). It has been recorded in the wild in the Asian Leopard Cat and also in the domestic descendants of Leopard Cat hybrids, the Bengal. |
