Acromelanism

Acromelanism is a genetic condition that results in pigmentation being affected by temperature. It results in point coloration where the extremities of an animal are a different colour to the rest of the body. It is commonly known for the coloration of Siamese and related breeds of cat, but can be found in many other species including dogs, rabbits, rats, mice, guinea pigs, minks, and gerbils. It is a specific type of point coloration.
Colorpoint patterns are where the extremities (paws, face, ears and tail) of the animal are colored differently compared to the rest of the body. The areas with different color may be referred as 'points' or being 'pointed'. Color can spread to the rest of the body, but is concentrated on the extremities.
Colorpoint patterns occur due to acromelanism, which is a type of partial albinism where pigmentation is affected by temperature. Cold temperaments trigger pigment development while warm temperatures decrease pigment development. The extremities are colder than the rest of the body which is why they darken.
The C locus in mammals is most often associated with the TYR gene, which controls the production of the tyrosinase enzyme that produces melanin.
The color point allele is at the C locus, where some forms of typical albinism are also located. Point is recessive for all animals. In cats, two copies of cs, one from each parent, are needed for points to be expressed.
Point coloration in cats originated in the Siamese and closely related Asian breeds, and is found in many Western-developed modern breeds. It is a form of partial albinism resulting from a mutation of tyrosinase, an enzyme involved with melanin production. The mutated enzyme is thermolabile; it fails to work at normal body temperatures, but becomes active in cooler areas of the skin. As a result, dark pigment is limited to the coldest areas of the body, that is, the extremities. Pointed kittens are born white, due to the warmth of uterus. As the kitten ages, the cooler areas darken while warmer areas remain cream to white in color. Colorpoints can usually be observed at four weeks.
As the expression of the gene responsible for the pointed pattern is regulated by temperature, pointed cats who live in cooler environments often show increased darkening of their fur relative to cats who live in warmer climates; in some extreme cases the entire coat will darken. Shaving a cat can also change the colour of the coat. Conditions that affect the temperature of the animal can change the colour too, e.g. pyrexia can inactivate tyrosinase and result in new growth being uncoloured and obesity may result in the whole body turning dark due to the decreased skin temperature.
Because of the pigment restriction caused by the temperature sensitive tyrosinase, pointed cats' eyes are always shades of blue because the blue layer in the eye common to all cats is not covered by another color. The back of the eye also lacks pigment, giving colorpoint cats' pupils a red and silver reflection in the dark, unlike a normally pigmented cat's shining silver-green or -blue.
The lynx point pattern is formed by mating a colorpoint cat with a tabby cat (or breeding cats that already possess the lynx point pattern). It is characterized by a mixture of the darkening (reduced) of point coloration with distinct tabby striping on the head, tail, and legs, and an otherwise uniform and comparatively pale body. It is an accepted pattern in some cat registries, but not others, for particular breeds (mostly Siamese-related).
Point coloration is inherent to the Siamese breed and some other closely related breeds. With most other breeds the colorpoints (including lynx) were brought into the breed lines long after their establishment, but there are exceptions. For example, lynx point was a feature of some of the foundation stock of the Siberian, now called the Neva Masquerade.
Different colours have special names: brown is known as 'seal point', red/orange as 'flame point', blue as 'blue point', and lavender as 'lilac point'. For the Tonkinese cat terms such champagne and platinum are used.
Rarely, dogs (Canis familiaris) are documented with point coloration or acromelansim. Recent documented evidence about acromelanism in dogs has only existed since 2017, when the first colorpoint dachshund was reported in the Czech Republic. More dogs have appeared since then, with the majority from the eastern Eurasian region and Canada. Genotyping the aforementioned dachshund revealed a mutation in the tyrosinase gene that results in a recessive colorpoint allele. A second tyrosinase variant is responsible for the colorpoint coat seen in a family of rescue dogs.
Some rabbits that appear to be pointed white lack the red eyes, which indicates they are of different breeding. The following such coat colors are examples of those created, not with the ch gene, but with the cchl, cchd, or cchm gene in conjunction with the e gene:
No pointed white rabbit can produce orange pigment; therefore, the points are always either black, blue, chocolate, or lilac. The resulting point coloration is sometimes in conjunction with a coat pattern, such as: white agouti, marten, broken, Vienna, or harlequin. (Such coats may not be recognized for showing.)
Breeds of rabbit that include varieties with point coloration include:
The C - Albino locus gene causes dilution of yellow and black coloration, causing Himalayan or Siamese markings depending on the allele affected.
Point colors in rats include:
There is only one type of guinea pig with a pointed coat. It is called the Himalayan, has either Black (a very dark brown) or Chocolate, tipped on the Ears, Nose, and Feet.