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Acromelanism

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Coloration of animal coat/fur
A cat with seal (black) point coloration.

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.

Description

Colorpoint patterns are where the extremities (paws, face, ears and tail) of the animal are colored differently 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.

C Locus in Mammals

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.

In cats

"Lynx point" redirects here. For the Intel Chipset, see Lynx Point.
New-born kittens suckling milk from their seal point mother cat. Colourpoint kittens are born solid-white.

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 the albino gene that affects 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 of the queen. 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 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 but with most other breeds the colorpoints (including lynx) were brought into some 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.

In dogs

This section needs expansion. You can help by adding to it. (March 2021)


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.

In rabbits and rodents

Californian rabbits with point coloration

Rabbit coat colors

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:

  • Blue point
  • Chocolate point
  • Lilac point
  • Pearl
  • Sable point
  • Sallander
  • Seal point
  • Siamese

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.)

Rabbit breeds

Breeds of rabbit that include varieties with point coloration include:

In fancy (domestic) rats

The C - Albino locus gene causes dilution of yellow and black coloration, causing Himalayan or Siamese markings depending on the allele is affected.

Point colors in rats include:

  • Sepia
  • Seal Point
  • Blue Point

In guinea pigs

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.

References

  1. Miller, W.H.; Griffin, C.E.; Campbell, K.L.; Muller, G.H. (2013). Muller and Kirk's Small Animal Dermatology7: Muller and Kirk's Small Animal Dermatology. Elsevier/Mosby. p. 620. ISBN 9781416000280. Retrieved 2015-05-17.
  2. Petrij, F (2001). "A Second Acromelanistic Allelomorph at the Albino Locus of the Mongolian Gerbil (Meriones unguiculatus)". The Journal of Heredity. 1 (1): 74–78. doi:10.1093/jhered/92.1.74. PMID 11336233.
  3. ^ Englar, Ryane E. (2019-07-03). "Changes in coat color". Common Clinical Presentations in Dogs and Cats. Wiley. pp. 135–139. doi:10.1002/9781119414612. ISBN 978-1-119-41458-2.
  4. Kumar CM, Sathisha UV, Dharmesh S, Rao AG, Singh SA (Mar 2011). "Interaction of sesamol (3,4-methylenedioxyphenol) with tyrosinase and its effect on melanin synthesis". Biochimie. 93 (3): 562–9. doi:10.1016/j.biochi.2010.11.014. PMID 21144881.
  5. D. L. Imes; et al. (April 2006). "Albinism in the domestic cat (Felis catus) is associated with a tyrosinase (TYR) mutation". Animal Genetics. 37 (2): 175–8. doi:10.1111/j.1365-2052.2005.01409.x. PMC 1464423. PMID 16573534.
  6. TICA. "Siberian Introduction" (PDF).
  7. Bychkova, Elina; Viktorovskaya, Olga; Filippova, Elizaveta; Eliseeva, Zhanna; Barabanova, Larisa; Sotskaya, Maria; Markov, Anton (2021). "Identification of a candidate genetic variant for the Himalayan color pattern in dogs". Gene. 769: 145212. doi:10.1016/j.gene.2020.145212. PMID 33039541. S2CID 222302410.
  8. Bychkova, Elina (15 February 2021). "Identification of a candidate genetic variant for the Himalayan color pattern in dogs". Gene. 769. doi:10.1016/j.gene.2020.145212. PMID 33039541. Retrieved 21 August 2024.
  9. Brancalion, L (2022). "Canine coat pigmentation genetics: a review". Animal Genetics. 53 (1): 3–34. doi:10.1111/age.13154. Retrieved 21 August 2024.
  10. Standard of Perfection 2016-2020. American Rabbit Breeders Association. 2015. ASIN B018GVBJK4.
  11. "Breed Standards 2016-2020" (PDF). British Rabbit Council. Archived from the original (PDF) on 21 January 2018. Retrieved 11 February 2018.
  12. "AFRMA - Rat Genetics, part 1".
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