Merle (dog coat)

Blue merle Border Collie puppy

Merle is a pattern in a dog's coat. "Merle" comes in different colors and patterns. Some have blue patches throughout and are considered blue "merle"; same with red, and chocolate, although some describe merle as only a "pattern". DNA testing identifies the "Merle" gene but not the variety of colors and patterns seen in the coats of the dogs with the gene. A dilute black (BB or Bb) dog is generally known as blue, but a dog with a merle coat does not test BB or Bb but still is a blue dilute from the "merle" gene[1] The merle gene creates mottled patches of color in a solid or piebald coat, blue or odd-colored eyes, and can affect skin pigment as well. Health issues are more typical and more severe when two merles are bred together, so it is recommended that a merle be bred to a dog with a solid coat color only.

Description

Merle can affect all coat colors. The merle forms of brown and black are usually called "red" (though this is not correct; red and merle are genetically different) and "blue" as patches of blue are formed throughout the coat. Merle is a dilution gene just like the blue, chocolate in a traditional color but without the color of blue DNA. Merle has its own colours and DNA; a blue merle is a dog who has blue patches on its body, red and some are chocolate or "cookies and cream". Dogs who are recessive red can still be affected by merle, but the patches are either hardly seen or, if the dog is a clear recessive red, are not visible at all.[2][3] Combinations such as brindle merle exist, but are not typically accepted in breed standards.

In addition to altering base coat color, merle also modifies eye color and coloring on the nose and paw pads. The merle gene modifies the dark pigment in the eyes, occasionally changing dark eyes to blue, or part of the eye to be colored blue. Since merle causes random modifications, however, both dark-eyed, blue-eyed, and odd-colored eyes are possible.[4] Color on paw pads and nose may be mottled pink and black.[5]

Breeds

Merle is a distinguishing marking of several breeds, particularly the Alapaha Blue Blood Bulldog, Australian Shepherd, and appears in others, including the Koolie, German Coolies in Australia, the Shetland Sheepdog, various Collies, the Welsh Corgi (Cardigan), the Pyrenean Shepherd, the Bergamasco Sheepdog, the Old English Sheepdog, and Catahoula Leopard Dog. In Dachshunds the merle marking is known as "dapple".[6] It is also present in the Pomeranian and Chihuahua, but is a disqualification according to the FCI, the Canadian Kennel Club [CKC] and other kennel club standards. It is only the American Kennel Club [AKC] that allows the registration of the 'merle.' In the Cocker Spaniel breed, it is a recognized pattern, although incorrectly listed as a 'color'.[7] The merle gene also plays a part in producing harlequin Great Danes and Beaucerons. In several breeds, such as the Pomeranian and Chihuahua, merle is an indicator of cross breeding by many breeders and associations.

The merle pattern has been found in the APBT and The American Bully, a newer breed. The UKC AND ADBA does not allow merle to be registered at all. If the dog was born in or before 2005 is the only way it can have ADBA paper work. ABKC considers merle a fault. However, they allow the registration and breeding of merle American Bullies.

The merle color pattern is also a new fad in the Boston terrier world. The color was likely introduced by another breed, such as the Chihuahua or the American Bully. Traditional breeders consider these dogs to be impure, regardless of the benefit of new genes to a closed pool. The Boston Terrier standard does not allow the showing of merle dogs.

Genetic basis

A deaf 8-month-old homozygous merle Australian Shepherd. She has blue eyes with starburst pupils and an eccentric pupil in her left eye.

Merle is actually a heterozygote of an incompletely dominant gene.[8] If two such dogs are mated, on the average one quarter of the puppies will be "double merles", which is the common term for dogs homozygous for merle,[9] and a high percentage of these double merle puppies could have eye defects and/or be deaf. Knowledgeable breeders who want to produce merle puppies mate a merle with a non-merle dog; roughly half the puppies will be merles, and none will have the vision or hearing defects associated with double merle dogs.[10]

In January 2006 scientists at Texas A&M University announced the discovery of a mobile genetic unit called a retrotransposon, responsible for the merle mutation in dogs.[11]

A phantom merle or cryptic merle is one with such small patches of merle—or none at all—that it appears to be a non-merle.[12] This is commonly seen in dogs who are recessive red, clear recessive reds in particular, though patches can still be seen in certain red dogs.[13] In America, a dog with the phantom merle coloring is described as being "cryptic for merle."

Merle modifiers

Certain "modifying genes" work in tandem (co-dominate expressive) with the merle gene to create a completely different look to the pattern.

Often mistaken for a "double merle", a harlequin merle (or just harlequin), is a dog that carries both the merle pattern gene and the co-dominate modifying gene (carried on a different locus) for harlequin. This causes most or all of the "blue" to be replaced with white, leaving a dog that is mostly white with black patches. All dogs exhibiting the harlequin pattern are also carriers of the merle gene. Common in Great Danes, (and registered as Harlequin) it is less commonly seen in other breeds such as the Catahoula Leopard Dog, Shetland Sheepdog and Collie, where the dogs are registered simply as merle.[14]

Patchwork or tweed, is a co-dominate modifying gene that turns the gray (in what would be a blue merle) or the liver (in what would be a liver or "red" merle) into varying shades of brown, gray and tan. This is the color pattern associated with the African wild dog and is very uncommon in pet dogs. It has been reported in the Australian Shepherd, Catahoula Leopard Dog and Border Collie.[14]

Health issues

Dogs with two copies of the merle gene (homozygous merle or "double merle") have an even higher chance of being born deaf.[15] The UK Kennel Club has acknowledged the health risk associated with homozygous merle and will stop registering puppies produced from merle to merle matings starting from 2013. Merle to merle mating is currently only forbidden in three breeds.[16] Recent research indicates that the majority of health issues occur in dogs carrying both piebald and merle genes. The piebald gene is indicated by white areas on the dog's coat as seen in the liver merle Catahoula pictured below.

The suppression of pigment cells (melanocytes) in the iris and in the stria vascularis of the cochlea (inner ear) leads to blue eyes and deafness. An auditory-pigmentation disorder in humans, Waardenberg syndrome, reflects some of the problems associated with heterozygous and homozygous merle dogs and genetic research in dogs has been undertaken with the goal of better understanding the genetic basis of this human condition.[11]

A Great Dane mix homozygous for merle displaying bilateral microphthalmia.
Australian Shepherd homozygous for merle

Ocular defects

Dogs who are homozygous for the merle pattern gene often have visual and auditory deficits.[12] These dogs are sometimes referred to as 'double merle' and sometimes incorrectly referred to as 'lethal white.' Ocular defects include microphthalmia, conditions causing increased ocular pressure, and colobomas, among others.[17] Double merle dogs may be deaf or blind or both, and can carry ocular defects in blue or colored eyes.[18] Currently no studies have been done to prove whether or not the merle gene affects the eyes, causing blindness.

Auditory defects

In one study of 38 dachshunds by a German researcher, partial hearing loss was found in 54.6% of double merles and 36.8% of single merles. 1 out of the 11 (9.1%) double merles was fully deaf while none of the single merles were.[19] Another study done by Texas A&M University found that of 22 double merles, 8 were completely deaf and two were deaf in one ear. Of 48 single merles, only one was deaf in one ear, and none were completely deaf.[19] In another study of 70 dogs, 15 of them Catahoula Curs, 4 of the Catahoulas were deaf, while 86% of the double merles of other breeds were deaf.[19]

Deaf, blind, and deaf and blind dogs can have good lives when properly cared for. There are a variety of internet groups dedicated to supporting carers of such dogs. Deaf dogs can compete successfully in agility[20] and there are many anecdotal reports of deaf/blind dogs earning their Canine Good Citizen certification, working as therapy dogs, and competing in dog sports like tracking or Nosework.

See also

References

  1. ↑ "What is Merle?". The White Aussies Project. Retrieved 28 October 2011.
  2. ↑ Schmutz, Sheila. "Schmutz - Merle coat color genetics". Sheila Schmutz. Retrieved 28 October 2011.
  3. ↑ Presberg, Carole. "Recessive Red Merle Border Collie". Carole Presberg. Retrieved 28 October 2011.
  4. ↑ McDonald, Lisa. "Aussie Eye Color". Lisa McDonald. Retrieved 28 October 2011.
  5. ↑ "Merle Coat, mottled paw pads". Perfect Peks Kennel. Retrieved 28 October 2011.
  6. ↑ "The Double Dapple". The Dachshund Magazine Online. Retrieved June 25, 2007.
  7. ↑ American Kennel Club, Cocker Spaniel.
  8. ↑ Sheila, Schmutz. "Schmutz - Merle". Sheila Schmutz. Retrieved 27 October 2011.
  9. ↑ "White Aussies Brochure" (PDF). White Aussies Project. Retrieved 27 October 2011.
  10. ↑ "Double Merle". White Aussies Project. Retrieved 27 October 2011.
  11. 1 2 Clark, LA; Wahl JM; Rees CA; Murphy KE (31 Jan 2006). "Retrotransposon insertion in SILV is responsible for merle patterning of the domestic dog". Proc. Natl. Acad. Sci. U.S.A. 103 (5): 1376–81. doi:10.1073/pnas.0506940103. PMC 1360527Freely accessible. PMID 16407134.
  12. 1 2 Coile, D. Caroline (1999). "Obtaining an Australian Shepherd". Australian Shepherds. Barron's. pp. 20–21. ISBN 0-7641-0558-2.
  13. ↑ Schmutz, Sheila. "Schmutz - Merle". Sheila Schmutz. Retrieved 27 October 2011.
  14. 1 2 "Dog Coat Colour Genetics". doggenetics.co.uk.
  15. ↑ Strain, GM; Clark LA; Wahl JM; Turner AE; Murphy KE. (Mar–April 2009). "Prevalence of deafness in dogs heterozygous or homozygous for the merle allele". J Vet Intern Med. 23 (2): 282–6. doi:10.1111/j.1939-1676.2008.0257.x. PMID 19192156. Check date values in: |date= (help)
  16. ↑ "Merle to merle mating in dogs". The Kennel Club. 18 May 2012. Retrieved 7 September 2012.
  17. ↑ Gustafson, JP; et al., eds. (2008). Genomics of Disease. p. 81. ISBN 978-0-387-76722-2.
  18. ↑ "Eye defects in Australian Shepherds". Retrieved 16 April 2011.
  19. 1 2 3 "George Strain on Merle" (PDF). Merle Poms. Retrieved 27 October 2011.
  20. ↑ "Deaf Dogs Roundtable". Retrieved 16 April 2011.
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