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Merle is a genetic pattern in a dog's coat and alleles of the PMEL gene. It results in different colors and patterns and can affect any coats. The allele creates mottled patches of color in a solid or piebald coat, blue or odd-colored eyes, and can affect skin pigment as well. Two types of colored patches generally appear in a merle coat: brown/liver (red merle) and black (blue merle). Associated breeds include Carea Leonés, Australian Shepherds and Catahoula Leopard Dogs. Health issues are more typical and more severe when two merle-patterned dogs are bred together.
Merle can affect all coat colors. The merle forms of brown are usually called "red" (though this is not correct; red and brown are genetically different), and black is called "blue" as lighter patches of black are formed throughout the coat and look slightly blue in color. 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. [1] [2] Combinations such as brindle and sable merle exist, but are not typically accepted in breed standards. [2]
In addition to altering the base coat color, merle also modifies eye color and coloring on the nose and paw pads. The merle allele modifies the dark pigment, resulting in eyes or parts of eyes turning blue. Since merle causes random modifications, dark-eyed, blue-eyed and odd-colored eyes are possible. [3] Pigmentation on paw pads and nose may be mottled by pink. [4]
Merle is a distinguishing marking of several breeds, particularly the Australian Shepherd and Catahoula Leopard Dog, and appears in others, including the Miniature American Shepherd, the Koolie in Australia, the Shetland Sheepdog, various collie breeds, the Cardigan Welsh Corgi, the Pyrenean Shepherd and the Bergamasco Shepherd. [5] In Dachshunds, the merle pattern is known as "dapple". [6] In Beaucerons the merle pattern is known as "harlequin". The merle allele also plays a part in producing harlequin Great Danes. Depending on the breed, registry or country, the pattern may be register-able with a registry but not allowed in conformation, and the pattern may not be recognized by the breed standard, disqualifying it from showing. [7] [5] There may also be additional requirements for the pattern such as the dog being allowed to have the pattern but must have completely dark eyes with no blue in them. [8]
Merle is sometimes introduced to other purebred dog breeds through crossbreeding, but these dogs are not purebred. In 2020 the UK Kennel Club (the oldest in the World) banned the registration of merle Poodles as it is not and never has been a colour pattern recognised in pure Poodles by the club. [9]
The merle allele is actually an incompletely dominant allele. [11] If two heterozygote merle dogs are mated, on the average, one-quarter of the puppies will be "double merles", which is the common term for dogs homozygous for the longest versions of the merle alleles, [12] and a high percentage of these double merle puppies could have eye defects or be deaf. Knowledgeable and responsible 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. [13]
In January 2006, scientists at Texas A&M University announced the discovery of a mobile genetic unit called a retrotransposon, responsible for the various merle mutations in dogs. [14] Merle can be tested for and identified through DNA. [15]
A phantom merle or cryptic merle is one with such small patches of merling, or none at all, that it appears to be a non-merle. [16] 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. [17] In America, a dog with the phantom merle coloring is sometimes described as being "cryptic for merle". A cryptic merle bred to a merle may produce merles, cryptic merles or non-merles, depending on whether the parents were heterozygous or homozygous for the merle alleles. [18] Another mutation for cryptic merle has been identified by Dr. Helena Synková, and has been given the working name of "atypical merle". It is recommended that if a breeder is unsure if their dog is merle or not, that they be tested for the merle allele lengths. [5]
Certain modifying genes work in tandem (co-dominate expressive) with the merle alleles to create a completely different look to the pattern.
Often mistaken for a "double merle", a harlequin merle (or just harlequin), is a Great Dane that carries both the merle pattern allele and the co-dominate modifying gene for harlequin, PSMB7 (also known as H). This causes most or all of the diluted "blue" color to be replaced with white, resulting in a dog that is mostly white with black patches. All dogs exhibiting the harlequin pattern also have the merle allele. Found only in Great Danes, (and registered as Harlequin). [19]
In 2018, a published paper identified six general categories of merle alleles that affect (or do not affect) the final coat color depending on the length of the SINE insertion within them. [20]
Dogs with two copies of some versions of the merle allele (called "double merle") have an even higher probability of being deaf and with vision impairments. [21] The UK Kennel Club has acknowledged the health risk associated with these particular homozygous merles and stopped registering puppies produced from merle to merle matings in 2013. Merle to merle mating is currently only forbidden in three breeds. [22]
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. [14]
Dogs who are homozygous for certain merle alleles often have visual and auditory deficits. [16] 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. [23] Double merle dogs may be deaf or blind or both, and can carry ocular defects in blue or colored eyes. [24] Currently no studies have been done to prove whether or not the merle gene affects the eyes, causing blindness.
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. One out of the 11 (9.1%) double merles was fully deaf while none of the single merles were. [25] Another study done by Texas A&M University found that of 22 double merles, eight were completely deaf and two were deaf in one ear. Of 48 single merles, one was deaf in one ear and none were completely deaf. [25] 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. [25]
Deaf and/or blind dogs can live well with proper care. Various internet groups are dedicated to supporting carers of such dogs. Deaf dogs can compete successfully in agility. [26]
The Catahoula Leopard Dog is an American dog breed named after Catahoula Parish, Louisiana. It became the state dog of Louisiana in 1979. It is recognized by the United Kennel Club under the name Louisiana Catahoula Leopard Dog, while the American Kennel Club Foundation Stock Service calls it Catahoula Leopard Dog. Both registries have assigned the breed a herding designation, although it has traditionally been used in hunting feral boars.
Roan is a coat color found in many animals, including horses, cattle, antelope, cats and dogs. It is defined generally as an even mixture of white and pigmented hairs that do not "gray out" or fade as the animal ages. There are a variety of genetic conditions which produce the colors described as "roan" in various species.
Collies form a distinctive type of herding dogs, including many related landraces and standardized breeds. The type originated in Scotland and Northern England. Collies are medium-sized, fairly lightly-built dogs, with pointed snouts. Many types have a distinctive white color over the shoulders. Collies are very active and agile, and most types of collies have a very strong herding instinct. Collie breeds have spread through many parts of the world, and have diversified into many varieties, sometimes mixed with other dog types.
The Australian Shepherd is a breed of herding dog from the United States. The name of the breed is technically a misnomer, as it was developed in California in the 19th century. It is believed to have its origins in sheepdog breeds from northwest Spain, as well as collies imported, alongside sheep, from Australia and New Zealand; the breed reportedly took its name from this trade. Originally used solely as a herding dog, the Australian Shepherd has become one of the most popular companion dog breeds in North America.
A dilution gene is any one of a number of genes that act to create a lighter coat color in living creatures. There are many examples of such genes:
Cat coat genetics determine the coloration, pattern, length, and texture of feline fur. The variations among cat coats are physical properties and should not be confused with cat breeds. A cat may display the coat of a certain breed without actually being that breed. For example, a Neva Masquerade could wear point coloration, the stereotypical coat of a Siamese.
Lethal white syndrome (LWS), also called overo lethal white syndrome (OLWS), lethal white overo (LWO), and overo lethal white foal syndrome (OLWFS), is an autosomal genetic disorder most prevalent in the American Paint Horse. Affected foals are born after the full 11-month gestation and externally appear normal, though they have all-white or nearly all-white coats and blue eyes. However, internally, these foals have a nonfunctioning colon. Within a few hours, signs of colic appear; affected foals die within a few days. Because the death is often painful, such foals are often humanely euthanized once identified. The disease is particularly devastating because foals are born seemingly healthy after being carried to full term.
A piebald or pied animal is one that has a pattern of unpigmented spots (white) on a pigmented background of hair, feathers or scales. Thus a piebald black and white dog is a black dog with white spots. The animal's skin under the white background is not pigmented.
Brindle is a coat coloring pattern in animals, particularly dogs, cattle, guinea pigs, cats, and, rarely, horses. It is sometimes described as "tiger-striped", although the brindle pattern is more subtle than that of a tiger's coat.
Point coloration is animal coat coloration with a pale body and relatively darker extremities, i.e. the face, ears, feet, tail, and scrotum. It is most recognized as the coloration of Siamese and related breeds of cat, but can be found in dogs, rabbits, rats, sheep, guinea pigs and horses as well.
Canine terminology in this article refers only to dog terminology, specialized terms describing the characteristics of various external parts of the domestic dog, as well as terms for structure, movement, and temperament. This terminology is not typically used for any of the wild species or subspecies of wild wolves, foxes, coyotes, dholes, jackals or the basal caninae. Dog terminology is often specific to each breed or type of dog. Breed standards use this terminology in the description of the ideal external appearance of each breed, although similar characteristics may be described with different terms in different breeds.
The cream gene is responsible for a number of horse coat colors. Horses that have the cream gene in addition to a base coat color that is chestnut will become palomino if they are heterozygous, having one copy of the cream gene, or cremello, if they are homozygous. Similarly, horses with a bay base coat and the cream gene will be buckskin or perlino. A black base coat with the cream gene becomes the not-always-recognized smoky black or a smoky cream. Cream horses, even those with blue eyes, are not white horses. Dilution coloring is also not related to any of the white spotting patterns.
The Landseer is a dog that originated in Canada. It is a black-and-white variety of the Newfoundland that is recognised as an independent breed in continental Europe.
The coat of the domestic dog refers to the hair that covers its body. Dogs demonstrate a wide range of coat colors, patterns, textures, and lengths.
The genetic basis of coat colour in the Labrador Retriever has been found to depend on several distinct genes. The interplay among these genes is used as an example of epistasis.
Splashed white or splash is a horse coat color pattern in the "overo" group of spotting patterns that produces pink-skinned, white markings. Many splashed whites have very modest markings, while others have the distinctive "dipped in white paint" pattern. Blue eyes are a hallmark of the pattern, and splash may account for otherwise "solid" blue-eyed horses. Splashed white occurs in a variety of geographically divergent breeds, from Morgans in North America to Kathiawari horses in India. The splashed white pattern is also associated with congenital deafness, though most splashed whites have normal hearing. Splashed white can be caused by multiple variants across two different genes, for which genetic testing is available.
A melanistic mask is a dog coat pattern that gives the appearance of a mask on the dog's face. The hairs on the muzzle, and sometimes entire face or ears, are colored by eumelanin instead of pheomelanin pigment. Eumelanin is typically black, but may instead be brown, dark gray, or light gray-brown. Pheomelanin ranges in color from pale cream to mahogany. The trait is caused by M264V (EM), a completely dominant allele (form) of the melanocortin 1 receptor gene.
Agouti is a type of fur coloration in which each hair displays two or more bands of pigmentation. The overall appearance of agouti fur is usually gray or dull brown, although dull yellow is also possible.
Dogs have a wide range of coat colors, patterns, textures and lengths. Dog coat color is governed by how genes are passed from dogs to their puppies and how those genes are expressed in each dog. Dogs have about 19,000 genes in their genome but only a handful affect the physical variations in their coats. Most genes come in pairs, one being from the dog's mother and one being from its father. Genes of interest have more than one expression of an allele. Usually only one, or a small number of alleles exist for each gene. In any one gene locus a dog will either be homozygous where the gene is made of two identical alleles or heterozygous where the gene is made of two different alleles.
The agouti gene, the Agouti-signaling protein (ASIP) is responsible for variations in color in many species. Agouti works with extension to regulate the color of melanin which is produced in hairs. The agouti protein causes red to yellow pheomelanin to be produced, while the competing molecule α-MSH signals production of brown to black eumelanin. In wildtype mice, alternating cycles of agouti and α-MSH production cause agouti coloration. Each hair has bands of yellow which grew during agouti production, and black which grew during α-MSH production. Wildtype mice also have light-colored bellies. The hairs there are a creamy color the whole length because the agouti protein was produced the whole time the hairs were growing.