Merle (dog coat)
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. Description 
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. Combinations such as brindle and sable merle exist, but are not typically accepted in breed standards. 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. Pigmentation on paw pads and nose may be mottled by pink. Breeds 
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. In Dachshunds, the merle pattern is known as "dapple". 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. 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. 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. Genetic basis 

The merle allele is actually an incompletely dominant allele. 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, and a high percentage of these double merle puppies could have eye defects and/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. 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. Merle can be tested for and identified through DNA. 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. 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. 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. 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. Modifiers 
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). 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. Health issues 

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. 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. 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. Ocular defects 
Dogs who are homozygous for certain merle alleles often have visual and auditory deficits. 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. Double merle dogs may be deaf or blind or both, and can carry ocular defects in blue or colored eyes. 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. 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. 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. 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. See also 
 List of dog coat patterns

References

External links 

 Merle dog coat color genetics
 Deafness in Dogs and Cats
 White Aussies Project
 Genetics of merle dogs
 : Basic, simple-language explanation of Australian Shepherd merle colorings
 Homozygous "Lethal White" Merles
 Border Wars - Double Merle Articles
 Banning of Merle 'pattern' Chihuahua in Canada
 Catahoula Coat Color Genetics - Introduction and History of Merle

Dog anatomy
Animal hair
Animal coat colors

https://www.americanspanielclub.org/about-the-breed/merle-color-in-cocker-spaniels/