Health and the Merle Pattern
Health and the Merle Pattern
The American Dog Breeders Association Inc. has done a review of the current report by Scot E. Dowd Ph.D. that was done for the AKC and UKC in regards to the merle color pattern. We have also contacted numerous breeders who have had direct experience with dogs containing this color pattern and have researched the pedigrees of dogs that have been registered with our association being described as ‘merle’. The Board Of Directors has reviewed this material and breeders reports and has spent many hours in discussion concerning what our policy should be in regards to this color pattern.
In talking to breeders and doing additional research, it is apparent that this color pattern is not "new’ to the breed. What is new to the breed is using the correct term merle to describe this color pattern. In 2001 at the urging of our fanciers, a list of ‘ACCEPTED TERMS TO DESCRIBE COLORS’ was adopted in an effort to enhance common understanding among fanciers of the breed. Up to this time these dogs with the merle color pattern were described as spotted, mottled, incorrectly as brindle , and in some incidences not addressed in the color description of the dog at all. In formulating the ‘ACCEPTED TERMS TO DESCRIBE COLORS’ within our breed, the ADBA sought out breeders and did a literature search of accepted canine references to come up with the terms that would be accepted to most accurately describe colors and color patterns. It was felt at that time, that the most accurate description of this color pattern is the term ‘merle’. In certain bloodlines we have verified merle dogs back four and five generations in the pedigree.
It is also true that the merle color pattern has not been a prevalent color pattern in the breed. We feel that this is because of its associated genetic weaknesses that occur in some individuals with this flawed gene. Prior to 1976, when the US Agricultural Department with the Animal Welfare Act of 1976, made it a federal offense to transport dogs across state lines for the purpose of fighting, many breeders were using the dogs for their ‘genetic task’ (dogfighting). Any dogs with physical weaknesses were eliminated from the breeding pool since they did not measure up. This is why bloodlines closest to their ‘genetic task’ will contain the most sound specimens of the breed. This is true in the American Pit Bull Terrier as well as all performance breeds. Since The Animal Welfare Act of 1976 breeders and fanciers have turned their attention to legal activities for this breed namely conformation shows and weight pulling events. The first dog shows for the American Dog Breeders Association were held in 1977 and weight pulling events started a few years later. Today breeders no longer have the original ‘measuring stick’ upon which to select brood stock, and thus selection is made using other criteria including color.
Before we go any further, here is some background information on the genetic components of color and genetic inheritance of color in the dog for common understanding:
There are nine different locations or loci on the chromosomes that have a combined effect to determine the color of a particular dog. At each locus, there can be found many variations or alleles that can be dominant or recessive in nature. Chromosomes come in pairs, so each dogs has two different alleles at each locus on the chromosome pairs, and depending on the dominance of the alleles contained at the locus determines the color of the individual. The genetic material of any mammal (including the dog) is inherited one half from the mother and one half from the father. The final characteristics of the offspring is determined by the new combination that results and the dominance or recessive nature of the alleles that they inherit.
What is Merle?
The merle color pattern is determined by the Locus M Pair and contains two alleles,( M) resulting in the merle pattern or (m) resulting in uniform pigment. The (M) being dominant and the ( m) being recessive.(For those used to the equine arena, this is the same allele that causes the dapple color pattern in horses.) The merle acts like other dilution alleles to lighten whatever color would otherwise have been expressed. However the (M) allele in the heterozygous state (Mm) the lighted effect is not spread evenly over the coat, but produces patches of undiluted color scattered over the dogs body. Example: the (Mm) genotype on an otherwise black dog produces a blue merle which is seen as a bluish gray dog with full color black spots. A dog with homozygous merle (MM), possessing two dominant merle (M) alleles at the Locus M Pair, is seen as a mostly white dog. The normal state of the merle locus is dual recessive (mm) and completely lacks the offending transposon allele resulting in normal color. ( We will discuss the term transposon and the physiological effects of this allele in a few minutes.) Knowing that the allele that produces the merle color pattern is dominant, when we see a merle dog, we know that one of its parents must be a merle. Knowing that the merle gene is dominant, breeding a merle dogs to a dog that does not have the affected merle allele (mm), the resulting litter would have the probability to produce 50% merles (Mm) and 50% unaffected pups (mm). Breeding two merle dogs (Mm) the resulting litter would have the probability of being 25% unaffected (mm), 50% merle (Mm) and 25% white (MM).
Health issues associated with the merle allele
The (M) allele is an excellent example of pleiotropy, the phenomenon where a single allele can cause distinct and seeming unrelated physical effects. Even in the heterozygous (Mm) dog the (M) allele is associated with deafness, eye defects, and problems with the dogs immune system. This can be explained by understanding a little bit about the early fetal development of the dog.
The process of coloration and color pattern in dogs begins with embryonic development. The specific cells that become the pigment producing cells come entirely from the same area of the embryo (neuronal crest) that the cells of the nervous system comes from. It stands to reason, that if you have defects in genes associated with color genetics you can have nervous system defects because both cells are derived from the same neuronal crest. This can explain why it is likely that certain dilute or patterned dogs, such as extreme piebalds, albinos, etc. as well as those that have the merle allele are prone to sensory, neurological and /or immunological problems. These defects have been observed and researched in other dog breeds (i.e. Australian Shepherds, Great Danes, Shetland Sheepdogs) that also carry these dilution alleles including the merle allele.
From this research it has been determined that the merle allele when expressed in the homozygous state (MM) is highly correlated to sensory, neurological and immune system defects in dogs. Some include distortion of the eye’s appearance, lack of the reflective substance (tapetum lucidum) that lines the back of the dogs eye. Dogs that lack this substance have night blindness and other visual problems. Other eye problems have been identified with the merle allele includes, small eyeballs, with a prominent third eyelids, and a physical cleft in the iris of the eye. Abnormalities of the eyes are a key indicator of other neurological defects. Deafness or a reduction in hearing has also been identified, as the merle color locus exerts effects on ear development. Excessive white or dilution in a dog of any color can be a warning sign of hearing problems. From talking to breeders who’s lines contain merle dogs, they relate that they are very aware of the potential health problems (sometimes being lethal to the affected pups) resulting in the