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) .
