Merle is a coat pattern seen in many dog breeds and is inherited in autosomal, incompletely dominant manner. It is characterized by irregularly shaped patches of diluted pigment and patches of solid pigmented coat color. Merle arises from a mutation in a gene involved in coat pigmentation called PMEL or SILV which is primarily located in pigment cells that produce and store eumelanin (black pigment). In the dog breeds evolution, a short transposable element known as SINE was inserted into a gene, thereby influencing gene expression and disrupting its function.
Merle affects only black pigment eumelanin, thus any black, liver, blue or isabella in the coat will be merled. Red pigment phaelomelanin is not affected by merle and will appear as normal. Thus red (genotype e/e) dogs that are genetically merles will appear as normal and will be hidden merles, only the eyes can be blue. They do not express merle but can produce merle offspring depending on the genotype of the mate. Similar case of hidden merle is with sable dogs (having at least one allele Ay) where only the tip of the hair can be merled while producing eumelanin and it is difficult to visually distinguish a merle from non-merle.
In addition to coat colour, the PMEL mutation can also affect the colour of eyes, nose, paw pads and skin. The merle gene modifies the dark pigment in the eyes, thus merle dogs typically have blue or partially blue eyes. Because of random modifications, both dark-eyed, blue eyed, and odd-colored eyes are possible. Colour on paw pads and nose may be mottled pink and black. In addition, PMEL mutation excites potential health concerns while modifying coat coloration PMEL gene exerts an important biological role in terms of hearing and vision impairments. It is hypothesized that combination of two copies of large enough merle alleles to produce the merle coat colour are thought to cause the death of pigment cells in the skin, retina and inner ear, resulting in a predominantly white coat color and hearing and vision problems. What is interesting about merle is that phenotypically merle dog can have offspring with different lengths of merle alleles and a merle dog could also have multiple merle alleles and these dogs would be called mosaics. They can have variants of alleles of different size found in different cells of their body. Mosaicism occurs due to inherent instability of SINE insertions and their susceptibility to mutations. This mosaicism may be present in only a small proportion of cells and may not be present in the analysed sample, making it difficult to always reliably detect mosaicism with genetic testing. The inheritance of an additional allele and its effect on phenotype cannot be predicted, as the distribution of mosaic cells throughout the body is different and not all cells are included in the test sample.
The presence or absence of the merle SINE insertion determines the possibility of observing the merle phenotype. However, the size of the insertion is very important. The SINE element has a poly-A tail (repeating nucleotide A) at the end, the length of which can vary as much as 80 nucleotides, resulting in a different sizes of merle alleles (M*) which correlates to the extent of the merle pattern observed. The longer the poly-A tail, the more diluted coat colour is. Due to the autosomal, incompletely dominant manner of inheritance only one copy of merle associated variant M* is sufficient for a visible effect.
In the past, our laboratory used the naming of alleles m, Mc and M as it was conducted by Clark et al. in 2006. We now use the nomenclature and size scale of the merle alleles as conducted by Langevin et al. in 2018. With a genetic test it can be determined whether a dog has copies of the M* allele and their lengths. The following are potential alleles that may be identified and lengths of their SINE insertions in base pairs (bp) (conducted by Langevin et al.) and some major phenotypic features:
- m: Wild type allele with no SINE insertion; coat is without Merle pattern
- Mc: Cryptic Merle (200 to 230 bp); coat is without Merle pattern-solid coat
- Mc+: Cryptic Merle (231 to 246 bp); coat is without Merle pattern-solid coat
- Ma: Atypical Merle (247 to 254 bp); coat is without Merle pattern or is diluted-brownish hue
- Ma+: Atypical Merle (255 to 264 bp); coat is not typical merle, merle is undefined, mostly diluted-brownish hue
- M: Merle (265 to 268 bp); Classic Merle with randomly diluted areas alternating fully pigmented areas
- Mh: Harlequin merle (269 to 280 bp); Minimal Merle, areas deleted to white, tweed
Because the boundaries between size ranges of M* alleles are set strict and defined on the basis of research on a smaller number of breeds, and because there are genetic differences between different breeds and even among individual dogs within the breed, coat color/pattern is not always characteristic for coat color/patterns of a given M* allele size range, especially when the size of the M*allele is at the border between two M* alleles size ranges. In addition, due to the complexity of the merle insertion variants and limitations of current molecular technologies, it is a challenge to determine the precise size of the M* allele. Nevertheless, determining the size of the merle insertion variants in our laboratory is accurate by +/- 1 bp. However, due the variable nature of the M* allele variant and a small difference in methodologies used between laboratories the results are challenging to compare among laboratories.
Regarding to the presence of tested mutation animals are classified in following genotypes:
- m/m (non-merle) - dog carries two copies of normal m allele are not expected to display a merle pattern. This dog will pass a copy of m allele to its entire offspring.
- m/M* - dog carries one copy of normal merle m allele and one copy of the M* variant allele. The size of the M* allele will determine the amount of dilute patches. As Merle is inherited in an incomplete dominant fashion only one copy of an M* allele is necessary for a dog to display some variation of the merle coat color/pattern. The dog will pass one copy of normal m allele to 50% of its offspring and one copy of M allele variant to 50% of its offspring.
- M*/M* - dog carries two copies of M* alleles; the size of both alleles will determine the variation of the merle coat color/pattern; dogs with two copies of smaller sized alleles display little to no merle pattern, while dogs with one or two copies of large sized alleles are expected to display a distinct dilution/white pattern. This dog will pass one copy of each M* allele variant to 50% of its offspring. Dogs that inherit two copies of the M allele insertion variant at certain size are at an increased risk to be an affected, “double merle” which may be prone to health problems.
- m/M*/[M*] or M*/M*/[M*] - dog carries an additional M allele and mosaicism for two different copies of the M allele. Each individual cell still only has two copies of allele, however, different cells on the body may have different sizes of alleles. The phenotypic impact of the additional allele cannot be predicted as distribution throughout the body may be variable. Similarly, it is possible for all alleles to be transmitted to offspring but depends on the alleles present in the egg and sperm cells, thus the heritability cannot be predicted.
Inheritance: incomplete autosomal dominant - read more
Mutation: SILV gene
Sample: EDTA whole blood (1.0 ml) or buccal swabs. Detailed information about sampling can be found here.