The intensity of the merle pattern is determined by the length of the poly(A) tail of a repeat element which has been inserted into the boundary of intron 10 and exon 11 of the PMEL17 locus in reverse orientation. This poly(A) tail behaves as a microsatellite, and due to replication slippage, longer and shorter alleles of it might be generated during cell divisions. The length of the poly(A) tail regulates the splicing mechanism. In the case of shorter tails, the removal of intron 10 takes place at the original splicing, resulting in a normal premelanosome protein (PMEL). Longer tails generate larger insertions, forcing splicing to a cryptic splice site, thereby coding for an abnormal PMEL protein, which is unable to form the normal fibrillar matrix of the eumelanosomes. Thus, eumelanin deposition ensuring the dark color formation is reduced. In summary, the longer the poly(A) tail, the lighter the coat color intensity of the melanocytes. These mutations can occur in the somatic cells and the resulting cell clones will shape the merle pattern of the coat. When they take place in the germ line, they occasionally produce offspring with unexpected color variations which are different from those of their parents.

A retrotransposon insertion in the SILV gene is associated with a peculiar phenotype of dog, known as a merle. It is characterised by various areas of their coat colour becoming diluted due to a malfunction in the eumelanin-producing pigment cells. Recent studies have shown that the exact size of the short interspersed element (SINE) insertion is in correlation with specific phenotypic attributes, but was not able to absolutely confine dogs to a certain colour pattern. Our study focused on the merle variations occurring in the Mudi breed. Altogether, 123 dog samples from 11 countries were tested and genotyped. The exact length of the merle alleles were determined by automated fluorescent capillary fragment analysis. The most frequent merle genotype in this Mudi sample collection was the 'classic' merle (m/M: 61.8%), whereas other variants, such as atypical (m/Ma and m/Ma+: 5.7%), harlequin (m/Mh: 13.8%), double merle (M/M: 0.8%) and mosaic profiles (17.9%) were also observed. The practical significance of testing this mutation is that, phenotypically, not only merle dogs are carriers of this insertion, but also the so-called hidden merle individuals (where the merle phenotype is fully covered by the pheomelanin-dominated colouration) are potentially capable of producing unintentionally homozygous 'double merle' progeny with ophthalmologic, viability and auditory impairments.

Background: The merle colour in dogs (merle: blackbird) is caused by a retrotransposon insertion in the SILV gene, resulting in malfunction in the eumelanin producing pigment cells. In homozygous (MM) individuals congenital auditory and ophthalmologic disorders are common, therefore the mating of two heterozygous (Mm) dogs should be avoided, and is in fact forbidden in case of breeds having this trait. Some of the colours, however are fully characterised by pheomelanin pigment (e.g. cream, beige, fawn), which are not affected by the merle gene, making it impossible to recognize the heterozygous individuals based on their phenotype (hidden merles). Objectives: The author's aim was to test a fast, reliable and cost-efficient genetic method in the Mudi breed, because in this breed along with the merle colour, white and fawn colours can also be found-which increases the chance for hidden merle individuals. Materials and Methods: In their study the authors used 23 hair or buccal swab samples collected from Hungarian and foreign possible hidden merle Mudi dogs, then multiplied the exon 11 of the SILV gene with polymerase chain reaction (PCR) method from the purified DNA. Agarose gel electrophoresis was used to detect the PCR products (M and m alleles) and to separate them by size. The authors compared their genotype results with the dogs' phenotypic traits (colour of the eyes and the fur). Results and Discussion: According to practical observations blue or partial blue eyes are solely caused by the merle gene in the Mudi breed, this observation was confirmed by the authors' DNA test, so in these simple cases hidden merle dogs can be detected by their phenotype. However, eye colour is not always affected by the merle gene: in this research 4 of the 17 brown eyed dogs tested proved to be heterozygous Mm. This indicates that the genetic test is the only reliable way to identify hidden merle individuals, which is important to prevent puppies with decreased viability.