Merle coloration in dogs

Merle pattern in dogs is associated with certain health issues, which should be known to each breeder. The breeder should understand and get acquainted with the problems and the depth of this phenomenon before he decides to introduce the merle pattern into his breed. Studies of the merle pattern have been conducted at several institutes in the world and so we can safely assume that other relations will be discovered.

The gene for merle pattern appears in many canine breeds, but this pattern (colour) is recognized only in some breeds as FCI standard. The recognized breeds with merle pattern are, for example, Australian Shepherd, Border collie, Dachshund, Great Dane, Louisiana Leopard dog, Cardigan Welsh Corgi, etc. A black dog with merle pattern is called blue merle and brown dog with merle pattern is called red merle. The light merle patches are irregularly spread over the body.

The merle-allele has a SINE insertion in SILV-gene, which can be of various lengths. The length of the inserted SINE sequence affects the way how the merle gene manifests itself in dog phenotype.   The M locus (merle) has several variants and some genotypes can be distinguished from each other only by a genetic test. The merle phenotype is inherited as an autosomal, incomplete dominant trait. The insertion contains polyT sequence of variable length.  The longer the polyT sequence is, the more the merle trait is expressed. The polyT sequence may become shorter or may expand from generation to generation. Therefore, the offsprings can have a different portion of merle coloration than their parents. The expression of polyT sequence is variable even in various cells of the dog – the dog looks like a merle mosaic.

.

Examples of merle coat color:

smooth collie

.

dachshund

.

Australian Sheperd - blue merle

.

Australian Sheperd - red merle

.

Variation of merle-alleles

m – Normal allele without merle. Dogs of m/m genotype (non-merle) have no merle pattern and are full coloured.

Mx - merle allele, where x is the number of T in the polyT sequence – the longer the sequence is, the more merle phenotype is expressed.

The rate of merle expression can be described as follows:

• The length of polyT sequence of approx. 35-62 T - cryptic merle (Mc) - The dog can have only a small merle patch, for example, at the end of the tail or on the edge of one ear or the merle colour can be concealed by white markings. These dogs carry the shorter version of the merle gene in one copy (m/Mc) or in two copies (Mc/Mc). Unlike the regular merle dogs, in the cryptic merle dogs no serious health problems have been described. The correct description of cryptic merle is a problem when registering the dog.  The Mc-allele can be proved only by a genetic test.
• The length of polyT sequence of approx. 63-73 T - dilution merle (Md)
• The length of polyT sequence of approx. 74-85 T - merle (M)
• The length of polyT sequence of approx. 86 T and more  - harlequin (Mh)

Hidden merle

The merle is not detectable to eye in light coat colours.  The designation “hidden” applies to dogs that are genetically Mx (merle) dogs, but the merle is expressed only in white coat (light coat colours, in general). Dogs with large areas of light coat colour are at increased risk of hidden merle. The hidden merle can be only proved by a genetic test.

.

Double merle

The M/M genotype (homozygous for merle) shows the merle pattern. These dogs are created when two dogs with the merle coat are bred together. Double merle dogs are very light, sometimes completely white, the colour of mucous membranes is pale to pinkish and the eye pigmentation is mostly blue. The dogs often have serious health problems (ophthalmologic and auditory problems, abnormalities of skeletal, cardiac and reproductive systems). For this reasons two merles should never be bred together.

The Mc/M genotype (cryptic merle / merle) is also risky for breeding. This genotype exhibits merle pattern and has usually no health problems (but health problems may occur!). Cryptic merle dogs must be bred only to non-merle dogs and when the dog is bred the cryptic allele may well expand to regular merle.

Genetic testing for merle gene is highly recommended so that severe health problems that may occur during incorrect or risky breeding of merle, cryptic merle and hidden merle dogs are avoided. The genetic tests reveal the merle, the hidden merle and the variant of cryptic merle. Breeders of breeds with sufficiently described merle pattern should pay increased attention to the breeding. In the litter, all puppies that appear as “non-merle” at first sight have to be properly checked for any less visible merle patches.