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Canine - Merle Gene (PDF version
 )Genetics of Canine Coat Colour
Canine coat colour is determined by the expression of a specific combination of genes.
A gene, the basic unit of heredity, is comprised of a unique sequence of DNA and directs
the production of a specific protein. Proteins are required for the structure, function and
regulation of the body’s cells, tissues, and organs. Genes are located within chromosomes.
Dogs have two sets of 39 chromosomes in every cell, one set inherited from each parent.
The location of each gene within a chromosome is referred to as its locus. While there is
more than 99% DNA sequence similarity between dogs, variations in DNA sequence do
occur in a small number of genes. Different forms of the same gene are called alleles.
Dogs can have two identical or two different alleles for a particular gene. If both alleles are
identical, then the dog is said to be homozygous at that gene; if both alleles are different,
then the dog is said to be heterozygous at that gene. The genotype of an animal is its
genetic identity, as identified by the alleles it carries; while the phenotype, or appearance, is
the expression of those alleles. Coat colour in dogs is usually controlled by a set of genes.
These include the colour genes, genes that affect the pigment colour of hairs, and the pattern
genes, those that affect the distribution of a particular colour. At least 20 genes have been
identified that affect coat colour in dogs.
Merle Coat Colour Patterning
The merle coat colour is characterized by patches of dilute pigment in combination with areas
of full pigmentation. Therefore, the merle gene acts to lighten whatever coat colour would
otherwise be expressed. However, unlike other dilution genes, the lightening effect is not
spread evenly over the coat, but is expressed as patches of diluted colour scattered over the
dog’s body. If the basic colour of the dog is black, the effect of the merle gene is a soft gray,
often referred to as “blue”. If the basic colour of the dog is red, the effect of the merle gene
is a pale red. The merle coat pattern is characteristic of a number of breeds recognized by
the American Kennel Club, including the Shetland Sheepdog, Collie, Border Collie,
Dachshund, Australian Shepherd, and Cardigan Welsh Corgi.
Genetic Inheritance of the Merle Gene
It is only recently that investigators at the Texas A&M University (reference: PNAS, 2006,
103(5):1376-81) discovered a mutation in the dog SILV gene and found it to be responsible for
the merle coat colour patterning in dogs. The merle gene (M) is inherited in an autosomal fashion.  In other words, the trait is not linked to gender and can be passed on from either the mother or the father. The gene is incompletely dominant, or a gene that has intermediate expression. A heterozygous dog, carrying
only one copy of the merle gene (Mm), expresses the characteristic diluted coat colour pattern. A non-merle dog
(mm) is normal in colour, while a homozygous double-merle (MM) is predominantly white. Punnett squares can be
used to determine the expected coat colour of offspring when breeding dogs of known genotype (i.e. coat colour genes have been identified). In the example illustrated, a non-merle
dog (mm), indicated in the vertical column, bred to a heterozygous merle (Mm), indicated in the horizontal column, will give rise to offspring with an expected frequency of 50% merle (Mm)
and 50% non-merle (mm). Dogs that carry the merle gene but do not show the characteristic
merle phenotype, are known as cryptic merles. These dogs may give rise to merle offspring.
It is suspected that the DNA sequence of the merle allele in the cryptic is shorter than the
allele expressed in the typical merle dog. The harlequin coat colour pattern in Great Danes is
produced through the interaction of the merle locus and the harlequin (H) gene. In harlequin
Danes, the merle background colour is diluted to nearly white with fully pigmented black
patches.
Health Problems Associated with the Merle Allele
Both heterozygous merle (Mm) and homozygous double merle (MM) dogs may exhibit
auditory and ophthalmic abnormalities including mild to severe deafness, increased
intraocular pressure, ametropia, microphthalmia and colobomas. The double merle genotype
may also be associated with abnormalities of skeletal, cardiac and reproductive systems.
Genetic Testing for the Merle Gene
With the recent discovery of the merle gene, a genetic test is now available that allows for the
identification of the merle allele. This technology is patent pending (U.S. Serial #
60/708,589) and available exclusively thru VITA-TECH. By testing dogs for this genetic trait,
it is possible to:
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