Genetics of Canine Coat Color

Canine coat color 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 color in dogs is usually controlled by a set of genes. These include the color genes, genes that affect the pigment color of hairs, and the pattern genes, those that affect the distribution of a particular color. At least 20 genes have been identified that affect coat color in dogs.

Merle Coat Color Patterning

The merle coat color is characterized by patches of dilute pigment in combination with areas of full pigmentation. Therefore, the merle gene acts to lighten whatever coat color 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 color scattered over the dog’s body. If the basic color of the dog is black, the effect of the merle gene is a soft gray, often referred to as “blue”. If the basic color 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, Cardigan Welsh Corgi, and Pomeranian.

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 color patterning in dogs. Texas A&M Article  

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 color pattern. A non-merle dog (mm) is normal in color, while a homozygous double-merle (MM) is predominantly white. Punnett squares can be used to determine the expected coat color of offspring when breeding dogs of known genotype (i.e. coat color 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 color pattern in Great Danes is produced through the interaction of the merle locus and the harlequin (H) gene. In harlequin Danes, the merle background color is diluted to nearly white with fully pigmented black patches.

Health Problems Associated with the Merle Allele

You should never breed two heterozygous merle (Mm) dogs together because it produces the homozygous double merle (MM).   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.

1. Ametropia: vision impairment due to a refractive error such that images fail to focus upon the retina.

2. Microphthalmia: a smaller than normal eye due to a defect occurring early in development. Affected dogs may have prominent third eyelids. Other eye defects are common in animals with this condition, including defects of the cornea, anterior chamber, lens and retina.

3. Coloboma: a defect in ocular tissue; a cleft or missing portion of components of the eye, most commonly affecting the iris.

The above information was borrowed from GenMARK Factsheet Merle Gene.pdf. GenMARK offers DNA testing of the Merle Gene for $95 per sample. If you have any doubts about your Pomeranian being a merle please have your Pomeranian spayed/ neutered or pay to have the Merle Gene DNA test. 

Merles that do not have the merle phenotype (appearance) may still have the merle genotype (Mm). These merles are called phantom merles or cryptic merles. Phantom merles should never be bred because they may produce phantom merle offspring.  Phantom merles may accidentally be bred to other merles (Mm) which would produce devastating (MM) merles with the health problems described above.  Please visit Texas A&M University Canine Genetic Research Division link for Cryptic Merle Photos.

Welcome to Cari's Pomeranians

Merle Gene Page