The Merle Factor in Dogs: Beauty with a Dark Side 
© Anna Darahan/ stock.adobe.com
The Merle genetic defect is particularly common in Australian Shepherds.
The Merle factor results in distinctive dogs with light patches and speckles. However, this is actually due to a genetic defect, which can be linked to incurable diseases. Here you’ll find all the essential information about the Merle factor in dogs.
Table of contents
What is the Merle Factor in Dogs?
The Merle factor (also called Merle or Merle genetic defect) is a particular coat pattern in dogs. Typically, Merle dogs have a significant amount of white in their fur, often in the form of patches. This popular coat colour can also be associated with one or two blue eyes.
This is because the causative Merle gene only lightens the brown-black pigment, Eumelanin. Yellow-red fur areas, which contain the pigment Phaeomelanin, remain unaffected.
How Does the Merle Factor in Dogs Develop?
The Merle coat colour stems from what is known as the Merle genetic defect. This arises due to a mutation of the Silver-Locus gene Pmel17 – a permanent alteration in the genetic makeup.
To understand the Merle genetic defect in dogs in more detail, it’s necessary to define so-called alleles. These are different versions of the gene that determine the expression of the Merle factor:
| Allele | Name |
| M | Merle |
| m | No Merle (wild type) |
| Mc | Cryptic Merle |
| Mc+ | Cryptic Merle plus |
| Ma | Atypical Merle |
| Ma+ | Atypical Merle plus |
| Mh | Harlequin Merle |
How is the Merle Genetic Defect Inherited in Dogs?
The Merle genetic defect has an autosomal intermediate (incomplete dominant) mode of inheritance. This means the dominant Merle allele (M) doesn’t completely suppress the recessive wild-type allele (m). Consequently, the recessive and normal wild-type allele retains some activity.
Your dog’s colour will depend on the combination of alleles it inherits from its parents.
© Nikol / stock.adobe.com
Which breeds are affected by the Merle genetic defect?
Several dog breeds are known to show instances of the Merle factor. Those most frequently affected include:
- Australian Shepherd
- Border Collie
- Collie (Short– and Longhair)
- Beauceron
- Catahoula Leopard Dog
- Chihuahua
- Dachshund
- Great Dane
- Norwegian Elkhound
- Welsh Corgi Cardigan
- Pit Bull
- Pomeranian
- Shetland Sheepdog (Sheltie)
Are There Animals of These Breeds Without the Merle Genetic Defect?
A great number of genes influence the coat colour of dogs. Consequently, many animals from the breeds mentioned above do not carry the Merle factor.
Are Merle Dogs More Susceptible to Illnesses?
Not every Merle-coloured dog is ill. Nonetheless, Merle dogs are at higher risk of developing certain diseases.
Since the melanocytes that produce Eumelanin are also involved in the development of sensory cells in eyes and ears, the Merle genetic defect in dogs can lead to corresponding sensory disorders:
Blindness
The Merle genetic defect can cause heritable eye problems and blindness in dogs. Pure-bred Merle dogs often suffer from malformations of the eyes, including:
- Dyscoria: irregularly shaped pupils
- Colobomas: split formation in the eye tissues
- Microphthalmia: abnormally small eyeball
Deafness
Dogs with the Merle gene have a higher risk of congenital hearing impairments, which are caused by pathological changes in the inner ear and dependent on genetic factors:
- 10 percent of purebred Merle dogs are unilaterally deaf
- 15 percent of purebred Merle dogs are bilaterally deaf
- 2.7 percent of mixed-bred Merle dogs are unilaterally deaf
- 0.9 percent of mixed-bred Merle dogs are bilaterally deaf
Are Merle Dogs the Result of Unethical Breeding?
It can’t be said that every dog with the Merle colour is the result of unethical breeding, as the Merle gene doesn’t lead to direct physical deformations. According to Section 11b Paragraph 1 of the Animal Welfare Act, it is “prohibited to breed vertebrates … if … breeding knowledge anticipates that as a result of the breeding … the offspring … will lack or have unfit or transformed body parts or organs for the species-appropriate use and suffer pain, suffering, or damage as a result”.
As a result, breeding two Merle marked dogs together is prohibited in countries such as Germany. When two dogs with at least one Merle allele are mated, it is highly likely that at least one of the puppies will be born incurably ill.
What is not prohibited is mating a mixed-bred Merle dog with a dog that does not have the defect.
Only Buy Puppies from Reputable Breeders
Unfortunately, there are occasions when dubious breeders or uninformed dog owners mate Merle carriers.
This is why it’s important to ensure that the breeder is reputable when buying a puppy.
Prevention Starts Before Breeding
To ensure that your dogs give birth to healthy puppies, responsible breeding is key. To prevent the birth of sick Merle dogs from the outset, you should have your parent dogs tested for genetic defects.
You can consult your vet or look for commercial providers of genetic tests. Several genomics laboratories now offer DNA analyses (genetic tests).
Does My Dog Carry the Merle Factor?
From the genetic test results of the parent dogs, you can determine whether they carry the Merle factor.
The following table shows typical examples of genotypes (composition of the Merle gene) and resulting phenotypes (coat colour):
| Genotype | Expression | Coat Colour in Combination with “m” |
| mm | Your dog is purebred and carries two wild type alleles and no Merle alleles | no Merle |
| Mm | Your dog is mixed-bred. It carries one normal allele and one Merle allele | Merle (light-dark dappled) |
| MM | Your dog is purebred and carries two Merle alleles | Merle (completely white) |
This article from zooplus Magazine is purely informative. If your pet is experiencing discomfort, contact your veterinarian for diagnosis and treatment.
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At the Justus-Liebig-University of Giessen I was extensively trained as a veterinarian and was able to gain experience in various fields such as small animal, large animal and exotic animal medicine as well as pharmacology, pathology and food hygiene. Since then, I have been working not only as a veterinary author, but also on my scientifically driven dissertation. My goal is to better protect animals from pathogenic bacterial organisms in the future. Besides my veterinary knowledge, I also share my own experiences as a happy dog owner and can thus understand and enlighten fears and problems as well as other important questions about animal health.
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