There are several potential health problems in dogs with merle coats. As much as this coat is appealing to many dog owners, this coat comes with some risks. Of course, not all dogs with merle coats will display these problems, but it's important being aware of them. 

Veterinarian Dr. Ivana shares information about what a what a merle coat is, dog breeds with this coat pattern, possible health problems associated with a dog's merle coat and the risks that come with breeding a merle dog to a merle dog.

Discovering the Merle Coat in Dogs 

Beauty is in the eye of the beholder – that is something we all know. However, we can all agree that dogs with merle coats are fantastic. Whether you think they are beautiful or unique, one thing is for sure – they are great.

Sadly, dangers are lurking beneath the unusual, yet beautiful color pattern. Keep reading, as this article will tell you everything you need to know about the intriguing merle coats in dogs – from inheritance through looks to associated health problems.

Firstly, what is a merle coat? Merle is a specific coat pattern rather than particular coat color. When someone says merle, the first thing that comes to mind is blue merle – because this expression is the most common. However, merle can also be red or chocolate.

Namely, a merle coat is a coat that is characterized by randomly distributed patches of lighter or diluted color against a background color. The patches vary in terms of shape, size, and color. 

In dogs with the merle-patterned coats the gene responsible for the coat pattern also affects the eyes, nose, and paw pads.

In terms of eyes, merle dogs can have differently colored eyes (odd-eyed), unusually dark eyes or blue-colored eyes. As for the nose and paw pads, they can be mottled black and pink.

Types of Merle Coats in Dogs 

There are two basic merle forms – blue (a dilution or lighter version from black) and red (diluted or lighter version from brown or liver).

Blue merles have distinguishable black, grey, and silver marbling with more or less pronounced copper points over the face.

On the other hand, red merles have light cinnamon to dark liver marbling with copper points distributed over the face and on the legs. 

Sometimes, tricolored dogs are mistaken for merle dogs. However, tricolored dogs lack merle’s marbling hallmark.

Red merles have light cinnamon to dark liver marbling with copper points distributed over the face and on the legs.

Red merles have light cinnamon to dark liver marbling with copper points distributed over the face and on the legs.

Is Merle Dominant or Recessive?

Merle coat occurs as a result of genetic mutation. The mutated gene responsible for the unique merle pattern is called PMEL17 or SILV.

Scientists refer to the merle coat pattern as incompletely dominant. Since genetics are complicated, it is necessary to explain what incompletely dominant means.

Incompletely dominant in this case indicates that the merle coat occurs when a dog inherits a single copy of the merle allele. That single copy causes color dilution.

However, when it comes to merle, there are three different alleles:

  •  Merle allele (M)
  •  Cryptic merle (Mc)
  •  Non-merle (m).

Merle dogs have one merle (M) and one non-merle (m) allele which is in genetic terms expressed as Mm.

What are hidden or cryptic merles? Hidden or cryptic merles are dogs that do not have the unique coat pattern, but are genetically merle (either MM or Mm).

This is possible because the merle pattern can sometimes be masked by other genes or even by heavy patching.

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A hidden or cryptic merle can be either solid-colored or have merle patches that are so faint they are almost impossible to notice. Popularly these dogs are called ghost or phantom merles.

The inheritance patterns of cryptic merles are unstable and highly unpredictable. Sometimes a cryptic merle (Mc) can produce a double merle (MM) and vice versa.

Health Problems in Dogs With Merle Coats 

The merle gene is responsible for more than just coat pattern. It is also associated with impairments in the auditory and ophthalmologic systems as well as with decreased efficiency of the immune system.

The reason behind this connection is the fact that during embryonic development the pigment and nervous system stem from the same cells. When the merle gene suppresses the coat pigment cells, it also inhibits the pigment cells in the iris of the eye and in the inner ear from fully expressing.

Therefore, merle dogs are particularly prone to eye and ear issues. Additionally, the blue eye coloration makes many diagnostic procedures more challenging. 

According to a study, deafness occurs in 9.2 of dogs with the merle allele. More precisely deafness appears in 25 percent of double merles and in 3.5 percent of single merles. Many other studies come up with similar results.

In addition to deafness, eye problems are prevalent among merle dogs. The most common eye-related abnormalities are:

  •  Microphthlamia (small eyes)
  • Microcoria (dog born without the muscles that allows the pupils to dilate)
  • Colobomas (missing pieces of tissue in structures that form the eye)
  • Night blindness
  • Clefts in the iris
  • Corectopia eyeballs not positioned in the center
  • Deformed third eyelids

Last but not least, it is not unusual for merle dogs to be born without eyes.

dog eyes

Health Problems in Double Merles 

 Health issues occur when a dog inherits two merle alleles and is genetically referred as double merle or MM. Double merle occurs when two merles breed with each other.

 A double merle dog will be white with differently-sized color patches. Because of the association with health issues, double merle dogs are often called “lethal white.”

A double merle dog carries two M alleles and will always pass the merle gene to the offspring. A single merle has one merle (M) allele and one non-merle (m) allele which means the merle gene can, but may not be passed on the offspring.

When two double merles are bred to each other all offspring will be merle. When a double merle is bred to a single merle, at least 25 percent of the offspring will be double merle (MM).

Did you know? If two merles are bred together it may result in a white coat, but unfortunately the defects associated with the merle gene can be doubled.

Dog Breeds With the Merle Gene

Which breeds can have merle coats? Naturally, the merle allele responsible for merle coat patterns is present in the following dog breeds:

  • Australian Shepherd
  • Beauceron (popularly known as harlequin)
  • Bergamasco
  • Border Collie
  • Cardigan Welsh Corgi
  • Catahoula Leopard Dog
  • Collie (in both Rough and Smooth coated)
  • Dachshund (popularly known as dapple)
  •  Great Dane (popularly known as harlequin)
  • Hungarian Mudi
  • Norwegian Dunker
  • Pyrenean Shepherd
  • Shetland Sheepdog.

Recently, the merle allele has been bred in other breeds, including:

  • American Cocker Spaniel
  • American Staffordshire Terrier
  •  Chihuahua
  •  Pomeranian

Concluding Thoughts 

Considering the array of health issues and their effect on the dog’s overall life quality, it is important to prevent merle-related issues in dogs. That is best achieved through responsible breeding.

The possibility of hidden merle dogs breeding with merle or non-merle dogs make the inheritance calculation exceptionally hard and rather unpredictable. Therefore, DNA screening is highly recommended for all dogs intended for breeding. 

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