What is a merle coat, and why is it sometimes considered to be a problem for dogs?

Merle is the name given to a certain specific coat pattern that can be found in dogs, rather than a particular colour. Most people who are familiar with the term “merle” think of blue merle, which is the most common expression of the pattern, but it can also be chocolate or red too.

Merle coats can appear in dogs that inherit a certain combination of genes from their two parent dogs, and you can find out if any dog (regardless of their coat pattern) carries a copy of the merle gene by means of DNA testing. However, the DNA test will only indicate the markers for the merle pattern, and won’t tell you what colours this might be expressed in.

The merle gene leads to a mottling pattern across the coat, which is reflected in the skin pigmentation too – and can also lead to blue eyes, or one blue eye.

Dogs with merle coats are often very beautiful, and the distinctive pattern that this creates within the coat can be highly appealing to potential puppy buyers. However, the gene that leads to the merle pattern does more than simply dictate the pattern of the coat itself – it can also lead to a range of potential congenital health issues in affected dogs, depending on the gene combination they inherit.

In this article we will look at the merle coat pattern in dogs in more detail, explaining how this pattern occurs, and why it can lead to health issues – and how. Read on to learn more.

What does a merle coat look like?

The gene that causes the merle coat creates patches of mottled colour in a coat that is otherwise piebald or solid, which extends to the colour and pattern of the skin underneath the fur too.

This means that a dog with a merle coat may have patches of solid colour or piebald markings interspersed with patches of merle, or they may be merle over most of their body.

Merle patterning can be found in cots of all colours, and expresses in shades of either blue, brown or black. The merle gene serves as a modifier that changes the basic colour and pattern of the dog’s coat, as well as the skin on their nose and the pads of their paws. It also modifies the eyes, particularly darker eye colours, which can lead to one or both eyes being blue.

How does a merle coat occur?

The origins of the merle coat colour have been narrowed down to a specific gene, which can be identified with DNA testing. This is a gene mutation, and merle is actually a heterozygote of an incomplete dominant gene, which is what dictates the end pattern of any dog who inherits the gene.

Dogs that inherit just one copy of the gene will have a merle coat – whilst dogs that inherit two copies will be what is known as “double merle,” because they are homozygous rather than heterozygous.

Double-merle dogs run a much greater risk of inheriting health problems along with the merle coat, including ocular defects and congenital deafness. Single-merle dogs, or those that are heterozygous, are less likely to be affected by such problems.

Another variant of this coat type is known as cryptic merle, in which only small patches of the merle pattern appear. Additionally, other genes that serve as modifiers can create a range of other merle patterns such as harlequin merle, which is commonly found in the Great Dane dog breed.

What dog breeds can have a merle coat?

The merle coat is recognised within the breed standard for certain dog breeds – despite the fact that it can be undesirable or even mean that puppies cannot be registered with The Kennel Club in others.

Breeds within which merle coats are commonly found and accepted include harlequin Great Danes, as mentioned above, and the Shetland sheepdog, the Dachshund (in which the colour is known as “dapple”) and the Australian shepherd dog.

However, in some dog breeds the merle coat is seen as a fault, which is either simply undesirable, or that may be forbidden within pedigree registered dogs. This means that a puppy with a merle coat that comes from a breed within which the colour is not permitted will not be eligible for Kennel Club registration.

This is because of the increased risk of health problems that occur in double merle dogs in particular, and the fact that breeding merle dogs within breeding programs is apt to increase the number of merles in the gene pool and so, increase the chances of two merle dogs being bred and passing on the double merle genes that can lead to problems.

Health issues associated with the merle coat gene

Dogs that inherit the merle gene have increased risk factors for congenital deafness in one or both ears, although this is not prevalent across single-merle dogs as a whole.

Dogs with the double merle gene may also inherit a range of other traits that occur due to the genes that lead to the merle colouration. Congenital deafness is one of these, and ocular defects or problems with the eyes is another. Improperly developed eyes and blindness often accompany the blue-eyed trait in double merle dogs.

Recent research has identified the combination of double merle with piebald colouring as the most likely pattern and colour to inherit health issues.

If you are considering buying a dog or puppy with merle patterning or merle parentage, it is important to find out first if this colour is permitted within the breed standard – and whether the dog in question is a single or double merle.

Find out about the potential problems that the merle gene can cause – particularly in the case of double merle dogs – for the dog in question. Additionally, remember that if you intend to breed from a merle dog, you will need to ensure that they are mated with a dog that doesn’t carry the merle gene, to avoid producing double-merle puppies.