Why Are Some Merle Dogs Deaf or Blind?

By Dr Duncan Houston

Merle coats can be striking, with patches of darker colour scattered through lighter areas. But the genetics behind that pattern can sometimes affect more than a dog’s appearance.

The greatest concern is with double-merle dogs, which inherit a merle-associated variant from each parent.

Many merle dogs have normal hearing and vision. Understanding the difference helps owners make informed decisions without assuming that every spotted coat comes with a disability.

Quick answer

Merle-associated genetic changes affect pigmentation, and pigment cells also have important roles in the inner ear and developing eye. In affected dogs, problems with these cells can contribute to permanent hearing loss, while abnormal eye development can impair vision. The risk is greater with certain double-merle genetic combinations, but not every merle or double-merle dog is deaf or blind. Hearing tests and an eye examination assess the individual dog more reliably than coat colour.

What does “merle” actually mean?

Merle is a coat pattern, not a breed. It creates irregular areas of diluted colour alongside more strongly pigmented patches. In Dachshunds, the pattern is commonly called dapple.

Merle is associated with a change in the PMEL gene, which is involved in the structures where pigment is produced and deposited within cells.

The genetics are more complex than a simple on-or-off switch. Different lengths of the merle-associated DNA insertion can produce different appearances, and some dogs carry a variant without an obvious merle coat.

This is why appearance alone cannot reliably establish a dog’s genetic status.

Why can pigmentation affect hearing?

Pigment-producing cells, called melanocytes, have functions beyond giving hair and skin their colour.

Some are found in a specialised part of the inner ear called the stria vascularis. These cells help maintain the chemical and electrical conditions needed for hearing.

In pigment-associated inherited deafness, melanocytes may be absent or fail to function normally. The inner ear can then lose the environment needed to support its delicate sound-detecting hair cells. Those cells degenerate, causing permanent hearing loss, often within the first few weeks of life.

The pale coat itself is not damaging the ears. The coat pattern and hearing problem can reflect related effects of the underlying genetics.

Hearing loss may affect one ear or both. A dog hearing through one ear can still respond to sounds, making the problem difficult to recognise at home.

Why can merle genetics affect vision?

The eye problem involves how the eye develops before birth.

Affected dogs can have merle ocular dysgenesis, a group of developmental abnormalities. These may include:

• Microphthalmia: one or both eyes are unusually small.

• Iris coloboma or underdevelopment: part of the coloured iris has not formed normally.

• Lens abnormalities: including cataracts or an abnormally positioned lens.

• Retinal abnormalities: the light-sensitive tissue at the back of the eye may be malformed or detached.

• Optic nerve abnormalities: affecting the pathway carrying visual information to the brain.

The effect depends on which structures are involved and how severely they are affected. An iris defect does not necessarily mean blindness, while a major retinal or optic nerve abnormality can seriously limit vision.

A blue iris alone is not the same as a malformed eye.

What is the difference between merle and double merle?

In a simplified example, a single-merle dog carries one conventional merle-associated variant and one non-merle copy. A double-merle dog inherits a merle-associated variant from both parents.

Double-merle dogs often have more extensive white areas, but their appearance varies. The likelihood and severity of hearing and eye problems also vary with the particular variants and other genetic factors.

A useful comparison is:

• Single merle: many dogs have normal hearing and eye development, although a normal-looking coat does not guarantee health.

• Double merle: greater concern for congenital hearing loss and developmental eye abnormalities, particularly with classic merle combinations.

• Either pattern: individual testing is needed to establish hearing, vision, and eye health.

A dog can have hearing impairment without significant vision impairment, or the reverse. Some have both.

Why is breeding merle to merle risky?

For the simplified mating of two dogs each carrying one classic merle variant, written as M/m × M/m, the expected outcomes for each puppy are:

Puppy’s genotype	Expected chance	Meaning
m/m	25%	Inherits neither parent’s merle variant
M/m	50%	Inherits one merle variant
M/M	25%	Inherits a merle variant from both parents

These are probabilities for each puppy, not a promise that every litter will contain those exact proportions.

The 25% figure refers to inheriting the double-merle genotype. It does not mean exactly 25% of the puppies will be deaf or blind. The resulting health effects vary.

The practical prevention message is to avoid merle-to-merle pairings. Normal hearing and vision in both parents do not eliminate the genetic risk to their puppies.

Can a dog carry merle without looking merle?

Yes. Some variants produce little visible pattern, and other coat-colour genes can mask its appearance. Mosaicism, where different groups of cells carry different versions of the merle insertion, adds further complexity.

The term cryptic merle is sometimes used for dogs with certain variants and little or no visible pattern. However, not every hidden or subtle pattern has the same genetics or risk.

For breeding decisions, use appropriate DNA testing and have the results interpreted by a veterinarian or genetics laboratory. Do not assume that a solid-looking partner is genetically non-merle.

Does a white coat or blue eye prove a dog is affected?

No. White coats and blue eyes can arise through different genetic pathways.

A dog with one blue eye can hear and see normally. A dog with brown eyes may have hearing loss. Likewise, extensive white markings do not prove that a dog is double merle.

Your vet needs to distinguish three separate questions:

1. What genetic variants does the dog carry?

2. Can each ear hear?

3. Are the eyes structurally healthy, and what vision is present?

A DNA result, hearing test, and eye examination provide different information. None should be treated as a complete substitute for the others.

How do vets check hearing and vision?

BAER testing for hearing

BAER stands for brainstem auditory evoked response. It measures electrical responses in the hearing pathway after sound is delivered to an ear.

Each ear is tested separately, allowing identification of dogs that hear through only one ear as well as dogs that are deaf in both.

Clapping, calling a name, or dropping an object cannot reliably provide this information. A dog may respond through the hearing ear or react to movement or vibration.

A veterinary eye examination

A veterinary ophthalmologist can assess the size and development of the eyes, examine the iris and lens, and look at the retina and optic nerve. Additional testing may be needed when internal structures cannot be seen clearly.

Some abnormalities are visible from the outside; others require specialist equipment. Early assessment helps establish useful vision, identify complications, and plan ongoing care.

Genetic testing

Merle testing helps clarify the variants a dog carries. It is particularly useful when the coat pattern is unclear or breeding is being considered.

It does not measure hearing or vision. A genetic result should be interpreted alongside clinical testing.

What should I do if I have a merle puppy?

Start with a veterinary health examination and discuss whether BAER testing and specialist eye screening are appropriate. This is particularly important if double merle is suspected, either parent’s genetics are uncertain, or you have noticed unusual eye development or responses to sound.

Bring any available:

• Parental genetic test results.

• The puppy’s own DNA results.

• Hearing or eye-screening reports.

• Information about apparent changes in behaviour or navigation.

Do not assume a puppy is simply stubborn if they consistently fail to respond to sound. Likewise, bumping into objects or struggling to follow movement deserves assessment.

Keep the focus on identifying what the puppy can do and what support they need.

When should I seek urgent veterinary help?

A congenital disability does not explain every new problem.

Seek urgent veterinary care for sudden vision loss, a painful or tightly closed eye, marked redness, new cloudiness, or a bulging eye. Conditions such as glaucoma can threaten sight and require immediate treatment.

Arrange prompt assessment for new hearing changes, ear discomfort, discharge, or difficulty with balance. These should not automatically be attributed to merle genetics.

For a comfortable puppy with longstanding suspected hearing or vision impairment, arrange testing rather than waiting for the problem to become more obvious.

Can merle-related deafness or blindness be treated?

Inherited sensorineural deafness caused by degeneration of the inner ear is generally permanent. There is no ear drop or supplement that restores those lost sensory cells.

Eye treatment depends on the abnormality. Some developmental defects cannot be corrected, but associated problems such as cataracts may be treatable. Eye discomfort or other complications still need veterinary care.

The goals are to preserve useful vision where possible, maintain comfort, and help the dog function safely.

Can a deaf or blind dog have a good life?

Yes. Many dogs with sensory impairments can learn, play, form close relationships, and participate in suitable activities. A study comparing dogs with and without congenital sensory impairments documented training, communication, and activity participation among affected dogs. It does not mean every dog has identical needs.

Practical support can include:

• Reward-based visual cues for a deaf dog who can see.

• Carefully taught touch cues when both hearing and vision are limited.

• Secure fencing and lead use where a dog cannot detect hazards.

• Consistent furniture placement and barriers around stairs or pools.

• Predictable routines, scent-based activities, and gentle introductions.

• Veterinary rechecks to identify discomfort or changing eye health.

Assess quality of life through comfort, enjoyment, relationships, and daily functioning. Deafness or blindness alone does not establish that a dog is suffering.

Common mistakes to avoid

• Assuming all merle dogs are deaf or blind.

• Assuming two healthy merle parents cannot produce affected puppies.

• Using coat colour instead of genetic testing when planning a mating.

• Treating a response to clapping as proof that both ears hear.

• Dismissing a new painful eye as part of an existing disability.

Frequently asked questions

Are single-merle dogs always healthy?

No dog’s health can be guaranteed from its coat pattern. Many single-merle dogs hear and see normally, but inherited eye conditions, hearing problems, and unrelated diseases can still occur.

Does every double-merle dog become completely deaf and blind?

No. Hearing and vision outcomes vary, and impairment may affect one or both sides. Clinical testing is needed to assess the individual dog.

Is a blue-merle dog more at risk than a red-merle dog?

The colour label alone is not enough to determine risk. The merle variants inherited, whether variants came from both parents, and other genetic factors matter more than the marketing name of the coat.

Will a healthy merle dog inevitably lose hearing or vision later?

No. Merle colour does not mean inevitable progressive deafness or blindness. However, some developmental eye abnormalities can lead to later complications, and unrelated age-related or acquired diseases can occur. Follow the monitoring plan recommended for your dog.

The genetic risk is important when planning breeding. For a dog already in your care, the next step is individual assessment and appropriate support, rather than assumptions based on appearance.

If you are unsure what your dog’s colouring or test results mean, ASK A VET™ can help you identify useful questions for your veterinarian. Hearing testing and an eye examination can guide a care plan suited to your dog’s abilities and needs.