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What is Merle Eye Anomaly?

A variety of coat and iris colors in dogs are the result of the Merle gene. A dilution gene, its presence can lighten the color of the dog’s coat, often creating a dappling effect as well as vary the eye’s colored part (iris). When it comes to the eye, a variety of colors can be seen as a result of the Merle gene, ranging from pale, light blue to green to amber. This same gene can also be responsible for developmental defects in the eye or eyes. The following breeds, among others, are known to have the Merle gene:

  • Australian Shepherd
  • Rough and Smooth Collies
  • Shetland Sheepdog
  • American Foxhound 
  • Catahoula Leopard Dog

The ocular effects of the gene can be minor, as in a blue iris, through severe as in complete blindness. The front or back part of the eye, or a combination of both can be impacted.

The Merle gene can lighten the color of a dog’s coat as well as lead to developmental defects in one or both of his eyes, ranging from minor to significant.

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Symptoms of Merle Eye Anomaly in Dogs

Dogs with the Merle gene (heterozygous or homozygous) can experience various symptoms with those considered double merles having the most severe eye abnormalities. These include:

  • Diluted or dappled coat
  • Varied color of the iris
  • A small eye (microphthalmia)
  • Pupil that is off-center (corectopia)
  • Cataracts
  • Blindness
  • Congenital hearing loss

It is found that double merles that have an excessive white coat particularly in the region of their head will experience the more significant abnormalities.


The Merle gene anomaly can be minor, as in the blue iris. The blue appearing in your dog’s eye can be a part of an eye that is otherwise brown and won’t impact his site. On the most severe end, the Merle gene anomaly can cause blindness. 

The front or back of your dog’s eye can be impacted; though the condition can impact both parts. Merle Ocular Dysgenesis is the name used when the entire eye is impacted.

Causes of Merle Eye Anomaly in Dogs

It is known that congenital anomalies in eyes can be inherited. In each dog, there are two copies of a gene; one from each of his parents. We will call the Merle gene “m” and the non-merle gene “M”. If your dog gets two copies of m, the gene for Merle, it is considered homozygous (mm) or double merle. A dog with two merle genes will be mainly white in color. If the dog inherits one Merle gene and one non-merle gene, he is considered heterozygous (Mm). The one Merle gene copy will dominate the non-Merle gene; the one copy will dilute your dog’s coat and lead to the possibility of different colored eyes. If your dog has no Merle genes, (MM) he will be a typical, full colored dog.

In Australian Shepherds, there are various eye abnormalities that result from the Merle gene that happen from an autosomal recessive trait, indicating that the condition is not sex-linked. A recessive trait is only expressed when homozygous, meaning that dogs with these abnormalities must be double Merle (mm). Double merle dogs may also experience different degrees of congenital hearing loss.

Diagnosis of Merle Eye Anomaly in Dogs

A diluted coat will tip off your veterinarian that your dog may have at least one Merle gene. When it comes to the Merle gene and ocular dysgenesis, any abnormalities that result will be present at birth, allowing for diagnosis at as young as six weeks of age. Your veterinarian will conduct a physical examination of your dog and may recommend that he be seen by a veterinary ophthalmologist, who can look for structural abnormalities in his eyes. 

Dog’s with one more two copies of the Merle gene may show changes in their iris (including thinning), a pupil that is off-center, iris coloboma (abnormal development of the iris that looks like a notch or cleft at the pupil’s edge) and the lens of his eye may be out of place. Persistent pupillary membranes or PPMs may be seen; these may be viewed as strands or sheets of tissue that start from your dog’s iris and connect to another part of the iris, lens or cornea. These can be minor or cause significant vision problems. Cataracts (focal or complete) can also occur as a result of the Merle gene. 

The back portion of your dog’s eye can also be impacted by the Merle gene. The sclera, which is the white of the eye, can be unusually thin (called scleral ectasia), leading to a staphyloma, which is a bulge that can be present in the front or back of your dog’s eye. The vascular layer at the back of your dog’s eye may not fully develop, call choroidal hypoplasia. Anomalies at the back of your dog’s eye can impact the optic nerve, in severe cases causing blindness. The development of the retina may also be impacted and the reflective layer at the back of your dog’s eye can be missing. 

The examination will determine whether there are eye abnormalities present in your dog and how severe they are. Your veterinarian will take those into account when considering whether your dog has at least one copy of the Merle gene.

Treatment of Merle Eye Anomaly in Dogs

Whether treatment is possible, and the type of treatment that is recommended, will be dependent upon the conditions that have resulted from the inheritance of the Merle gene. For some of the abnormalities that occur, there are no treatment options, while for others, treatment may be possible. 

Dogs that have the Merle gene should not be bred, even if they are only displaying a mild form of the condition as they can still pass the gene on and their offspring can experience it in a more severe form. Merles should never be bred with one another as this would lead to an increased chance of double merles.

Recovery of Merle Eye Anomaly in Dogs

Recovering from treatment and managing your dog’s condition will depend on the extent of the problems he is experiencing as a result of the Merle gene. It is important that you attend follow up appointments as recommended by your veterinarian so that changes in your dog’s condition can be monitored and treatment provided when possible.

Merle Eye Anomaly Questions and Advice from Veterinary Professionals

Great Dane
6 Months
Mild condition
0 found helpful
Mild condition

Has Symptoms

Red Bumps

My double Merle Great Dane is 6 months old. She was born deaf but seems to see just fine. She is prodiminantly white white some Merle patches. Eyes are extremely blue and get red. More importantly, she is almost constantly developing these little stye like pockets on and around her eye lids. Warm compress helps and they drain on their own but another one comes after. What does this mean? Been going on for 2-3 months.

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Whippet mix
2 Years
Moderate condition
0 found helpful
Moderate condition

I am considering adopting a Whippet with blue eyes and corectopia. Can this condition be corrected? Her pupils are small and dropped, and not in the center of the dogs iris. It doesn't seem to effect her vision, however it gives her a strange appearance.

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Border Collie
4 Months
Fair condition
0 found helpful
Fair condition

Has Symptoms

Marble Eye

I have a 4 month (almost 5 month) old Border Collie with blue eyes. Her left eye is starting to marble where half the eye is still blue but the top of her eye is turning Amber/Gold. Does this mean she will two different color eyes? Or will the one just stay marbled? I had her eyes checked with the vet for vision problems but the vet said her sight it fine. Could this cause issues as she gets older?

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bichon poo
4 Weeks
Mild condition
1 found helpful
Mild condition

Has Symptoms

Eye Redness

I was called today by the lady who has one of my puppies (not yet 8 weeks). She informed me that it looked like her eye had been poked out so she took her to the vet. The vet said due to her Merle gene (she is a bichon poodle with marbled color) she is blind in one eye. I have been researching about the Merle gene and about homozygous vs heterozygous merle gene dogs. In my
Case, will the puppy lead a healthy life
Even though she Is blind In one eye and has the merle gene (not
Sure if she is MM or Mm —-I know MM would have caused more issues)?

Dr. Callum Turner, DVM
Dr. Callum Turner, DVM
3320 Recommendations
Normally in these cases unilateral or bilateral loss of vision may occur, but in many cases a pup may be otherwise healthy; obviously shouldn’t be bred. Regular check ups and monitoring of general health is important. Regards Dr Callum Turner DVM

Hi. I'm currently waiting to get my cocker spaniel puppy in a week. He has a black/ brown eye and a light blue eye. Can a cocker spaniel have the gene?

Thank you!!!!!

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Australian Shepheed
5 Years
Moderate condition
0 found helpful
Moderate condition

My double Merle has had minimal vision since joining our family 4 years ago. Her right eye is normal sized the left eye considerably smaller both blue. Occasionally her right eye would turn green or yellow. We thought there might havebennsome trauma to the eye. Her vet ophthalmologist would prescribe prednisolone acetate drops and typically the eye would turn blue within two days.

Eventually we were instructed to give one drop to each eye daily. What now seems to have happened is there is active bleeding into each eye. She was put on a round of oral pred and also an injection of pred to her eye (or right above it).

The condition is worsening and no one can tell us what is happening or why. She appears to be totally blind now and she has blood just floating around in her eyes.

We were told to continue with the drops twice daily and come back in a month. What is happening?

Dr. Callum Turner, DVM
Dr. Callum Turner, DVM
3320 Recommendations
There are many causes for hyphema (blood in the anterior chamber of the eye) in dogs which congenital eye deformities are one possible cause; also secondary conditions to the eye deformity may also contribute to the symptoms. I cannot say specifically what the cause is or an appropriate treatment, you should discuss with your Ophthalmologist as they would be more experienced on this than me. Regards Dr Callum Turner DVM

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9 Weeks
Moderate condition
0 found helpful
Moderate condition

Has Symptoms


Can blindness be corrected in a 9 wk old meril Chihuahua..his pupils are fixed open and do not respond to light or dark. He seems to see something. Like large objects in a well lit room. Original owner did not get a diagnosis from a vet..

Dr. Michele King, DVM
Dr. Michele King, DVM
1611 Recommendations
Thank you for contacting us about Beasley. Unfortunately, the gene that controls the Merle coat color is also often tied to blindness or eye abnormalities, and deafness. It is unlikely that the condition can be corrected, no matter the age. There are veterinary ophthalmologists that would be able to do a thorough eye exam on him as they have specialized equipment to do so, and it might be a good idea to have him seen by one in your area, so that you know what kind of vision he has and how it might progress. In the meantime, make sure you keep him safe, as he is a dog that can't be left outside unattended. Blind dog typically do quite well in protected environments, as long as they know where things are, sometimes it isn't noticeable that they are blind as other systems tend to become more acute. I hope that he does well.

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