Double Merle Eyes: Understanding the Risks and Health Effects in Dogs

This pet health content was reviewed for accuracy by a veterinarian, Dr. Pippa Elliott, BVMS, MRCVS. It was last reviewed and updated on October 29, 2024

Understanding the Merle Gene in Dogs

The Merle gene is a pattern that dilutes a dog’s coat color, often lightening it and adding patches of white. However, its appearance can vary widely:

• Dark Coats: May appear bluish and often show blue coloring in the eyes.
• Light Coats: Can have patches or hints of pink, orange, or white, with eyes that may appear blue. Even the nose and paws may display color variations.

Some dogs with the merle gene may have “odd eyes,” with one blue and one brown eye—or even a combination of blue and brown within the same eye.

Dog Breeds Affected by the Merle Gene

Breeds commonly affected by the merle gene include: Australian Shepherds, Collies, and Sheepdogs Dachshunds, Pomeranians, and Chihuahuas

• Pit Bull Terriers, Cocker Spaniels, and Great Danes

For these breeds, the merle gene can lead to varied coat and eye colors, but it also brings potential health issues when presented as a double-merle gene.

Risks Associated with Double Merle Gene

A single merle gene may cause harmless changes like unique fur or eye color. However, when two merle gene carriers are bred together, their offspring may carry the double merle gene, which can lead to:

• Double Merle Eyes: Dogs with double merle eyes are often blind, deaf, or both.
• Physical Limitations: Some double-rule dogs may be born without eyes, paws, or other essential organs.

These health issues make it crucial for breeders and potential dog owners to understand the risks.

Common Eye Abnormalities Linked to Double Merle Eyes

Double-rule eyes often come with a range of abnormalities due to the genetic effects on pigmentation and eye structure. These abnormalities can significantly impair a dog’s vision or even lead to complete blindness. Common eye issues in dogs with double merle eyes include:

• Microphthalmia: An abnormality where the eyes are unusually small, often affecting sight.
• Coloboma: A structural defect in the eye that may cause gaps or holes, impacting vision.
• Retinal Dysplasia: Malformations in the retina that can lead to poor vision or blindness.

These eye abnormalities are prevalent among double-merle dogs, highlighting the importance of responsible breeding to avoid such risks.

How Double Merle Gene Affects Inner Ear Development

The double-merle gene doesn't just impact double-merle eyes; it can also disrupt the development of a dog’s inner ear. The same pigment cells responsible for eye color also influence the structure of the inner ear. When these cells are underdeveloped, the dog may experience hearing impairments or complete deafness.

• Inner Ear Malformations: Leads to partial or complete hearing loss.
• Cochlear Dysfunction: Affects how sound waves are processed, contributing to deafness in many double-merle dogs.

Dogs with the double merle gene are at high risk for congenital deafness, underscoring why pairing two merle carriers is discouraged.

Identifying Signs of Vision and Hearing Impairments in Double Merles

For dog owners or breeders, understanding the potential impairments associated with double-rule eyes is essential. Double-rule dogs may exhibit visible signs indicating blindness or deafness, which can be subtle or severe. Signs of Vision Impairment

• Difficulty navigating spaces or bumping into objects
• Disorientation in unfamiliar environments
• Lack of reaction to visual stimuli
• Signs of Hearing Impairment
• Unresponsiveness to sounds, including calling or clapping
• Startling when approached or touched unexpectedly
• Signs of confusion or difficulty locating sounds

Recognizing these signs early allows for better management and support for affected dogs.

Safe Breeding Practices to Prevent Double Merle Health Issues

Preventing the health issues associated with double merle eyes and other complications requires responsible breeding practices. Here are essential steps breeders can take to avoid producing double-merle dogs:

• Pair Merle Carriers Only With Non-Merle Dogs: This reduces the risk of double-merle genetics, lowering the chances of producing offspring with severe health issues.
• Genetic Testing: Ensure both breeding dogs are screened to confirm only one carries the merle gene.
• Educate Prospective Owners: Inform potential adopters or buyers about the risks of breeding two merle dogs, reinforcing why it should be avoided.

By following safe breeding practices, breeders can avoid creating double-merle dogs, which ultimately prevents the risk of blindness, deafness, and other physical impairments.

Responsible Breeding Practices for Merle Dogs

Some breeders aim for the blue-merle coat color by breeding a single merle carrier with a non-merle carrier, which lowers the risk of double merle-related defects. Here’s what to ask if you’re interested in a merle-coated dog:

• Inquire About Parent Genetics: Confirm that only one parent carries the merle gene.
• Understand the Risks of Double Merle Eyes: Ensure the breeder is informed about the gene to avoid potentially harmful pairings.

Avoid Breeding Two Merle Dogs Together

If you plan to breed a merle dog, it’s crucial to avoid pairing it with another merle carrier. While a colorful coat is desirable, the risks associated with double-merle eyes—including blindness and deafness—are substantial.

In other words, don’t breed two merle dogs. A unique coat is attractive, but it’s not worth compromising the dog’s health.

Frequently Asked Questions (FAQ)

References

• “Merle Eye Anomaly.” Wag Walking. https://wagwalking.com/condition/merle-eye-anomaly
• “Double Merle Eye Abnormalities.” Double Merles Info. https://www.doublemerles.info/eye-abnormalities
• “Double Merle Defects: Eye Abnormalities.” Australian Shepherd Health & Genetics Institute (ASHGI). https://www.ashgi.org/eyes-double-merle-defects