Why do some dogs have blue eyes
Why do some dogs have blue eyes? The answer lies in the merle gene, which suppresses pigment in the iris, but the risks of double-merle breeding are significant.
Blue eyes in dogs are primarily caused by the merle gene, which suppresses pigment in the iris, though rare genetic mutations can also cause the effect. This is not a sign of albinism or poor health in single-merle dogs, but it does signal a specific genetic mechanism that can lead to serious defects if two carriers are bred together.
The merle gene suppresses pigment
The merle allele, an incompletely dominant gene linked to the PMEL gene, creates mottled patches of color in a solid coat and simultaneously modifies eye color. It works by randomly suppressing melanocytes, the cells responsible for dark pigment, which results in eyes that are blue, odd-colored, or a mix of both. This same mechanism affects the nose and paw pads, often leaving them mottled with pink rather than solid black or brown.
While the term "blue" is used for the coat color in black-based merles, the genetic effect on the eyes is a lack of dark pigment rather than a blue dye. Dogs with recessive red coats can carry the merle gene, but the mottling may be so faint it is invisible, or the eyes may still turn blue depending on the specific allele length.
Double merles carry high health risks
When two heterozygous merle dogs are bred, approximately one quarter of the puppies will inherit two copies of the merle allele (homozygous MM), a condition often called "double merle" or "lethal white." These dogs have a significantly higher probability of hearing and vision impairments because the suppression of melanocytes affects the inner ear and the development of the iris more severely than in single-merle dogs.
Studies indicate that a high percentage of double merles are deaf or have eye defects such as microphthalmia, colobomas, or increased ocular pressure. A 1977 study in dachshunds found that 54.6% of double merles had mild to severe deafness, compared to 36.8% of single merles. More recent data suggests that while single merles can have blue eyes without issues, double merles face a much steeper risk of being completely deaf or blind.
Specific breeds and the "dapple" pattern
The merle pattern is a distinguishing marking in several breeds, including the Australian Shepherd, Catahoula Leopard Dog, and Australian Cattle Dog. In Dachshunds, this pattern is known as "dapple," while in Great Danes, a specific modifier gene creates the harlequin pattern, which is distinct from standard merle but still relies on the presence of the merle allele.
Breed registries vary on how they treat the merle gene. The UK Kennel Club banned the registration of merle poodles in 2020, and the Pembroke Welsh Corgi Club of America states that merle is not a naturally occurring or accepted color for their breed. In some breeds, the pattern is registerable but disqualifies a dog from conformation shows if it does not meet specific eye color requirements.
When blue eyes indicate a defect
While a single merle dog with blue eyes is generally healthy, the presence of blue eyes combined with excess white on the head is a strong indicator for deafness in breeds like the Border Collie. In double merles, the risk extends to ocular defects that can cause blindness, though no studies have definitively proven that the merle gene alone causes total blindness in all cases.
Breeders avoid mating two merle dogs specifically to prevent the production of double merles, a practice supported by the UK Royal Kennel Club's 2013 ban on registering puppies from such matings. Genetic testing can identify the specific length of the SINE insertion within the merle allele, helping breeders determine if a dog is a cryptic merle or a carrier before breeding.
Genetic testing clarifies the cause
Because the merle allele is a mobile genetic unit involving a retrotransposon, the specific mutation can vary. Scientists have identified six general categories of merle alleles that affect the final coat color depending on the length of the insertion. This variability explains why some dogs appear to be non-merle but carry the gene, producing merle puppies when bred to a known carrier.
Testing is recommended whenever a breeder is unsure if a dog is a cryptic merle, as these dogs can produce double merles or cryptic merles depending on the parents' genotypes. Understanding the specific allele is crucial for predicting not just coat color, but the potential for auditory and visual deficits in the litter.