Why do some dogs have different colored eyes
Why do some dogs have different colored eyes? The answer lies in the merle gene, which suppresses pigment in the iris, but other factors like genetics and age also play a role.
Dogs with different colored eyes, a condition called heterochromia, most often have the merle gene, which suppresses pigment in the iris, though other genetic factors and age-related changes can also cause it. This phenomenon is not a sign of illness in most cases, but a direct result of how specific alleles modify melanin production during development.
The merle gene is the primary cause
The merle pattern, controlled by alleles of the PMEL gene, creates mottled patches of color in a coat and frequently results in blue or odd-colored eyes. The allele modifies dark pigment, causing parts of the iris to turn blue or creating a split appearance where one eye is blue and the other is brown, or both are different shades. This happens because the gene suppresses melanocytes, the cells responsible for pigment, in the iris and inner ear during development.
Dogs with the merle allele can have dark-eyed, blue-eyed, or odd-colored eyes depending on the random nature of the modification. While the pattern is most visible in breeds like the Australian Shepherd, Catahoula Leopard Dog, and Cardigan Welsh Corgi, it can appear in others like the Shetland Sheepdog and Miniature American Shepherd. In Dachshunds, this pattern is known as "dapple," and in Great Danes, it interacts with a modifier gene to create the harlequin pattern.
Genetic combinations create variations
The severity of eye color changes depends on how many copies of the merle allele a dog carries. Standard merle dogs are heterozygous, meaning they have one copy of the allele, and they may have one blue eye, two blue eyes, or one of each. When two merle dogs are bred together, approximately one quarter of the puppies are born with two copies of the allele, known as double merles or homozygous merles.
Double merles often have more extensive white markings and a higher probability of having eye defects or being deaf. The suppression of melanocytes is more profound in these dogs, sometimes resulting in entirely blue eyes or a complete lack of pigment in the iris. Because of the associated health risks, including auditory and visual impairments, responsible breeders avoid mating two merle-patterned dogs.
Other genetic and non-genetic factors
Not all eye color differences stem from the merle gene. Some dogs may have a "cryptic merle," where the patches are so small they are invisible on the coat but still affect the eyes. These dogs can pass on the allele to produce merle offspring, including double merles, if bred to another carrier. Additionally, certain modifying genes can alter the expression of the merle pattern, creating unique eye colors that might be mistaken for double merle traits.
Age can also play a role in eye color changes. Some dogs, particularly those with blue eyes, may develop a brownish tint as they age, though the underlying genetic cause remains the same. In rare cases, a condition known as "blue-eyed merle" can result in a dog with completely blue eyes and a high likelihood of hearing loss, a condition that mirrors Waardenburg syndrome in humans.
Health implications and testing
While heterochromia itself is often just a cosmetic trait, it can be a marker for underlying health issues, particularly in double merles. Studies have shown that dogs with the merle genotype are disproportionately predisposed to hearing and vision impairments. A 1977 study in dachshunds found that 36.8% of merle dogs and 54.6% of double merles had mild to severe deafness, compared to none in non-merle groups.
Genetic testing can identify the merle allele and its specific variations, helping breeders make informed decisions. The UK Kennel Club stopped registering puppies from merle-to-merle matings in 2013 due to these health risks. For owners, understanding the genetic background of their dog can provide insight into potential health needs, ensuring that any vision or hearing changes are addressed promptly.
When to consult a vet
If a dog's eye color changes suddenly or is accompanied by other symptoms like cloudiness, redness, or vision loss, a veterinary examination is necessary. While heterochromia is usually a stable genetic trait, sudden changes can indicate underlying health problems such as uveitis or glaucoma. Regular check-ups are essential for dogs with known genetic predispositions to ensure their long-term well-being.