The condition where a dog has different colored eyes is called heterochromia. Derived from Greek words meaning “different color,” this term describes a variation in the pigmentation of the iris. Heterochromia is relatively common in dogs, often resulting in a striking appearance where one eye is a deep brown and the other a bright blue. This difference in eye color is generally a congenital trait, meaning the dog is born with the condition.
Classifying the Different Types
Heterochromia is classified into distinct types based on the visual pattern of the iris. Complete heterochromia, also known as heterochromia iridum, is the most recognizable form. In this type, each eye is a single, solid, and entirely different color from the other (e.g., one fully blue eye and one fully brown eye).
Sectoral heterochromia occurs when only a segment of one iris displays a different color than the rest of that eye. This results in a marbled or split appearance, such as a patch of blue within an otherwise brown iris. This variation can manifest in one or both eyes.
The third form is central heterochromia, which involves a different color radiating outward in a ring around the pupil. This often creates a halo effect, where the inner ring of the iris is one color and the outer area is another.
The Genetic Explanation for Uneven Color
The underlying cause of heterochromia is an uneven distribution or absence of the pigment melanin within the iris. Melanin concentration determines eye color. Dogs with high amounts of pigment in the stroma will have brown eyes, while those with no melanin in that area will have blue eyes. In heterochromia, one eye or a section of an eye lacks melanin, resulting in the lighter, blue coloration.
The condition is primarily hereditary, linked to specific genes that influence the placement of pigment-producing cells (melanocytes) during development. The Merle gene, which creates marbled coat patterns, is strongly associated with heterochromia because it disrupts the normal migration of melanocytes to the developing eye. Similarly, the Piebald gene, responsible for large white patches on the coat, is also linked to this variation. A known genetic factor is a 98.6-kb DNA duplication specifically associated with blue eyes and heterochromia in some breeds.
Breeds Commonly Affected and Health Status
While heterochromia can appear in any dog, it is more frequent in certain breeds that carry associated coat color genes. The Siberian Husky is widely known for the condition, often displaying complete heterochromia with one blue and one brown eye. Other commonly affected breeds include the Australian Shepherd, Border Collie, Australian Cattle Dog, and Dalmatian.
Hereditary heterochromia is usually considered a cosmetic trait and does not affect the dog’s vision or overall eye health. However, the genes causing this condition are rarely associated with a higher risk of deafness in certain breeds. Dalmatians with heterochromia, especially those with sectoral variation, may have an increased chance of being deaf due to the genes responsible for their white coat and pigment distribution. If the eye color difference is acquired suddenly later in life, rather than being present from birth, it may signal an underlying health issue like injury or inflammation and warrants a veterinary examination.