Many breeds of dogs have brown, hazel or even yellow eyes, but there are also some breeds that are more prone to blue eyes, commonly referred to as glass eyes. The breeds most commonly associated with glass eyes include Australian Shepherds, Blue Heelers, Alapaha Blue Blood Bulldogs, Catahoulas, Siberian Huskies, Corgies, Dachshunds and Shetland Sheepdogs. Any other dog breed could potentially have a blue eye or glass eye, but in many breeds the rare blue color is considered a disqualification in the show ring, but does not mean that the dog cannot be registered. Glass eyes do not indicate damage to the eye and there is no indication that the dog is not able to see equally well through any colored eye, despite many myths to the contrary. There is no breed standard that only allows for blue eyes, but it is favored in many of the breeds by breeders looking for flash in their breeding line.

Glass eyes are entirely a genetic component and are most typically associated with the merle coloration. Merle coats can be either blue or red with patches of a darker blue or red over lighter colors, often gray, with white and black areas also noticed in the coat. This coat combination is a double recessive gene that means that both parents must possess the merle gene to produce merle puppies. In some cases the parent may be brown eyed, solid colored or parti-colored dogs that don't appear to have the merle coloration. There is some concern in some breeds such as the Great Dane that two merles should not be bred together as the resulting litter has a far greater chance of serious genetic conditions such as blindness, deafness and neurological disorders. The Great Dane breed standard disqualifies merle coat colors for this reason.

It is important to note that not all blue eyed or glass eyed dogs are merle. The Siberian Husky blue eye is actually dominant genetics rather than recessive, meaning that puppies are much more likely to have blue eyes, but they can also have brown eyes or what are known as parti-eyes which are blue and brown on the same iris. Very often Huskies, as well as other breeds known for blue eyes, will have one blue eye and one brown eye, which is known as bi-eyed.

The blue eye pattern can lead to some very interested color combinations and markings. Depending on the breed the color markings in the eyes will be called by different terms. In the Catahoula Leopard Dog a true glass eye is all blue but may have light and dark patches within the iris, but it cannot be completely white. A glass eye with larger spots of brown, very dark blue, black or any other eye color such as green or amber is known as a marble eye. The same color combinations with small spots of color throughout the iris is known as a flecked eye.

It is important to realize that many breeds of dogs will have puppies that are born with blue eyes, but then the eyes darken to brown shortly after the puppy's eyes open and the puppy matures. Glass eyes are blue all through the dog's life, although there may be some darkening and lightening or even addition of flecks through the iris as the dog matures.