Collies, with their striking beauty and intelligent demeanor, are loyal, loving companions who capture many hearts. However, collies have a long list of potential eye conditions related to genes and breed that are exacerbated by the merle gene’s presence. Pets with one copy of the gene can have a beautiful, unique coat color, but those with two copies can suffer from vision and hearing deficits. The Veterinary Vision Center team explains how the merle gene impacts eye health in collies.

What is the merle gene in dogs?

The merle gene is a particular coat color gene that codes for unique merle or dapple coat colors in certain breeds, including collies, Australian shepherds, dachshunds, and many others. Dogs’ merle coats are highly variable in their pattern, color dilution, and color distribution with varying spots and swirls, because the gene’s behavior is difficult to predict. Although we can test to determine the merle gene’s presence, it can mutate in individuals, and a merle dog’s offspring may look completely different from the parent. 

Dogs with two copies of the merle gene are known as double merles. Breeders must carefully choose which dogs to breed together to ensure no double merles, because these dogs express the most extreme end of the merle spectrum, their coats contain little pigment, and they are born mostly white with multiple vision and hearing abnormalities.

Eye disorders in merle collies

Merle collies with only one gene copy, and collies with other coat colors, may be born with normal eyes and vision, or they may suffer from one of many genetic eye disorders common to the breed. Single merle dogs often have one blue eye or a blue section in a brown eye, which is not harmful. 

Double merle collies are more likely to suffer from eye health issues than the general collie population. The following conditions commonly occur in collies and increase in likelihood or severity if the collie carries merle genes:

  • Microphthalmia — Dogs with microphthalmia are born with abnormally small eyes and may suffer from underdeveloped internal eye structures. Vision may be normal, or these pets can be completely blind.
  • Persistent pupillary membrane (PPM) — PPMs are strands of tissue that bridge across eye structures and may affect vision or normal eye functions.
  • Cataracts — Affected dogs can be born with cataracts or lens opacities that prevent light and visual information from reaching the retina.
  • Iris abnormalities — The iris (i.e., the colored eye portion) may be underdeveloped, thin, or missing areas, causing abnormal pupil shapes.
  • Posterior segment abnormalities — The vascular cell layer, light-detecting retina, and optic nerve can be underdeveloped, resulting in varying degrees of vision impairment.

Double merle collies with eye disease typically have a mostly white coat and little to no pigment on their head or face fur. The same merle genes that affect eye health can also impact a dog’s hearing ability. Severely affected dogs may be born blind and deaf.

Other genetic eye disorders in collies

Collies are prone to several other genetic eye problems, whether or not they carry or express the merle gene. Common problems include:

  • Distichiasis — Distichiae are abnormal eyelashes that can rub on the cornea.
  • Corneal dystrophy — This condition causes cloudy white patches in the layers of the cornea (i.e., the normally clear window at the eye’s front). 
  • Nodular episclerokeratitis — This condition causes inflammation, discoloration, and lumps on the white portion of the eye (i.e., sclera) that may also involve the cornea.

Genetic testing in merle collies

Genetic testing is available to detect some of the collie’s most common genetic eye disorders, but, genetics are complicated. Many genes behind these disorders have not yet been identified, so testing cannot guarantee future eye health or the health of a particular dog’s offspring. The merle gene is complex and can vary in its expression. However, we know that dogs with two merle gene copies are at far higher risk of eye problems than those with only one copy, which genetic screening can help to identify. If you plan to acquire a collie from a breeder, ensure the parents have been thoroughly screened for genetic issues and receive frequent eye examinations.

Living with a vision-impaired merle collie

Most blind or partially blind collies were born with their eye abnormalities, and unlike sighted pets who lose vision later in life, they learn to effectively navigate their world using their other senses. Although you should take certain safety precautions, such as keeping your collie in a fenced area or on a leash, raising a blind dog is similar to raising a sighted one. Avoid moving furniture when possible, keep floors clear of debris, and let your pet map your home on their own four feet rather than lifting and carrying them, which can be disorienting.

Collies are loving, loyal, and overall healthy family dogs. Although genetic eye disorders and the merle gene plague the breed, genetic testing and responsible breeding can reduce these problems over time. If you own or breed collies, we recommend routine eye examinations and breeding stock screenings to catch problems in the early stages and avoid passing them on. Contact Veterinary Vision Center to schedule a complete eye examination for your collie puppy, adult, or senior.