Veterinarians can learn a lot about an animal just by looking at their coat and eye color. That is because certain inherited disorders are associated with these traits, which have been described in a recent review article published in the Canadian Veterinary Journal.
Take coat color-related sensorineural deafness for example. This inherited disorder has been commonly diagnosed in dogs, cats, horses, llamas and alpacas. Typically these animals have white, merle or dapple-patterning in their fur although that alone is not indicative of whether the animal will develop deafness. Animals that also have blue eyes or eyes with different colors (one blue eye and one green eye, for example) are at more risk for developing this type of inherited deafness.
Lethal white foal syndrome (photo above from greystonevet.com) is another example. These purely white foals are the offspring of frame overo horses (image below from article).
This is an autosomal recessive trait so both parents must carry the gene for this syndrome to develop. The lethal nature of the syndrome is due to the poorly developed intestinal tracts of these offspring leading to an inability to pass feces. Unfortunately, a successful treatment has not yet been developed so these foals are euthanized shortly after birth to prevent suffering.
Congenital Stationary Night Blindness
Congenital (i.e. inherited) Stationary Night Blindness affects humans as well as dogs, mice and horses. In Appaloosa horses the condition is related to coat color and is caused by a defective gene (transient receptor potential cation channel number 1, TRMP-1) in both the skin and retina resulting in both night blindness and a leopard-like coat pattern. Visit this website for more information on this disorder.
Lavender foal syndrome (AKA: coat color dilution) is a lethal disorder found in Arabian foals like the one pictured above (photo from: The Onderstepoort Veterinary Genetics Laboratory). The coat color of these foals appears a dilute lavender, silver, pink or pewter. These animals are euthanized shortly after birth since they are afflicted with a debilitating combination of extreme muscle spasms (opisthotonus), paddling movements of the limbs and rigid extended muscles, crossed eyes (strabismus), spontaneous rolling of the eyes (nystagmus), and an inability to stand upright after lying on their sides. Animals may also experience exaggerated spinal reflexes and responses to touch. This is an autosomal recessive trait that has been recently linked to a defect in the myosin 5A gene.
You have all probably seen a Siamese cat, like the one above shown in the article, with inherited strabismus or nystagmus. The Siamese coat color pattern is a form of albinism and is related to temperature such that the extremities (arms, legs, tail, face, tips of ears) have color whereas the body is poorly pigmented. As for the vision of Siamese and other albino cats, a misdirection of nerves leads to problems with how visual inputs are processed in the brain causing the animals to have poor stereoscopic vision and blindness near their noses. Other misguided nerves in the Siamese cats cause poor coordination of eye and head movements. This inherited disorder has been linked to mutations of the tyrosinase gene.
Multiple congenital ocular anomoly syndrome
This syndrome is yet another example of how coat color may signal a disorder in horses. It affects many different breeds and can manifest as cysts in the eyes as well as retinal detachment. As you can imagine, the horses suffer from varying degrees of abnormal vision. The disorder is commonly seen in horses with silver coat colors like the silver-colored Rocky Mountain horse shown above.
Since these disorders seem to be so common to horses, I can’t help but wonder if they arose from modern breeding practices.
AA Webb, CL Cullen. Coat color and coat color pattern-related neurologic and neuroophthalmic diseases. Can Vet J. 2010 June; 51(6): 653-657.