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.
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First, unfortunately, for your "list of websites with more information on coat color related disorders", the "click here" goes to an Arizona State login -- for those of us not at Arizona State, not much use, I'm afraid.
Second -- does the "silver-colored Rocky Mountain horse shown above" referred to in the paragraph on multiple congenital ocular anomoly syndrome, actually refer to the foal in the lethal white foal syndrome picture? If not, then I think you may be lacking another picture.
Interestingly, had run into the lethal white foal syndrome before, but did not know about the night blindness associated with the appy leopard coat. I do actually want to know more, these things fascinate me.
It's fascinating how certain things can be so linked. Although I do suspect, as you suggested, that modern breeding practices have something to do with it: take the case of a condition called "stargazing" in captive-bred corn snake populations. There was a time when it was thought to be linked to the genes for a particular (very pretty) colour morph known as "sunkissed". But in truth, it was merely coincidental that this disorder first arose in sunkissed-morph corns (and it has been traced to one particular breeder's captive populations), and it's modern breeding practices that have caused it to become somewhat epidemic in the captive-bred world (fortunately this is not a condition that is thought to cause any suffering or pain to the affected individuals, although it does make them virtually worthless to breeders). Breeding for selective traits requires inbreeding, which also can unintentionally select for undesired and undesirable traits. The same sort of thing can happen in the wild, of course -- but it generally requires a population bottleneck of some sort, where a group of individuals become cut off from others of their species (or become the only ones left of their species) and are forced to interbreed more closely than they would normally choose to do.
There are also homozygous conditions colloquially called lethal white in dogs and guinea-pigs, although both have a better chance of survival than a LW foal. In dogs, it's caused by the merle white-pattern, and in cavies by roan/dalmation. Neither is similar genetically to LW in horses.
Certainly all three examples of LW are homozygous for genes that would carry a significant selection disadvantage in the wild, even a single copy. There was (at least in the UK) a strong traditional prejudice against white-marked horses for the very reason that messing with pigment expression has unexpected effects: I'm only in my twenties, but I can remember being taught old wives tales that may or may not have had a grain of truth in them associating white markings and blue eyes with poor vision, deafness, and general poor health.
However, lavender foal syndrome is fully recessive: prior to the genetic test (which has only become available in the last year or two) the only way to tell if your lines carried it was test breeding or family history. Although it is more associated with some lines than with others, it can turn up almost anywhere in arab or arab-derived breeds. Unlike, say, HYPP (a de novo mutation in a very successful sire), it hasn't been linked to any historic carriers yet, and it does occasionally turn up in desert-bred stock as well.
I suspect the horse examples are more prominent and better researched because of the relatively high cost to the breeder, and the relatively high economic value of horses. In the particular case of guinea-pigs, breeding roan-to-roan (or roan-to-dalmation) has been known to result in 1/4 LW offspring for a long time now, but as far as I'm aware no-one has isolated the gene or offers testing services; sadly, also, some irresponsible breeders deliberately perform the cross either because it reduces the number of solid-coloured culls that need to be found pet homes, or because they can sell any surviving LWs as a novelty.
Sorry if this is too full of blather: I need more coffee.