What color-associated health issues occur in Aussies?
Dogs that have two copies of the merle gene virtually always have abnormally developed eyes which frequently are blind. They may also be deaf due to lack of pigment in the inner ear.
Areas of white markings typically cover pink skin. If the skin is exposed (eye rims, lips, nose) or sparsely covered with hair (sometimes the top and end of the muzzle) there is risk of UV sun damage if the dog spends a lot of time in intense sunlight.
The type of deafness found in double merles can also occur in dogs that have white markings on or around the ears. The functional defect is the same in merles and non-merles (lack of pigment in the inner ear) but caused by different genes.
Dilute, a non-standard color that sometimes occurs in the breed, is associated with skin problems in some breeds. However, this does not appear to be a consistent problem in dilute Aussies.
Blue eyes are not more susceptible to eye defects and disease but they are more sensitive to bright light (just as they are in blue-eyed people.) There is no clear association of blue eyes to deafness in Aussies; deaf Aussies virtually always have too much white on the head or are double merle.
Why is the merle gene sometimes lethal?
It isn’t. The myth arose from some very early research that indicated that there might be very serious health issues in double merles other than with the eyes and ears defects we are familiar with. This has proven not to be the case.
Will double merles have health problems other than being blind or deaf?
The only health issue other than those of the eyes and ears is a risk for sun damage to pink areas of skin if the dog spends a lot of time in sunlight. Some very early studies of double merles indicated that they might have or develop a wide range of health issues, however there is no definitive research supporting those early suppositions and anecdotal evidence from owners of double merle Aussies does not support it, either.
What kind of eye defects do double merles have?
Dogs that inherit two copies of the merle gene (“double merles”), usually have multiple eye defects, generally with associated vision loss which often goes to the point of blindness. Any part of the eye can be affected. Irises are frequently deformed, sometimes grossly so. Pupils may be subluxated (off-center). The lens may be subluxated (out of place). The retina may be abnormal, and the optic nerve may be improperly developed. The entire globe of the eye may be abnormally small (microphthalmia). Most double merles will have one or combination of these defects in each eye. All of this can be avoided by not breeding merle dogs to each other.
Are eye problems common in Aussies because we have so much merle?
Dogs with two copies of the merle gene usually have multiple and significant problems with the eyes. Dogs with only one or no merle genes will not have the eye problems typical of a double merle.
Aussies do have a number of eye diseases, but these are independently inherited and can be found in any color dog. Even though some of these bear some resemblance to the problems seen in double merles, they are genetically distinct. Puppies who have only one merle parent and normal merles from two merle parents are be no more likely to have eye problems than any other dog.
Are dogs out of many generations of merle to merle breeding more likely to have or throw eye problems?
No. The number of generations of merle to merle breeding has no bearing on whether or not a dog will have or carry eye problems. Except for the problems that occur in double merles, the fact a dog is merle is only a coincidence if it happens to have other eye diseases. The only exception is iris coloboma, which is much more frequent in merles but still occurs in non-merles, too.
Does breeding merle-to-merle increase the possibility of deafness in the breed?
Only in the puppies that inherit two copies of the merle gene. This can be avoided by not breeding merles to each other.
Is it safer to breed two blue merles together than two red merles?
The problem with merles isn’t whether they are blue or red but in the breeding of any two merles to each other. The base coat color doesn’t matter.