So, if you see a white collie with a predominantly white head,
chances are high that they have hearing and/or sight problems to some
extent. Which is why only the most studious of breeders should breed for
white collies, imho.
Actually, that isn't the whole truth. There are "white" collies, and then there are "white" collies. The problem whites you are describing are properly called homozygous, or double merle. They are also sometimes (unkindly) called lethal white or defective white. The double merles occur when two blue merle collies are bred. Some of the pups are tri (no merling), some are blue (tri + one merle gene) and some are doubles (tri + two merle genes). Double merles can also be produced when two sable merles are bred.
White is a whole other ball game. White is not linked to problems; merle (or rather the doubling of it) is. Not always, but enough to make breeders take pause. My dog is a double merle, she's a normal eyed possible noncarrier and can see and hear just fine. Her litter had two doubles in it, four tris and no blues! The color on her registration certificate is "white merle", but in researching her pedigree I've found a double merle that was registered as "white" and a tri-headed white that was registered as "white".
The easiest way to think of collie colors is that they can be two colors: black (tri) or brown (sable). They may have no merle, one merle, or two merle genes; they may have no white, one white, or two white genes. Brown is dominant over black: ie, a brown dog may carry the black gene.
To make things more complicated, a double merle may also be a white - if two blue-headed whites were bred. This would be awfully rare, though. In any case, there are ways to tell if a dog is a blue headed white or a double merle. The double will always have blue eyes; blue merles may have blue, brown, or mixed eyes. The double will usually have a predominantly white head; whites almost always have a fully colored head.
Black + one merle gene (no white) = blue merle.
Black + one merle gene + one white gene = white-factored blue.
Black + one merle gene + two white genes = blue headed white.
Black + two merle genes = double merle (sable merles are really uncommon)
Some examples of collie color:
Double merle (normal eyed)
Double merle (deaf dogs, aussies) - I'd say that all of these are double merles, some are not as obvious as others. Especially when you get mixes, like "Kip". Some, like "Missy" have a lot of color, but I'd still wager that they are double merles. Some of them have visual impairment or are totally blind. "Uma" is a double red merle - the result of breeding two red merles or a red merle to a blue merle.
White puppies - look at the dog on the very bottom, "Bandit". She is from a white x white breeding - the dad is a sable headed white and the mom is a blue headed white. If you didn't know her breeding, it would be possible to mistake her for a double merle at first glance. But, she has brown eyes.
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