Originally Posted by Spiritwind
Honestly I think "lethal" white was a name given to the white offspring of a merle x merle breeding. Years ago they would just automaticly put the solid white pups down, thinking they were sickly (other than being just deaf or blind).... and I guess on occasion you might get other issues that pop up with them.... but I personally prefer the term Double Merle or Double Dilute, I think it describes the dog and its color better.
Also, not all double merles are deaf or blind... some can both see and hear just fine.
These "lethal" whites are geneticly NOT white.... they are a result of doubling up on the merle (a dilute gene) gene. They are not white, they are merles (In the smooth Collies case pictured above and probably also in the danes case - Blue Merles geneticly)...
Merle is an incomplete dominat dilution gene... which is how you get the silvery/grey coat (for blue merles) mixed with the black patches... a blue merle is a diluted black dog... a red merle is a diluted red dog etc.. ie. You breed Tri-color Collie to Blue Merle Collie = Blue and Tri pups, but the blue merles are the result of the merle gene mixed with the tri.... it dilutes the tri into that silvery color in spots.. in other areas you get the black patches where it did not dilute the color.
When you breed Merle to Merle you double up on the dilution gene which dilutes the color even more and gives you a dog who appears white... this also causes the possible vision and hearing problems.
The smooth collie in the picture i posted is from a sable merle to a blue merle breeding. Both parents are champions.... and this double merle smooth Collie has been bred.... his deafness and blindness is the result of his color.. so this is not something that is passed to offspring. I know the person who bred and owns this dog.... and he has produced some beautiful puppies. When a double merle is bred to a tri-color the litter will be 100% normal blue merles. I personally would NEVER purposely breed merle to merle because of what could happen... but I do know people who have.....