In the Aussie breed there's a gene that carries the blue merle color. The color is highly prized, but it also carries blindness and deafness with it as well as other potential health problems. Some very unethical breeders will breed two blue merles together in hopes of getting blue merle pups. Very often there will be pups in the litter born with these defects and they are usually killed - hence the term "lethal white" for the gene. I had one of these Aussies for many years. I rescued him literally minutes before he would have been put down at the shelter. No one wanted a blind puppy but me. O'Reilly was one of the most joyful, most fearless, loving, wonderful creatures I've ever known. If you've got a fenced in yard where a blind dog pup can run safely, SAVE A LIFE! They can be wonderful companions.
And if anyone tries to come in uninvited in the dark . . . you couldn't ask for a better guardian, lol!