Double merle or homozygous merle occurs when the offspring of two merled parents inherit two copies of the dominant merle gene. The odds of this are 25% for each pup born from such a litter. Double merles often have excessive white and can have hearing and vision problems as a result of having two copies of the merle gene. Homozygous merles can be deaf or blind, or express iris colobomas and microphthalmia. Not all homozygous merles are affected, but most are, making the breeding of two merles a very touchy subject. Some breeders euthanize mostly white pups, while others may attempt to sell them as "rare" white Aussies without disclosing the potential for health defects. A large percentage of homozygous merles sold eventually end up in rescue and shelters, as the average family is ill-prepared to take on a deaf or blind pet. However, deaf or blind Australian shepherds can make wonderful pets given a home prepared for their special needs. They are an intelligent breed, which generally learn hand signals with ease.