The Australian Shepherd, often known simply as the 'Aussie', is a medium-sized breed of dog that was, despite its name, developed on ranches in the United States during the 19th century. In this case, the deafness and blindness are linked to having two copies of the merle gene, which disrupts pigmentation and produces these health defects. Also, great variety is seen in the Aussie's eye color, and they are often heterochromatic. An early nickname for the breed was 'ghost-eye dog'. Although some Aussies are born with naturally bobbed or partially bobbed (stubby) tails, the majority are born with full, long tails. Breeders have historically docked the tails when the puppies are born. In the United States and Canada, the standard calls for a natural bob or docked tail not to exceed four inches as a defining characteristics. Any natural tail length is permitted when showing in Europe, where docking has been banned in most countries, including the United Kingdom.