What is Merle
The most important thing to remember is that merle is a color pattern, just like spotting, and not a color by itself. Merle dogs can come in a variety of colors. There are also numerous breeds of dogs carrying the merle gene, including many herding breeds, dachshunds, catahoula leopard dogs, great danes, and even chihuahuas.
Merle can be thought of as a defect to the color genes. A Blue Merle dog should actually be Black, but the merle gene makes the black hairs gray. In essence, the merle trait hinders the pigment from getting to the hairs. It is not a mixture of white and black hairs, though the gray coloring can vary in intensity. Merle is also not linked to the white trim (face markings, collar, legs). There are several other genes that make up the white trim on a dog.
Merle is a dominant gene, meaning when a dog has the merle variant, it will show up in the hair. In some cases where a dog has a lot of white trim, the merling can be hidden (called a phantom merle).
All genes come in pairs. A merle dog has one merle gene and one solid gene. When two merle parents breed, the offspring can receive a merle gene from both parents (double merle or homozyogous merle). And, since merle is essentially a defect, two merle genes is double-defect.
In most animals, pigment of the skin/hair is closely linked to the fibers that also make up vision and hearing. In double merles, this double defect often results in dogs that are fully or partially blind, deaf, or both. Many have underdeveloped eyes (iris hypoplasia, iris coloboma, microopthalmia) or missing eyes (anopthalmia).
These handicaps, while easily overcome for many dogs, are seen as taboo in the breeding/show world. Many breeders will “cull” or kill double merle puppies. This is why double merles are often called “lethal whites”. Double merles are otherwise happy, healthy dogs, and the white pigment does not directly translate to other health issues that would be lethal. “Lethal white” is also a term in horses, but is a completely different condition unrelated to the merle gene.
Why do we have double merles?
Responsible breeders are aware of the dangers of double merles and will only breed merle dogs to solid-colored dogs. However, merle dogs have become most desirable in the Aussie world, and many misguided, unknowledgeable, or irresponsible breeders will breed two merles together thinking it will yield more merle puppies. They consider culling double merle puppies a necessary deed, even though genetics proves the percentage of merle puppies to be the same. Some very misguided breeding operations will even purposely breed two merles together in order to have a double merle female in their breeding dogs, ignoring her specialized needs with her handicaps, since she will always pass a merle gene to her offspring.
How do you train a deaf/blind dog?
We get the question a lot of “but how do you communicate with them?” First, most double merles only have partial handicaps. For those dogs who are either completely blind or deaf, sign-language commands or touch-commands are often used. Many trainers will also utilize items around the house, such as flash lights to get a deaf dog’s attention. It is actually very easy to communicate with a handicapped dog. And, since Aussies in general are typically velcro dogs (attached to your side and looking to you for approval), they eagerly learn how to please you.