What is a double merle?
A dog that inherits two dominant versions of M is a double merle. In Australian Shepherds these dogs usually will have excessive amounts of white, be deaf in one or both ears, and have serious and generally blinding eye defects. In a few severe cases the eyes have been missing altogether. The effects can range from moderate to severe.
Breeds with merle but little or no white, like the Catahoula, and also working line Aussies where the line has little or no white markings, generally are not deaf (no white on the head), and tend to have only minor merle eye defects, if any.
What goes wrong with double merles?
The same early embryonic tissue layer that gives rise to nerves, skin, eyes, and ears also gives rise to melanocytes. The merle gene causes changes in cells that arise in melanocytes (pigment cells) during development. Pigment cells occur in the eyes and inner ear, as well as in the skin and hair follicles. Merle causes a reduction of pigment. If a dog has only one copy of the merle gene, it will have the merle color pattern, leaving some areas with full color (black or red) while other areas are a lighter shade. If the dog has two copies of the gene, some cells will have no pigment at all, which leads to deafness when they are located in the inner ear, or will develop abnormally, as happens in various structures within the eye.