We talk about merle a lot on this website, so it seemed like a good idea to explain what it is (and isn't). The most important thing to remember is that merle is a color pattern, not a color by itself. Merle dogs can come in a variety of colors (see links to the left). There is a partial list of the breeds of dogs carrying the merle gene on our links page.
The most commonly seen variation is called blue merle (this is the coloring often considered "typical" of Australian Shepherds). A blue merle is actually a black dog, with the black broken up into irregularly shaped patches by gray. Merle is the gray (lighter part) of the coat's coloring, not the black patches. The lighter part of the coat can vary considerably, from very light (powder blue) to very dark, almost black (steel blue).
Although you will sometimes see merle defined as a mixture of black and white hairs (referring to blue merle dogs), this is not actually the case. Although there will be some variation in the shades of gray on a given dog, it is definitley a third color, and not a mixture of black and white. Dog without the gray are more likely to be roansG.
Some dogs will be heavily merled, where most of their coat is gray, with only a few patches of black. Other dogs will be lightly merled, where their coat is mostly black, with small areas of merle. These dogs are often referred to as "phantom (or crypticG) merles" because they appear at first glance to be a solid color. These dogs are still genetically merle though, and will reproduce as such.
Merles are also commonly red (brown), and any merle dog may or may not have copper (tan) points and white trim. It is also possible to have merle in other colors (such as sable, yellow, fawn, and dilute blue or red), but they are harder to identify as such without experienced help. All of the above information applies to any color of merle dog. Harlequin Danes are also merles, even though only the solid color remains. It is theorized that the harlequinG gene removes the intermediate color (usually gray in Danes), but this is not known for sure.
(Remember, all pictures with blue borders are links.)
The White Aussie Project 2003-2005