What is a double merle?
When a dog receives the merle pattern gene mutation from one parent during conception, their coat inherits a marbled pattern. This pattern is desirable in a lot of breeds, and AKC standard in several. When two merle-patterned dogs are bred together, each puppy in the litter has a 25% chance of inheriting that gene from both parents. The resulting offspring is referred to as a double, or homozygous merle.
Double merles often have a predominantly white coat. Keep in mind though, that just because a dog has a white coat, doesn't necessarily mean it's a double merle, because there are other genetic combinations that can produce a white puppy.
The American Kennel Club (AKC) recognizes "merle" as an acceptable pattern in the following breeds:
Great Dane (harlequin acts the same)
The merle gene is appearing in more breeds every day. The following breeds are often seen in merle, but do not formally recognize it as an acceptable pattern, or they are not AKC recognized breeds:
Alapaha Blue Blood Bulldogs
American Staffordshire Terriers/"Pitbulls"
This shows the genetic possibilities of EACH puppy in a litter bred from two merle dogs.
As you can see each puppy has a 50% chance of being born merle, a 25% of being solid/non merle, and a 25% chance of being a double merle.
As you can see, there is no risk of double merles when breeding a solid dog to a merle.