Why You Shouldn't Breed Two Merle Dogs Together

The term "merle" refers to a distinct coat pattern seen in several dog breeds, characterized by patches of diluted pigment creating a mottled or marbled appearance against a base coat color. Merle patterns can occur in a range of base colors, including black (producing blue merle), red or liver (producing red merle), and sometimes chocolate or tan. The pattern is caused by a dominant gene known as the SILV or M-locus gene, which randomly dilutes areas of pigment in the coat, eyes, and sometimes skin.

Breeds in which merle is a naturally occurring and accepted coat pattern include the Australian Shepherd, Border Collie, Rough Collie, Shetland Sheepdog, Cardigan Welsh Corgi, Dachshund, and Great Dane, among others. In these breeds, merle is a legitimate and breed-recognized pattern that can be shown and bred responsibly. However, merle has also been introduced through crossbreeding into breeds where it does not naturally occur, such as French Bulldogs, Chihuahuas, and Pomeranians. In these instances, the merle gene was not part of the breed's original genetic makeup and is considered a product of undisclosed or controversial crossbreeding.

The most critical health concern associated with the merle gene is the double merle condition, which occurs when two merle dogs are bred together. Puppies inheriting two copies of the merle gene (homozygous merle) face a dramatically increased risk of severe health defects including blindness, deafness, and microphthalmia (abnormally small eyes). Responsible breeders avoid merle-to-merle pairings for this reason. Prospective owners interested in merle-patterned dogs should verify that the puppy comes from a single merle to solid color pairing and that the breeder understands and manages the genetic risks associated with the pattern.