Merle (M Locus)

General Information: The M-Locus (M) Merle gene, associated with the PMEL gene, leads to a unique coat pattern that includes irregular patches of diluted pigment and solid color, often accompanied by blue or heterochromatic eyes and mottled skin on the nose and paw pads. The expression of the Merle pattern is an autosomal incomplete dominant trait, meaning a single copy of the Merle allele (M) will produce the merle appearance. However, dogs that inherit two copies of the allele (M/M), known as "double merles," often have a predominantly white coat and are at a higher risk for serious health issues, including sensory impairments like deafness and blindness. These effects are due to the Merle gene’s impact on melanocytes, the cells responsible for pigment production. While visually striking, the Merle pattern requires careful management to prevent these adverse health outcomes in breeding practices.

Gene / Testing Information: Genetic testing for the M locus (M) is crucial for identifying carriers of the Merle gene and managing breeding decisions to prevent health complications associated with the "double merle" condition. Testing reveals whether dogs carry zero, one, or two copies of the merle variant, helping breeders avoid mating two Merle carriers, which can lead to a 25% chance of producing double Merle offspring per pregnancy. These double Merle puppies are at high risk for severe auditory and visual disabilities due to pigment dilution throughout the body, including in the inner ear and eyes. To maintain the health of future litters and uphold breed standards, it is recommended that Merle dogs be bred only with non-merle partners to ensure no offspring receive two copies of the Merle gene. This approach helps preserve the desired aesthetic traits of the Merle pattern while minimizing the risk of congenital defects.