The merle gene (in the M locus) dilutes random sections of the coat to a lighter colour (usually grey in a black-pigmented dog), leaving patches of the original colour remaining. The patches can be any size and can be located anywhere. The edges of the patches may appear "torn" unlike the rounded spots of a Dalmatian. Merle affects only eumelanin, meaning that any black, liver, blue or isabella in the coat, eyes or nose will be merled, whether it's the whole of the body or in patches. Phaeomelanin (red) is not affected at all and will appear as normal. Merle is dominant, meaning all "normal" merles are heterozygous (Mm).

A homozygous merle is actually a "double" merle (MM), which can result in serious health issues. Including deafness, blindness, sun sensitivity and skin cancer. For this reason two merles should never be bred together, as this will result in some double merle puppies. In many countries, these matings are banned from registration.

As reported by Clark et al. (2006)(citing Sorsby and Davey (1954), "Dogs having Mm and MM genotypes typically have blue eyes and often exhibit a wide range of auditory and ophthalmologic abnormalities" As breeding merle dogs can be very challenging in avoiding producing merle x merle puppies, it is especially important to take breeding advice.

Additional genetic information: OMIA unique variant codes 1103-1107 are related to specific variations in phenotype. 1103 - no merle pattern, solid coat; 1104 - no merle pattern, solid coat; 1105 - no merle pattern, diluted, brownish hue; 1106 - muted, undefined, diluted brownish hue; 1107 - minimal merle, areas deleted to white, tweed