Merle only affects black pigment and it does so in patches. These diluted patches result in a mottled appearance. Merle is also an incomplete dominant, meaning that dogs that have two copies of the Merle gene are more profoundly affected than dogs that carry only one copy.
Merle dogs can have health problems, with the worst cases being found in dogs who are homozygous. Most single Merles have no noticeable health problems and live happy productive lives.