Double-dapple dachshunds have inherited two copies of a gene that causes irregular lightening or dilution of the base coat color, giving a dappled or marbled appearance. Dogs with a single copy of the gene are usually healthy, but dogs with two such genes may have defective eyes or ears.
Dogs of other breeds with the dappling gene are usually called "merles." In heterozygous dapples, the dappled pattern is usually seen in the coat or revealed in an eye that is completely or partially blue, but dappling can be hidden in red or cream coat colors caused by the recessive gene ESPS. Genetic testing can reveal if a dog has the merle or dapple gene. Puppies born from a mating of two dapples may be normal but are at high risk of blindness, deafness and internal organ deformities.