they have a single M (merle) dominant allele and an m (non-merle) allele. MM genotypes are double-merle or double-dapple whereas mm genotypes are non-merle.
The reason we typically use the term dapple for dachshunds instead of merle is that the Mm genotype in dachshunds expresses itself through white spots on the dog’s non-white coat.
In contrast, other dog breeds with the Mm merle genotype have darker spots on their lighter coat. Both the merle and the dapple coat types are caused by the same gene.
but because accidentally or intentionally breeding two dapple (Mm) dachshunds together can lead to double dapple (MM) dachshunds being born in their litter.
the Mm merle genotype that causes the pretty dapple/merle coats of dachshunds and other dogs. That’s similar to how we got people with blonde hair some ~11,000 years ago.