The merle pattern is produced when a dog has a single copy of the M< allele. All merle dogs have the genotype Mm — meaning they have one allele for merle and one allele for non-merle. All non-merles are mm. If you breed a merle (Mm) to a non-merle (mm) you will on average produce a litter in which a half of the puppies get the M allele so are Mm (merle) and half get the non-merle allele so are mm.
But if you breed two merles together (Mm X Mm) you will produce on average a quarter mm (non-merle), a half Mm (merle) and a quarter MM (double-merle; also called double-dapple). And double merles don’t look like merles. Instead, they’re mostly white with merle patches. But the main reason you want to avoid producing MM dogs is that they often have visual and auditory problems.
Was that confusing enough?!!