Piebald, or Extreme White Spotting describes a variable degree of white spotting ranging from tiny white spots up to completely white animals. The absence of pigment is caused by the absence of skin melanocytes ("leucism"), not by failure in the biochemical reactions required for pigment synthesis. As summarised by Körberg et al. (2014), Little "described four different alleles at this locus with phenotypic effects ranging from solid (S, . . .), to a completely white coat, caused by homozygosity for the Extreme white allele (sw, . . .). The two intermediate phenotypes were named Irish spotting (si, . . .) and piebald (sp, . . .). Irish spotting is characterized by modest white spotting, often present as a white collar and a white belly, as demonstrated by breeds such as the Bernese Mountain dog and Basenji. Piebald-coloured dogs display limited to extensive white spotting and the phenotype is observed in several breeds, including the Beagle and Fox Terrier." (From OMIA)