Polar overdominance is closely related to parental imprinting - a phenomenon where only one (maternal or paternal) allele is expressed in the offspring. In contrast to gametic imprinting, polar overdominance is not very well known and only few examples of this phenomenon have been reported in mammals. One of them is the callipyge phenotype in sheep, which appears in the offspring only when the mutated allele comes from the father and the wild allele comes from the mother. In review, latest concepts on the molecular mechanism underlying the callipyge phenotype are presented and the new examples in mammals are discussed.