An inheritable muscular hypertrophy was recently described in sheep and shown to be determined by the callipyge (CLPG) gene mapped to ovine chromosome 18. We demonstrate in this work that the callipyge phenotype is characterized by a non-Mendelian inheritance pattern, referred to as polar overdominance, in which only heterozygous individuals having inherited the CLPG mutation from their sire express the phenotype. The possible role of parental imprinting in the determinism of polar overdominance is envisaged.