POLAR OVERDOMINANCE AT THE OVINE CALLIPYGE LOCUS

An inheritable muscular hypertrophy was recently described in sheep an d shown to be determined by the callipyge gene mapped to ovine chromos ome 18. Here, the callipyge phenotype was found to be characterized by a nonmendelian inheritance pattern, referred to as polar overdominanc e, where only heterozygous individuals having inherited the callipyge mutation from their sire express the phenotype. The possible role of p arental imprinting in the determinism of polar overdominance is envisa ged.