The callipyge phenotype in sheep is an inherited muscular hypertrophy that affects only heterozygous individuals who receive the CLPG mutation from their father. The CLPG mutation is a single nucleotide substitution in what is probably a long-range control element (LRCE) within the DLK1-GTL2 imprinted domain. Recent results suggest that the unique mode of inheritance of callipyge, referred to as polar overdominance, results from the combination of the cis-effect of the CLPG mutation on the expression levels of genes in the DLK1-GTL2 imprinted domain, and the trans interaction between the products of reciprocally imprinted genes.