Technical Abstract: A causative mutation in sheep, named callipyge, with large effects on lean and fat development as well as eating quality of meat was recently discovered. Expression of muscle hypertrophy is inherited in a unique parent-of-origin manner referred to as polar overdominance. Specifically, animals exhibiting characteristic muscle hypertrophy must inherit the mutated allele from the sire, and not from the dam, making callipyge a unique phenomenon. We identified the specific causative mutation by sequencing key inbred animals identical-by-descent for a 210-Kb region of distal sheep chromosome 18 known to contain the mutation. Genetic location of the mutation was subsequently identified as a single nucleotide polymorphism in an imprinted domain region between the DLK1 and MEG3 genes. Imprinting status of several genes in this region was not changed by the mutation but expression levels of several transcripts have been altered. Paternally expressed transcripts are up regulated when the mutation is present on the paternal gamete and maternally expressed transcripts are up regulated when the mutation is present on the maternal gamete. Discovery of the causative polymorphism for this muscle hypertrophy condition marks the starting point for future research efforts to determine the mechanism by which this single base change leads to such marked phenotypic alterations. New insights into basic biology of imprinting regulation in this important region of the genome for livestock and humans will certainly be an outcome of basic research in this area. In this textbook chapter research investigating the genetic mechanism of the callipyge locus and the associated effects on carcass composition and meat quality traits was summarized. Current gaps in research knowledge and how this information could be used to capture the benefits and mitigate the antagonisms associated with this mutation were identified.