POLAR OVERDOMINANCE AT THE CALLIPYGE LOCUS IN SHEEP

Abstract

In 1983, a male Iamb exhibiting pronounced muscle development in the hind quarters was born into a commercial Dorset flock in Oklahoma. The ram was callec Solid Gold and in matings with normal females, produced lambs expressing the unusual phenotype. This muscle hypertrophy trait is referred to as callipyge [<Gk calli- beautiful + -pyge buttocks] . Animals demonstrating the callipyge phenotype produce leaner, higher yielding carcasses but there is some concern with decreased tenderness of the loin. Genetic characterization of the locus has demonstrated a unique mode of inheritance termed “polar overdominance”, in which only heterozygous offspring inheriting the mutation from their sire express the phenotype The three other genotypes are normal in appearance. Progeny data indicate that reactivation of the maternal callipyge allele occurs after passage through the male germ line, although this reactivation is not absolute. The callipyge gene has been mapped to the distal end of ovine chromosome 18.