In 1983, the occurrence of a ram exhibiting an exceptional muscular development was reported in a flock of Dorset sheep. This unusual phenotype was transmitted to some of the offspring, and subsequent matings involving hypermuscled male descendents of the founder ram and normal ewes clearly demonstrated a 50%-50%, sex-independent segregation ratio of the trait. It was therefore postulated that a dominant mutation (CLPG versus the normal clpg allele) at the autosomal “callipyge“ locus was responsible for this hypertrophy. Using the same family material, the callipyge locus was unambiguously mapped to the distal part of ovine chromosome 18 (Cockett et al., 1994).

In this paper, we present very strong evidence that the callipyge locus is subjected to a previously undescribed form of parental imprinting. It is indeed demonstrated that animals only express the callipyge phenotype if they have inherited the CLPG mutation from their sire, not if it is transmitted by the dam.