Cockett et al., AM Symposium

The Callipyge Phenomenon: Evidence for Unusual Genetic Inheritance

N. E. Cockett*, S. P. Jackson^†, G. D. Snowder^‡, T. L. Shay*, S. Berghmans^§, J. E. Beever**, C. Carpenter^#, and M. Georges^§
*Department of Animal, Dairy and Veterinary Sciences, Utah State University, Logan 84322-4700; ^†Department of Animal Science and Food Technology, Texas Tech University, Lubbock 79409; ^‡USDA, ARS, U.S. Sheep Experiment Station, Dubois, ID 84323; ^§Department of Genetics, University of Liége, 4000 Liége, Belgium; **Department of Animal Sciences, University of Illinois, Urbana 61801; and ^#Department of Nutrition and Food Sciences, Utah State University, Logan 84322-8700

ABSTRACT In 1983, a male lamb exhibiting a pronounced muscular hypertrophy, particularly noticeable in the hind quarters, was born into a commercial Dorset flock in Oklahoma. The ram was premonitorily called Solid Gold. He subsequently produced offspring expressing the unusual phenotype, which is referred to as callipyge (Greek: calli- beautiful + -pyge buttocks). Animals demonstrating the callipyge phenotype are all descendants of this founder ram. These animals 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.

Implications The isolation and characterization of the callipyge gene will lead to many exciting areas of study. Elucidation of the gene may allow better understanding of the relationship between muscle development, fat accumulation, and tenderness. Possible manipulations of the gene may lead to improvement of carcass composition in other livestock species. Several aspects of callipyge muscle development are unique from other hypertrophied conditions in livestock, in that the hypertrophy is not apparent until 3 wk of age or older and it is primarily expressed in the hind quarters of the animal. Finally, polar overdominance resembles imprinting because of the observed parent-of-origin effect, but it differs from conventional imprinting in that homozygous CLPG/CLPG individuals do not express the phenotype. This novel phenomenon may apply to other complex traits, including inherited diseases in humans.

Key Words: Sheep, Callipyge, Genes

AM Symp. 221-227