An inheritable muscular hypertrophy was recently described in sheep and shown to be determined by the callipyge gene mapped to ovine chromosome 18. Here, the callipyge phenotype was found to be characterized by a nonmendelian inheritance pattern, referred to as polar overdominance, where only heterozygous individuals having inherited the callipyge mutation from their sire express the phenotype. The possible role of parental imprinting in the determinism of polar overdominance is envisaged.

A mutation causi muscular hypertrophy, with associated leanness and improved feed efficiency, has been recently identified in domestic sheep (Ovis aries). Preliminary results indicate that an autosomal dominant gene may be responsible for this economically advantageous trait. We have exploited the conservation in sequence and chromosomal location of DNA markers across Bovidae to map the corresponding callipyge locus to ovine chromosome 18 using a battery of bovine chromosome 21 markers. Chromosomal localization of the ovine callipyge locus is the first step toward positional cloning of the corresponding gene.Increasingly, health concerns expressed by consumers determine the quality standards for meat. Currently, the fat content and lean composition of meat products are emphasized. In this work, we report the mapping of a gene affecting both the production efficiency and quality of meat in domestic sheep (Ovis aries). In 1983, a sheep producer identified a ram that possessed extreme muscling. The animal showed little external fat but had unusual muscling in its hind quarters. This phenotype was transmitted by the founder male to part of his offspring and to his descendants in later generations, suggesting an inheritable neomutation.Subsequently, a study was initiated to more rigorously determine the segregation mode ofthis muscular hypertrophy condition (1). In the study, 150 Rambouillet ewes with normal phenotypes were mated to rams that showed the muscular hypertrophy phenotype; the rams were themselves offspring of crosses between normal ewes and muscular hypertrophy males. Of the 200 lambs produced from the ewes, 97 (48.5%) expressed the muscular hypertrophy phenotype and segregation of the muscular hypertrophy phenotype was equal between the sexes. These data indicate a single autosomal gene is responsible for the muscular hypertrophy condition. The name callipyge (from Greek calli-, beautiful; -pyge buttocks) and symbol CLPG are proposed for this gene, and the existence of two alleles, CLPG and clpg, is suggested; CLPG/CLPG and CLPG/clpg animals are heavy muscled and clpg/clpg animals are conventional in appearance.Growth and carcass characteristics of animals expressing the callipyge trait were also analyzed in the study (2, 3). Birth weights, weaning weights, and rate of gain were not significantly different between the heavy muscled and normal phenotypes. However, muscle mass was 32.2% greater in lambs with muscular hypertrophy than in normal lambs. This increase in muscle mass was limited almost exclusively to the pelvic limb with little increase in the thoracic limb. Carcasses of heavy muscled animals were also remarkably lean, with 7.8% less fat than carcasses from normal animals.To characterize this potentially advantageous gene, we sought to identify genetic markers linked to the CLPG locus. As relatively few markers have been mapped in sheep, contrary to the situation in cattle, we exploited the conservation in sequence and chromosomal location of markers across Bovidae and used a battery ...

The objectives of this study were to determine the model of inheritance of the callipyge gene and to evaluate the growth, ADFI, feed efficiency, reproductive performance, and wool growth of sheep that are heterozygous for the callipyge gene. Ewes (n = 236) with a normal muscle phenotype and genotype were mated to three heterozygous rams that expressed the callipyge gene. Lambs (n = 311) were subjectively classified at weaning (90 to 120d) according to muscle phenotype by a panel of three evaluators working independently. The callipyge muscle phenotype was expressed in 150 lambs, whereas 161 lambs expressed a normal muscle phenotype. The percentage of lambs expressing the callipyge muscle phenotype (48.2%) did not differ (P > .1) from the expected 50%. Growth rate was similar for lambs of both phenotypes regardless of sex. Feed efficiency was superior (P < .05) for both male and female lambs with the callipyge muscle phenotype. Average daily feed intake was lower for male (P < .02) and female (P < .004) lambs with the callipyge muscle phenotype. Grease fleece weight and staple length at 12 mo were superior (P < .03) for ewes with a normal muscle phenotype. These results indicate that the callipyge gene in sheep is dominant when inherited from the paternal parent and lambs expressing the callipyge gene have increased feed efficiency and reduced ADFL.

Paternal half-sibling Rambouillet ram lambs (n = 18) representing two muscle phenotypes were slaughtered at 54.5 kg to evaluate carcass characteristics and composition. Lambs were produced from a sire that was heterozygous for the callipyge gene. Carcasses were broken into wholesale and retail cuts to evaluate percentage bone-in retail yield of carcasses at various fat trim levels and percentage of boneless retail cuts. Retail cuts were trimmed to .6 and then to 0 cm fat trim and bones were removed to determine boneless, closely trimmed retail cut yield. Chemical composition was determined using proximate analysis. Lambs expressing the callipyge gene had higher dressing percentages (57.3 vs 53.9), leg (14.4 vs 11.0) and conformation (14.4 vs 11.0) scores, and larger longissimus muscle (LM) areas (17.6 cm2 vs 10.3 cm2). All other carcass measurements were similar between phenotypes except marbling score, which was higher (417.8 vs 325.6) for controls. Lambs expressing the callipyge gene had a higher (40.2 vs 32.9) percentage boneless retail yield than controls. Retail yield of the boneless shoulder did not differ between phenotypes (8.9 vs 8.0). All other percentages of boneless retail cuts were higher (P < .02) for lambs expressing the callipyge gene. Carcasses from lambs with the callipyge gene had higher protein (16.6 vs 15.2), moisture (63.6 vs 58.6) and ash (.85 vs 77) percentages and lower fat (18.9 vs 25.4) percentages than controls. These data suggest that ram lambs expressing the callipyge gene have an advantage in retail yield and carcass conformation when compared to normal-muscled half-siblings.

Paternal half-sibling Rambouillet ram lambs (n = 18) representing two muscle phenotypes were slaughtered at 54.5 kg to evaluate the effect of the callipyge gene on muscle mass. Lambs were produced from a sire that was heterozygous for the callipyge gene. Nineteen muscles were dissected from the right side of each carcass to evaluate muscle weights relative to carcass weight. Excised muscle mass as significantly higher (42%) for lambs exhibiting a callipyge muscle phenotype than for half-siblings. In the pelvic limb, all excised muscles except the peronius tertius were larger in lambs expressing the callipyge gene (P < .001). In the torso, the longissimus (P < .001), psoas major (P < .001), and psoas minor (P < .01) were larger in lambs with the callipyge phenotype. In the thoracic limb, the biceps brachii (P < .001) triceps brachii (P < .002), and extensor carpi radialis (P < .01) were larger in lambs with the callipyge phenotype. Total pelvic limb (P < .001) torso (P < .001), and thoracic muscle weights were higher (P < .01) in lambs with the callipyge phenotype. Callipyge lambs had a higher (P < .01) percentage of excised muscle weight in the pelvic limb and torso and a lower (P < .01) percentage in the thoracic limb when compared to controls. These data indicate that the magnitude of expression of the callipyge gene is dependent upon the location of the muscle on the body and that the increased muscle mass was concentrated in the leg and loin.