Genetic improvement of overall reproductive success in sheep: A review

Reproduction is an economically important complex composite trait in sheep. Genetic improvement of composite traits can occur by selection for individual components traits, some combination of individual component traits, or by direct selection for the composite trait. This review discusses the responses of selecting directly for litter weight weaned compared to selection responses for one of its component traits. Litter weight weaned is concluded to be a biological selection index determined by environmental factors under which it is selected for. Selection for litter weight weaned can result in a balanced biological composite trait with favorable responses in component traits such as fertility, number of lambs born, lamb survival, lactation, and lamb growth. It is concluded that selection to improve reproductive efficiency under most production and environmental systems would benefit from selection for a composite trait such as litter weight weaned rather than for a single component trait

The callipyge mutation and other genes that affect muscle hypertrophy in sheep

Genetic strategies to improve the profitability of sheep operations have generally focused on traits for reproduction. However, natural mutations exist in sheep that affect muscle growth and development, and the exploitation of these mutations in breeding strategies has the potential to significantly improve lamb-meat quality. The best-documented mutation for muscle development in sheep is callipyge (CLPG), which causes a postnatal muscle hypertrophy that is localized to the pelvic limbs and loin. Enhanced skeletal muscle growth is also observed in animals with the Carwell (or rib-eye muscling) mutation, and a double-muscling phenotype has been documented for animals of the Texel sheep breed. However, the actual mutations responsible for these muscular hypertrophy phenotypes in sheep have yet to be identified, and further characterization of the genetic basis for these phenotypes will provide insight into the biological control of muscle growth and body composition