Myostatin acts as a negative regulator of muscle growth. Mice and cattle deficient for myostatin have a dramatic increase in skeletal muscle mass (McPherron et al., 1997, Arnold et al., 2001). Natural mutations in the myostatin gene have been seen in several cattle breeds and in one mouse line and in cattle the associated phenotype is referred to as ‘ Double-muscling’ (DM). Although the direct DM effect seems to be positive in meat producing livestock with increased muscle and decreased fat there are reports of negative side effects in reproductive and other traits in cattle, with an indication of substantial interaction with the genetic background (Wiener et al., 2002). Thus it may be possible to select for the beneficial traits, while minimising the impact of the negative effects. Although negative side effects are not fully understood, breeding companies in the meat-producing sector are devoting considerable effort to high-throughput screening for mutations in this gene, hoping to find variants associated with increased muscularity. It is important that the impacts of such mutations on a wide range of production and welfare traits are fully explored before it becomes a focus of selection in meat producing livestock. The objectives of the work presented here were to utilise marker assisted introgression of a myostatin-deficiency producing partially recessive allele (Compact, MstnCmpt-dl1Abc
) to estimate its effects on traits of growth and fitness on the genetic background of an extreme high growth line, as a model of a highly developed livestock breed. Fitness effects as assessed by the segregation ratio are reported here.