Genetic selection has been and continues to be the major tool to change body composition in species raised for the production of meat.[1,2] Previously, fat was a feedstock for many by-products, but most uses were supplanted by oil-derived products. Fat is also an energy-dense product and was important as food in earlier times when humans expended large amounts of energy to accomplish tasks that in developed countries are now performed by or assisted by machines. In developing and developed countries, as modernization progresses, there is less need for energy-dense diets. The tendency of modern societies toward being overweight or obese has changed the demand toward lean meat. Genetic selection for less body fat and more muscle is successful because of the high and moderate degree of heritability of fat and muscle deposition, respectively.1-3-1 For example, in pigs selected for and against backfat thickness, it changed approximately 3.5% per generation.1-4-1

As more is known about the role of specific genes or combinations of genes in the regulation of body composition, genetic selection can be directed toward those genes.[5] The double-muscled condition in cattle results from mutations in a protein, myostatin, involved in differentiation and growth of muscle. Selection for these types of mutations may lead to more muscular animals. Selection for less function of genes associated with adipose tissue accretion may lead to leaner animals. Animal growth is complex and regulated by a multitude of genes, so selection for a single gene may not produce the desired effects. Determination of anonymous markers in the genome associated with variation in quantitative traits (quantitative trait loci) allows discovery and mapping of genetic loci that should be valuable in future selection projects. Fine mapping of these regions can then be used to discover a particular gene or genes involved with the desired traits.

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