Quantitative and molecular genetic determination of protein and fat deposition.

After 30 years of selection, breeding of the pig breed sus scrofa Piétrain has resulted in reduced backfat thickness (from 3.2 to 1.9 mm) and increased loin muscle area (40 to 60 cm2) which indicates high genetic determination of these body composition traits. The use of sophisticated quantitative genetic methods that include all genetic relationships of large populations has led to a high response to selection of these traits. Selection on feed intake, lean and fat tissue growth using nonlinear functions to optimise these traits during the entire growth period in a biological model offers the opportunity to further improve total genetic potential. Protein and lipid deposition rates during the entire growth period have to be known for this biological model to be applied; thus knowledge of the genetic background of these traits is of high economic value. With the use of molecular genetic methods, such as candidate gene and genome scan approaches, the identification of genes for obesity and growth can be obtained. In sus scrofa, candidate genes associated with obesity and growth include Leptin Receptor, Melanocortin-4 Receptor, Agouti related protein, Heart fatty acid binding protein 3, and Insulin-like growth factor 2. Some of these candidate genes also explain variation in obesity levels in humans. Initial genome-wide scans have identified quantitative trait loci (QTL) on chromosomes 1, 4, 5, 7 and X for obesity and on chromosomes 1, 4, 7, 8, 13 and 18 for growth. Physiological candidate genes and predispositional QTL for obesity are not always located on the same chromosome; this is known the "polygenic paradox". Use of a nonlinear growth function is recommended in order to give more insight into the physiological regulation of obesity traits. Sus scrofa is an excellent model organism to examine the genetic regulation of obesity. The conservation of DNA sequence and chromosomal segments between sus scrofa and homo sapiens will permit easy transfer of results to human studies.