Cavanagh et al. Genetics Selection Evolution 2010, 42:36 http://www.gsejournal.org/content/42/1/36 R E S E A R C H G e n e t i c s Se l e c t i o n Ev o l u t i o n Open Access Mapping Quantitative Trait Loci (QTL) in sheep. III. QTL for carcass composition traits derived from CT scans and aligned with a meta-assembly for sheep and cattle carcass QTL Colin R Cavanagh1,2, Elisabeth Jonas1, Matthew Hobbs1, Peter C Thomson1, Imke Tammen1, Herman W Raadsma1* Abstract An (Awassi × Merino) × Merino single-sire backcross family with 165 male offspring was used to map quantitative trait loci (QTL) for body composition traits on a framework map of 189 microsatellite loci across all autosomes. Two cohorts were created from the experimental progeny to represent alternative maturity classes for body composition assessment. Animals were raised under paddock conditions prior to entering the feedlot for a 90-day fattening phase. Body composition traits were derived in vivo at the end of the experiment prior to slaughter at 2 (cohort 1) and 3.5 (cohort 2) years of age, using computed tomography. Image analysis was used to gain accurate predictions for 13 traits describing major fat depots, lean muscle, bone, body proportions and body weight which were used for single- and two-QTL mapping analysis. Using a maximum-likelihood approach, three highly significant (LOD ≥ 3), 15 significant (LOD ≥ 2), and 11 suggestive QTL (1.7 ≤ LOD < 2) were detected on eleven chromosomes. Regression analysis confirmed 28 of these QTL and an additional 17 suggestive (P < 0.1) and two significant (P < 0.05) QTL were identified using this method. QTL with pleiotropic effects for two or more tissues were identified on chromosomes 1, 6, 10, 14, 16 and 23. No tissue-specific QTL were identified. A meta-assembly of ovine QTL for carcass traits from this study and public domain sources was performed and compared with a corresponding bovine meta-assembly. The assembly demonstrated QTL with effects on carcass composition in homologous regions on OAR1, 2, 6 and 21. Background Sheep production is a major contributor to global food production and sheep are one of the few sources of meat with little cultural and religious restriction in con- sumption. Body composition traits in sheep, primarily muscle mass and fatness, are economically important to the sheep meat industry. There are numerous methods to predict body composition in sheep. Much of the var- iation that exists in sheep body composition is expressed as between- and within-breed differences. In order to understand the genetic architecture of these economic- ally important traits it is essential to accurately define the phenotypes which describe carcass composition [1]. * Correspondence: herman.raadsma@sydney.edu.au 1ReproGen - Animal Bioscience Group, Faculty of Veterinary Science, University of Sydney, 425 Werombi Road, Camden NSW 2570, Australia Full list of author information is available at the end of the article Live-weight is considered as a standard measurement of body mass, but is a poor indicator of body composi- tion due to the inability to distinguish between different stages of physiological maturity. Body weight may be used as indicator of body composition in animals of similar genetic backgrounds and at the same physiologi- cal maturity, however, at different maturity stages the accuracy is greatly reduced [2,3]. Improved predictions of carcass composition can be determined by using ultrasound. Such scans provide a basis to estimate breeding values for eye muscle area and subcutaneous fat depth [3-5]. Increased accuracy and prediction of full body carcass characteristics can be achieved using com- puted tomography (CT) [6,7] but this is not routinely implemented due to cost constraints. In addition to the difficulties in obtaining accurate carcass measurements, generation intervals are large, © 2010 Cavanagh et al; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.