Cavanagh et al. Genetics Selection Evolution 2010, 42:36 http://www.gsejournal.org/content/42/1/36 Page 6 of 14 Figure 1 QTL map of the entire genome for body and carcass weight and dressing percentage. high, but CT or virtual dissection is more precise and reliable [48]. Our study confirmed the high correlation between carcass weight and estimates of carcass weight from scanning [49]. Compared to ultrasound, the stan- dard errors of the predicted values are lower [48,50]. Vester-Christensen et al. [51] and Young et al. [48] have proposed that CT scanning should be an essential refer- ence tool for body and carcass composition. The use of the more precise phenotypes derived from CT measures will also lead to better phenotypes for genetic analysis. Heritabilities for CT-derived traits have been found to be moderate to high [48,52,53]. Theoretical predictions of the genetic progress by incorporating CT traits into selection indices suggest increases in response by 50% or even 100% when combining different measurement methods [6]. The sheep in our study were managed as two cohorts. These cohorts differed significantly in carcass weight and stage of maturity and were considerably heavier than animals in studies published previously [49]. Ani- mals investigated here were taken to a greater stage of maturity to measure specific effects on fat and fat distri- bution. Sheep from cohort 1 had similar muscle/carcass lean weights compared to meat sheep [54] and Norwe- gian lambs [49]. However, for both these studies, the proportion of muscle was higher than in our study, largely due to differences in fatness and stage of devel- opment (age, maturity). For the same reasons, the pro- portion of bone in the carcass was lower in our study than in studies presented by Young et al. [54] and Kongsro et al. [49]. The main focus of our project was the study of fat characteristics in the carcass. Therefore, older and con- sequently more mature sheep were used. Adjusting body composition traits for body weight at the time of scan- ning was considered the best method to accurately mea- sure tissue groups independently of their body mass. Animals from the second cohort had higher fat content