Boman et al. Genetics Selection Evolution 2010, 42:4 http://www.gsejournal.org/content/42/1/4 Year 1 The first year, all ewes of ten commercial NWS flocks were genotyped at the c.960 position. In essence, for each ewe homo- or heterozygous for c.960delG, an age-matched control ewe without the mutation from the same flock was also included in the study. All ewes were mated to a ram without the mutation (n = 34). Two flocks were excluded from the study due to the low numbers of ewes carrying the mutation (4 and 6, respectively). The remaining flocks were genetically well tied, since six belonged to the same ram circle, one was a former member of the circle and one had a history of rams purchased from the circle. A total of 200 ewes (100 case/control pairs) were included in the study, and each flock was represented with 18 to 28 ewes. In six flocks, ultrasound scanning to count the number of foetuses had been performed, thus only pregnant ewes were included in the experiment. The first priority was to include all homozygous ewes, thereafter the youngest heterozygous ewes within each flock. The numbers of ewes and rams per genotype are given in Table 1. The selected ewes’ lambs born this year were genotyped. Year 2 It was decided to replace two of the flocks from year 1, by another flock. This flock was in an adjacent ram circle, having genetic ties to the original experimental flocks because common AI rams had been used and local elite rams had been exchanged. Basically, the same sampling strategy as in year 1 was followed; 100 ewes with the c.960delG mutation and 100 without were included. In both groups, ewes with a low esti- mated overall breeding value were sampled, since these are not relevant for producing replacements. Prediction of the breeding value, is described by Eikje et al. [5]. Each flock was represented with 20 to 30 ewes. In addition, we balanced the groups with respect to age and flock as in year 1. All ewes were artificially insemi- nated with frozen semen from rams heterozygous for the c.960delG mutation (n = 7). For the ewes that returned, a local ram carrying the mutation was used. The numbers of ewes and rams per genotype are given in Table 1. The selected ewes’ lambs were also geno- typed in year 2. Table 1 Number of ewes and rams (local/AI) per genotype and year Sex c.960 Year 1 Year 2 GG 100 101 Ewes sampled G(delG) (delG)(delG) 96 4 96 3 GG 29/5 Rams G(delG) (delG)(delG) 10/7 1/0 Guanine (G) is found in the mutated position (c.960) in the wild type; in year 2, a local ram serviced ewes that returned Page 2 of 7 Management and slaughter The experiment did not interfere with normal manage- ment; for example, the farmers were allowed to move lambs to a foster mother or providing supplemental feeding. In year 1, the farmers decided if and when to slaughter the lambs, while in year 2 all experimental lambs were intended to be slaughtered. At approximately four months of age, the lambs were gathered and transferred from the rough grazing pasture to the farm. Subsequently, the weaning weight of the lambs was measured and the farmers selected the lambs to be sent directly for slaughter, and those to be kept on rich pasture, for finishing. Live weight was used as a guide to decide when to slaughter the lambs according to common practise. Some farmers shipped lambs only twice in the season, while others shipped them more frequently, depending on management choices and flock size. The lambs were all slaughtered in the same commer- cial abattoir, and carcass classification was carried out according to the EUROP classification system in Norway [6], which is on a 15-point scale, a value of 15 being the meatiest or fattiest class, respectively. Statistical analysis Data on growth and carcass traits were retrieved from the national sheep recording system (SRS). The data were analysed univariately for weight gain per day from birth to weaning, weaning weight, carcass weight gain per day from birth to slaughter, carcass weight, carcass conformation class and fat class (Yijklmno), with the fol- lowing linear model, using DMUAI in the DMU soft- ware package [7]: Yijklmno  Gi  GDj  S k  R l  ADm  fy n  io  e ijklmno where Gi is the fixed effect of the ith genotype class (1, ..., 6; see Table 2), GDj is the fixed effect of the jth genotype class of the dam (1, ..., 5; as in Table 2, except the class homozygous for c.960delG), Sk is the fixed effect of the kth sex class (male or female), Rl is the fixed effect of the lth rearing class (1, 2, ≥3 or bottle lamb), ADm is the fixed effect of the mth age of dam class (1, 2, 3, 4 or ≥5), fyn is the random effect of the nth flock-year class (1, ...., 15), io is the random additive genetic effect of the oth animal and eijklmno is the ran- dom residual term. The pedigree file comprised a total of 3292 animals, a pruned subset retrieved from the SRS for the experimental animals, comprising all known ancestors in six generations. In the statistical model, the effects of sex, rearing class and age of dam were factors that we a priori believed to affect the traits since they are taken into account in the