Boman et al. Genetics Selection Evolution 2010, 42:4 http://www.gsejournal.org/content/42/1/4 Table 4 Solutions ± standard errors for various traits and allelic effects Allelic effect a2360 d2360 Weight gain/d from birth to weaning (g) -3 ± 4 -2 ± 5 Weaning weight (kg) -0.6 ± 0.5 -0.2 ± 0.6 Carcass weight gain/d from birth to slaughter (g) 1 ± 2 -2 ± 3 Carcass weight (kg) 0.2 ± 0.2 -0.3 ± 0.3 Carcass conformation class (scale 1-15) 0.30.015 ± 0.1 -0.1 ± 0.2 Carcass fat class (scale 1-15) -0.40.000 ± 0.1 -0.2 ± 0.1 a960 2300..000021  7 2.800..000071  0.9 3 ± 3 0.900..001384  0.4 2.600..000000  0.2 1.500..000000  0.2 d960 270.001 ± 8 2.80.004 ± 1.0 4 ± 4 -0.2 ± 0.4 -1.70.000 ± 0.3 0.50.010 ± 0.2 Page 6 of 7 int. -7 ± 9 -0.5 ± 1.2 -1 ± 5 0.3 ± 0.5 0.80.014 ± 0.3 0.0 ± 0.3 Additive (a) and dominance (d) effect for mutations in position c.2360 and c.960 respectively, and the interaction effect (int), when both mutations are present. The P-value of genotype classes contrasted with the wild type (GG_GG) is presented as superscript, while the P-value for G(delG)_GG, G(delG)_AG and (delG) (delG)_GG contrasted with GG_AA is given in subscript. The P-values are given only for significant findings (P < 0.05). percentage, indicated by the enhanced carcass confor- mation class for this group (Table 3). The reduced weaning weight and weaning weight gain per day (Table 3) also show that the group homozygous for c.960delG grows slowly. However, it is likely that a possibly enlarged dressing percentage, together with the fact that slaughter information was discarded for slow growing lambs in this group (Table 2), explain why the carcass weight gain per day is closer to that of other groups than expected from live weight gain. The effects of the c.2360G>A mutation have also been examined in other studies. Before this mutation was reported, Laville et al. [10] had investigated the effect of the corresponding QTL in Belgian Texel sheep. They reported a QTL effect that increased conformation scor- ing and carcass weight, and reduced the fat score. Kijas et al. [9] had found that under Australian conditions, the g.+6723G>A mutation (equals the c.2360G>A muta- tion) had significant effects on slaughter measurements of muscling and fatness, but only minor impact on live weight and growth. These results correspond well with our findings. Similarly, Hadjipavlou et al. [11] had studied the effect of the c.2360G>A mutation on Charollais lambs, and did not find any effect on live weight. With an animal model, AA animals were found to have significantly lar- ger muscle depth than AG and GG animals, while AG and GG animals were not significantly different. None of the fat depths were significantly different. They con- cluded that the effect on phenotype depended on the genetic background, a point that is clearly demonstrated in our material for carcass conformation class, showing that animals heterozygous for the c.2360G>A mutation are strongly influenced by the genotype at the c.960 position. Conclusions In NWS, increased muscle mass and reduced carcass fat are caused by the c.960delG and the c.2360G>A muta- tions. The impact of c.960delG is more important com- pared to c.2360G>A, and displays dominance effects. In the rough grazing environment of this experiment, lambs homozygous for the c.960delG mutation experi- enced reduced growth rate. Acknowledgements We thank the producers that participated in the field experiment and Hans Vestjord for helping with collecting blood samples. Silje Karoliussen is acknowledged for excellent technical help. The project has received funding from the Research Council of Norway (project no 173923/I10) and Marketing levies (paid by producers). Author details 1Department of Animal and Aquacultural Sciences, Norwegian University of Life Sciences (UMB), PO Box 5003, N-1432 Ås, Norway. 2The Norwegian Association of Sheep and Goat Breeders, PO Box 104, N-1431 Ås, Norway. 3Animalia - Meat and Poultry Research Centre, PO Box 396 Økern, N-0513 Oslo, Norway. 4Centre for Integrative Genetics (CIGENE), Department of Animal and Aquacultural Sciences, Norwegian University of Life Sciences (UMB), PO Box 5003, N-1432 Ås, Norway. Authors’ contributions IAB carried out the experiment, performed the statistical analysis and drafted the manuscript. DIV was responsible for genotyping of the animals, and improved the manuscript, jointly with GK. All authors participated in planning the experiment, read and approved the final manuscript. Competing interests The authors have been granted a patent in the UK on the diagnostic method of gene testing for the c.960delG mutation (GB2433320). 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