Boman et al. Genetics Selection Evolution 2010, 42:4 http://www.gsejournal.org/content/42/1/4 c.960 c.2360 Weight gain/d from birth to weaning (g) Weaning weight (kg) Carcass weight gain/d from birth to slaughter (g) Carcass weight (kg) Carcass conformation class (scale 1-15) Carcass fat class (scale 1-15) c.960 c.2360 Weight gain/d from birth to weaning (g) Weaning weight (kg) Carcass weight gain/d from birth to slaughter (g) Carcass weight (kg) Carcass conformation class (scale 1-15) Carcass fat class (scale 1-15) Table 3 Solutions ± standard errors for various traits and genotype classes, resulting from mutations at c.960 or c.2360 GG 357 ± 12 50.1 ± 1.7 136 ± 5 21.4 ± 0.6 7.4 ± 0.3 6.0 ± 0.3 GG 361 ± 12 50.2 ± 1.7 143 ± 5 22.1 ± 0.6 8.30.000 ± 0.3 5.00.000 ± 0.3 G(delG) GG AG 352 ± 11 49.4 ± 1.6 134 ± 5 21.3 ± 0.6 7.7 ± 0.3 5.40.001 ± 0.2 AG 349 ± 12 48.9 ± 1.8 140 ± 5 22.3 ± 0.7 9.300..000000  0.4 4.400..000000  0.3 Page 4 of 7 AA 350 ± 12 49.0 ± 1.8 137 ± 5 21.8 ± 0.7 8.10.015 ± 0.4 5.10.000 ± 0.3 (delG)(delG) GG 31200..000021  16 44.600..000071  2.2 142 ± 8 23.300..001384  0.9 12.500..000000  0.5 3.000..000000  0.4 Guanine (G) is found at both mutated positions in wild types, while (delG) and adenine (A) respectively, are found when 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). Solutions are given with the following restrictions; genotype of dam class GG_GG, male, twin and age of dam = 3. different from the genotype GG_AA, while the geno- types G(delG)_AG and (delG)(delG)_GG resulted in sig- nificant (P < 0.001) effects, towards more meaty and less fatty animals. The wild type group had a carcass confor- mation class and fat class of 7.4 and 6.0, respectively; homozygotes for the c.2360G>A mutation had 8.1 and 5.1 respectively; and homozygotes for the c.960delG mutation showed the largest effect with 12.5 and 3.0, respectively (for illustration; see Figure 1). The effect of the ewe’s MSTN-genotype on her lamb (s) was close to zero and non-significant for all traits (results not shown). The allelic effects are given in Table 4. The mutation in c.2360 showed a significant additive effect only on carcass conformation (0.3) and fat class (-0.4), and no significant effect of dominance. The mutation in c.960 significantly affected all traits, except for daily carcass weight gain. For this mutation, there were also signifi- cant dominance effects for four of these traits. For car- cass conformation class, a significant interaction between the mutations was estimated. Discussion The results show that both the c.2360G>A and c.960delG mutations affect conformation and fat class in NWS lambs, yielding a carcass with less fat and increased muscle mass (Table 3 and 4). The effect of the c.960delG mutation is larger than that of the c.2360G>A mutation. This is in line with the results obtained by Boman et al. [2], who suggest this is most likely due to the different functional impact of the two mutations. The effect of the c.2360G>A mutation, as compared to the wild type, is slightly more pronounced in this experiment, compared to the material reported by Boman et al. [2]. However, in the experiment reported here, we were able to study more than one flock environment, a larger number of lambs in all MSTN-groups, and the farmers only partially decided which lambs to slaughter. In addition, the statistical model also accounted for the proper number of lambs following the ewe at weaning, rather than the number of lambs born. There were no overlap between rams and years. It is possible that the genetic contribution from the rams and the flock-year effects may have been confounded, but this will not affect the relative size of effects of gen- otype classes. Also, lambs homozygous for the c.960delG mutation were only produced the second year. As the five other genotype classes were produced both years, this lack of complete cross classification should not be a problem. Since the c.2360G>A-mutation is already segregating in NWS at a medium frequency (Table 2), we hypothe- sise that in the future this mutation will reach near-fixa- tion in NWS, as in the Texel breed [1,9]. Therefore we tested the other MSTN groups against the group homo- zygous for c.2360G>A, in addition to testing against the wild type. In Norway, live weight is the most important criterion for deciding when to slaughter lambs. Thus, the higher carcass weight for the homozygous c.960delG mutation group may be explained by enlarged dressing