The aim of this study is to determine the relationship between 3 previously described polymorphisms (SNPs) located at 3 candidate genes (CAST, TNNI1, MYOG) and slaughter traits in different weight groups of commercially housed hybrid pigs. A total of 402 pigs [Large White × (Landrace × Duroc) sows line × Pietrain boar line] were tested using PCR-RFLP method. After 111 and 118 d in fattening (DF) their carcasses were qualified in a slaughterhouse where the hot weight (HW), backfat thickness (BF) and loin diameter (LD) were measured, lean meat % (LM %), live weight (LW) and average daily gain (ADG) were calculated. Because of the wide range of LW (82–144 kg) heterogeneity of experimental population had to be reduced, therefor animals were divided into 3 groups based on mean (116.1 kg) and SD (10.2 kg) of LW as follows: low (L) (<mean-1 SD, n = 63), medium (M) (mean ± 1 SD, n = 276) and high (H) (>mean+1 SD, n = 63). The effects of SNPs on carcass quality traits were analyzed in the 3 classes separately using the least square analysis of the GLM method with Tukey-Kramer correction (SAS 9.1) including sex and DF as fixed effects and LW as covariance factor. According to our results the examined SNPs show different effects on carcass quality at different weight. Pigs with the GG genotype of CAST (Ser66Asn) gene show the worst BF and LM % (P < 0.05) values in the L group, but no significant results were found in the other 2 groups. The TNNI1 gene has no significant effect on traits. The SNP in 3′ UTR region of MYOG gene influenced BF and LM % (P < 0.05) values, but only in the H group. The marker-assisted selection plays crucial role in animal husbandry, despite economically important traits show multifactorial inheritance with a strong environmental influence. Our results prove that the effect of polymorphisms depends on LW, moreover, the LW strongly influences the carcass quality traits, therefore if the test population shows a high degree of variability the use of weight categories should be considered.