To investigate the genetic basis of carcass composition in beef cattle a whole genome scan was conducted to discover quantitative trait loci for marbling and rib eye area. Simmental, Angus or Simmental x Angus steers were obtained from five ranches in Montana
representing calving years between 2005 and 2009. Steers were transported after weaning to the University of Illinois Beef Research Unit where feedlot performance and carcass data were collected. During the five year period, DNA samples were collected on more than 2300 steers. A total of 1,672 parentally verified steers representing 127 sires were genotyped using the Illumina® BovSNP50 BeadChip. After implementation of quality control measures 49,601 SNPs were available for analysis. A within family association analysis was performed through the QFAM permutation procedure of PLINK. Preliminary results for the analysis with marbling and rib eye area revealed significant associations (P ≤ 0.01) with 266 and 383 SNPs respectively, representing loci on each of the bovine autosomes and of those significant associations two and one were highly significant (P ≤ 0.001), respectively.