Although the synergetic effects of multiple marker loci regarding quantitative traits such as blood glucose level have attracted interest, previous conclusions have been based on assumptions that each marker locus behaves independently of the other, leading to approximation. To cope with this problem, this paper focuses on the effects of multiple genetic factors and tries to find significant marker combinations by using conjunctive rules regarding genotypes at multiple marker loci. Application of the proposed method on the OLETF model rat of non-insulin dependent diabetes mellitus (NIDDM) has found significant combinations of marker loci with respect to oral glucose tolerance (OGT).