The objectives of this study were to determine the effects of a single gene and composite genotype of the casein gene family, including the beta-lactoglobulin gene (beta-LGB), acyl-CoA: diacylglycerol acyltransferase 1 gene (DGAT1), growth hormone gene (GH), and luteinizing hormone receptor gene (LHR) on milk yield, milk composition, the percentage of fat, protein, solids-not-fat, and total solid in crossbred Holsteins. A total of 231 crossbred Holstein cows were examined for the study. The genotype of the beta-casein gene was analyzed by allele-specific polymerase chain reaction, while the alpha-S1, alpha-S2, kappa-casein, DGAT1, beta-LGB, and GH genes were analyzed using a polymerase chain reaction-restriction fragment length polymorphism assay. The association between genes and milk yield and milk composition was analyzed. Three pairs of genes, for which significant associations were detected, were beta + kappa-casein, DGAT1 + beta-casein, and GH + beta-LGB. In the single-gene model, most loci are significantly associated with traits. A significant association between the composite genotype and the traits was detected in all composite genotypes. GH + beta-LGB appears to be the most suitable variants for improving milk production and percentage of milk protein. Overall, the effects of the composite genotype and single gene were different. A physical or functional relationship between genes is necessary for investigating gene markers.