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ABSTRACT. 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 + kappacasein, DGAT1 + beta-casein, and GH + beta-LGB. In the single-gene model, most loci are significantly associated with traits. A significant 2561-2571 (2015) 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.

The objective of this study was to analyze and investigate the genotype frequency and the association between Acyl-CoA:diacylglycerol acyltransferase1 gene, DGAT1 gene, and milk yield (MY), milk composition, protein yield (PY), fat yield (FY), solid not fat yield (SNF), total solid (TS), the content of fat, protein, solid not fat, and total solid, (%Fat,%Prot,%SNF,%TS) in two herds of crossbred Holstein dairy cattle in Thailand. Two hundred and twenty-seven crossbred Holstein cows were used and their blood samples were taken for the study. PCR-RFLP was used to identify the allele and genotype of DGAT1 gene. A general linear model and the least square method were used to estimate the least square mean and additive, and the dominant effect of the gene on the traits and the least significant differences were used to compare the mean of each trait between genotypes. Two alleles (K, A) and three genotypes (AA, KA, KK) were detected, the highest allele and genotype frequencies were A and AA, respectively. The least mean squares of each genotype were compared and significant differences between genotype were detected. Genotype KK has the greatest effect on all milk composition content traits, while genotype AA has the greatest effect on yield traits. Highly significant additive gene effect was detected. From the results, it can be concluded that the DGAT1 gene can be used as a gene marker for assisted selection in milk composition traits.

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