Polymorphism of the b-lactoglobulin (b-LG) gene in 101 cows belonging to the Holstein herd and a superior cow, producing more than 150 Kg milk/day, together with four offsprings was investigated by the Polymerase Chain Reaction-Restriction Fragment Length Polymorphism (PCR-RFLP) method. In the Holstein herd, the alleles A and B of the b-LG gene had frequencies of 0.53 and 0.47, respectively. The genotypes AA, AB and BB of the b-LG gene were estimated to have frequencies of 0.257, 0.544 and 0.198, respectively. Genotypes were distributed according to the Hardy-Weinberg equilibrium. Results indicated that the b-LG genotypes significantly affected (P< 0.01) milk yield (genotype AA being more effective than genotype BB). The superior cow and her progenies were all heterozygotes (AB).

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