Effect of polymorphic variants of growth hormone (GH), β lactoglobulin (β LG), and Pit1 genes on milk yield was analyzed in a Holstein herd. Genotypes of the cows for these genes were determined by polymerase chain reactionrestriction fragment length polymorphism (PCRRFLP) method. Allele frequencies were 0.884 and 0.116 for L and V variants of GH, 0.170 and 0.830 for A and B variants of Pit1, and 0.529 and 0.471 for A and B variants of β LG, respectively. GLM procedure of SAS software was used to test the effects of these genes on milk yield. Results indicated significant effects of these genes on milk yield ( P < 0.05). Cows with LL genotype of GH produced more milk than cows with LV genotype (P < 0.05). Also, for Pit1 gene, animals with AB genotype produced more milk than BB genotype (P < 0.05). In the case of β LG gene, milk yield of animals with AA genotype was more than BB genotype (P < 0.01). Therefore, it might be concluded that ho mozygote genotypes of GH (LL) and β LG (AA) were superior compared to heterozygote genotypes, whereas, the heterozygote genotype of Pit1 gene (AB) was desirable.

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