Selection in dairy breeding is basically aimed at milk production and composition. Recently, a number of potential candidate genes have been recognized. Candidate genes are selected on the basis of known relationships between physiological or biochemical processes and the trait. Occurrence of allelic variation in the regulatory and structural regions of those genes may influence the diversification of quantity and composition of milk. Polymorphism of nucleotide sequences in these regions may influence the gene expression or a sequence of the product. Therefore the importance of milk production effect and administration of growth hormone (GH) on cattle production, prompted us to examine the presence of restriction fragment length polymorphism at the GH gene as a genetic marker information. A total of 257 Holstein cows from four herds in Isfahan province were genotyped. Two variants of bovine growth hormone gene, L and V, were identified with Alu- I enzyme. The frequency of L and V allele was 0.74 and 0.26, respectively. Distribution of genotypes was 0.541 for LL , 0.401 for LV and 0.058 for VV genotypes. The studied populations were in Hardy-Weinberg equilibrium. The SAS software was used for this analysis. The formula of general linear model (GLM) included the effects of growth hormone genotype (LL, LV , and VV) and herd (n=1 - 4). The statistically significant differences between individuals of different herds were found at birth weight, gestation length, milk, fat and protein yield. On the basis of the presented significance levels, GH genotype affected gestation length significantly. According to the results of the present study and earlier results of other, it might be concluded that highly strong relationship between GH genotype and milk production or reproduction traits could not be detected. The stimulated growth hormone release is heritable in dairy cattle and is affected by the Leu/Val polymorphism in the GH gene. Hence, they may be useful physiological indicators for selection on milk production.
