During the last decades, strong genetic progress achieved in milk yield led to decreased reproductive performance in dairy cows. Declined fertility is believed to result from the negative energy balance (NEB), caused by high milk production combined with limited feed intake in early lactation. Recently, much effort has been devoted to understand the role of the leptin and its receptor in regulating food intake and reproduction in ruminants. Leptin is secreted by adipose tissues and acts especially through its receptor on the hypothalamus, the center of energy homeostasis, as well as on ovarian follicular cells, on placenta and mammary glands. Genes encoding leptin (LEP) and leptin receptor (LEPR) are located in bovine chromosomes 4 and 3, respectively. Inside the LEPR gene, discover a missense mutation T945M. The C to T is located in exon 20 at position 115, and causes the threonine to methionine amino acid substitution in the intracellular domain of the LEPR-b isoform, at residue 945. It might be responsible for the efficiency of the signaling pathway in cells and, as a consequence, for the modulation of the leptin effects. The objectives of this study were to estimate the allelic and genotypic frequencies of leptin receptor gene in Holstein cows and also evaluating the association between gene polymorphism and the milk and reproductive traits. For this purpose, blood samples were collected from 395 cows of five Holstein dairy herds in Isfahan province. Number of Samples varied from 64 to 96 among the herds. DNA was extracted using salting out procedure. The DNA quality and quantity were determined using electrophoresis. The 400 bp fragment of the gene was amplified by specific primers and Thermocycler. PCR products were detected by electrophoresis on 1%TAE agarose gel.Amplified fragment digested via TaqI restriction enzyme in 8 to 14 hours at 37 ? C. The genotype Frequencies of BB, PB and PP were 0.11, 0.49 and 0.40, respectively. The frequencies of alleles P and B were 0.648 and 0.352, respectively. The genotypes were distributed according to the Hardy-Weinberg equilibrium. Shannon’s and Nei’s diversity indices showed that gene diversity is the same in all herds. The analysis by GLM procedure (SAS software) did not reveal any significant effect of the T945M polymorphism on adjusted 305-day milk, fat percentage, protein percentages, lactation length and conception rate (P 0.05). Key words: Leptin receptor gene, Polymorphism, Performance traits, Dairy cows.
