Molecular biology techniques are promising help in genetic improvement since they permit the identification, mapping and analysis of polymorphisms of genes encoding proteins that act on metabolic pathways involved in economically interesting traits. The IGF axis, which consists of insulin like growth factors I and II (IGF-I and IGF-II), and their associated binding proteins and receptors (GHRHR, GHR, IGF-IR and IGF-IIR), plays a key role in the metabolism and physiology of mammalian. The insulin-like growth factor-I receptor (IGF-IR) mediates the biological actions of IGF-I and IGF-II. This receptor is structurally similar to the insulin receptor, consisting of two ? and two ? subunits, which combine to form a heterotetramer; IGF binding activates a tyrosine kinase signal pathway. IGF-IR plays an important regulatory function in mammary gland development and milk secretion. IGF-IR gene in cattle was mapped on chromosome 21 The IGF-IR polymorphism is located in a noncoding region (intron) of the IGF-IR. The objectives of the present study were to estimate the allele and genotype frequencies of the IGF-IR gene polymorphism in two genetic groups of dairy cattle and to determine associations between this polymorphism and milk yield and performance or reproductive traits. For this purpose, genotyping was performed on 266 Holstein cows from Bos Taurus descent and 41 Najdi cows from Bos Indicus descent. A 625 bp IGF-1R gene was amplified by PCR using bovine specific primers. RFLPs in this segment was studied using Taq I restriction enzyme. Genotype AA was characterized by the presence of two restriction fragments of 580 and 45 bp. Genotype BB was determined by the presence of three fragments of 410, 170 and 45 bp. Heterozygous individuals (AB) presented four fragments of 580, 410, 170 and 45 bp. Genotype BB was not detected in any of the breeds studied. The frequencies of genotypes AA and AB in Holstein and Najdi cows were 1.0 and 0.0; 0.707 and 0.293, respectively. Frequencies of allele A and B were 1.0 and 0.0 in Holstein and 0.854 and 0.146 in Najdi, respectively. The A allele of IGF-1R appears to be fixed in cattle of Bos Taurus descent. Therefore, this polymorphism is not useful in studies on the identification of quantitative trait locus` in Bos taurus . In this study, traits of interest were analyzed using the GLM procedure of SAS. The results showed no significant associations between genotypes and milk yield and compositions, calving and pregnancy length (P 0.05). Animals carrying the AA genotype presented a longer lactation period (P 0.05) and genotype AB has a heavier birth weight and more dystocia than AA genotype (P 0.01 and P 0.05, respectively). Genotype AA has a more calving interval than genotype AB (P=0.11)
