Alpha-1-antitrypsin (AAT) as a strong protease inhibitor plays a major role in the protection of tissues against proteolytic destruction by neutrophil elastase. Existence of this protein in mammary gland may increase the survival of milk proteins such as lactoferrin and lysozyme. The biological role of AAT in tissues like mammary gland, and results from previous quantitative trait loci (QTL) studies has provoked the investigation of the AAT as a candidate gene influencing milk production traits. The present study assessed the association of a single nucleotide polymorphism (SNP) at position 5504 of the AAT gene with 305-day milk yield, milk fat and protein percentage and somatic cell score (SCS) in 408 Iranian Holstein cows. Polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) technique was used for genotyping. The effect of the genotype on the traits of interest was analyzed using the general linear model (GLM) procedure of SAS software. The frequencies of alleles A and B were 0.633 and 0.367, respectively, and genotypes showed a significant deviation from Hardy-Weinberg equilibrium. Results of association analysis showed a significant relationship between the SNP in the AAT gene and fat and protein percentage. Cows of genotype AB had higher milk fat percentage than those of genotype AA. Cows with genotype AA showed lower milk protein percentage compared to those carrying genotype AB. Considering the association of this polymorphism with fat and protein percentage, the SNP has potential to be implemented as a marker in breeding programs for these traits.

Key words: AAT gene, SNPs, Candidate gene, Dairy cattle, Milk-related traits