Abstract
Factor XI deficiency is a hereditary disease with an autosomal recessive mode of inheritance. The disorder is characterized by extended bleeding time and predisposition to various hemorrhagic conditions in affected animals. Despite the economic importance of the Holstein Friesian breed in dairy cattle breeding, the prevalence of the F11 gene mutation is still neglected by many farmers. The project aims to determine the frequency of the mutation in exon 12 of the F11 gene in 114 Holstein Friesian cows reared in private farms in Bulgaria. The PCR analyses confirmed that 104 animals were homozygous normal (91.23%), 9 were identified as heterozygous carriers (7.89%) and 1 was characterized as homozygous affected (0.88%). The calculated allele frequency of the mutant allele is 0.0482. Clinical data confirmed the association between homozygosity and reduced health status. The results obtained are consistent with reports from other countries and emphasize the need for genetic screening and selection control to limit the spread of the disease in dairy cattle.