Isolation of genomic DNA is one of the basic steps in many different molecular analyses. There are a few reports on methods of DNA isolation from milk, but many of them are time consuming and expensive, and require relatively large volumes of raw milk. In this study a rapid, sensitive, and efficient method of DNA extraction from milk somatic cells of various mammals (cattle, sheep, goats, horses) is presented. It was found that milk is a good source of genomic DNA, and to obtain a sufficient amount and quality of DNA, suitable for molecular analysis such as PCR, 10 mL of raw milk is sufficient. Thanks to this method, stress in animals can be reduced during collection of researched material. Therefore, this method could be widely used in molecular analyses.

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Bovine mastitis is a widespread disease of the mammary gland, highly contributing to the increase in veterinary costs in dairy industry. In the present study, the genetic polymorphism within bovine L-selectin gene was analysed and its impact on clinical mastitis occurrence, somatic cell score (SCS), and milk production traits in Polish Holstein-Friesian cows was examined. Polymorphism within L-selectin gene, molecule responsible for neutrophil attachment to endothelium, might have a potential role in immune response to bacterial infections and udder health. Two hundred and six Polish Holstein-Friesian cows were genotyped by polymerase chain reaction-restriction fragment length polymorphism method. Two single nucleotide polymorphisms mutations within the coding sequence of L-selectin gene were identified (c.165G>A and c.567C>T). The effect of c.165G>A and c.567C>T mutations on SCS was highly significant (P = 0.0019 and P = 0.0003, respectively). Strong associations (P ≤ 0.0001) were also observed between L-selectin polymorphism and milk production traits (milk yield, milk fat percentage, and milk protein percentage). However, the polymorphism in the analysed gene had no influence on the resistance or susceptibility of cows to clinical mastitis (only the tendency toward significance, P = 0.06 for c.567C>T mutation was found). Potential exploitation of the information on the identified associations in genetic selection needs to confirm the obtained results in further investigations.

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The major histocompatibility complex in cattle (BoLA) is regulated by genes that are closely related to the development of the immunological response to pathogens. The most polymorphic BoLA-DRB3.2 locus was analysed in 209 black-and-white Holstein-Friesian cows in Poland in order to a better explanation of influence of MHC on immunity to diseases in dairy cattle. A total of 23 alleles were identified, among which the *24, *16 and *22 alleles were observed with the highest frequency. These alleles were analysed in terms of their association with the occurrence of mastitis, ovarian cysts, retained placenta and uterine abscesses as well as their contribution to production traits (milk yield, protein and fat percentage in milk). It was determined that the BoLA-DRB3.2 *22 and *16 alleles were associated with a lower risk of clinical mastitis; however, a statistical significance was observed only for the *22 allele. Clinical mastitis was observed at a frequency lower by 8% in cows with one copy of the *22 allele compared to cows with 0 copies of the allele. The presence of the *22 allele in the genotype was also associated with higher milk yield, although this association was not statistically significant.