At the current stage of genetic studies of cattle, more and more attention is being drawn to autochthonous breeds. Native cattle have a number of prominent phenotypic traits and have preserved unique genes and their combinations lost by modern commercial breeds, which would be valuable to use in selective programs. We surveyed polymorphism of the Ukrainian autochthonous Gray breed according to alleles of exon 2 of the BoLA-DRB3 gene. The uniqueness of the gene lies in the broad variability of its allele variants. Significant informativeness at DNA level is quite important for genetic studies. We surveyed allele polymorphism using the PCR-RLFP method on DNA isolated from 88 samples of blood of cows and 5 samples of sperm. We identified 28 alleles, of which 23 variants were nomenclature ones and 5 (jba, *jab, *jbb, *nad and *nda) were “without established nomenclature”, their share accounting for 8.9%. Four alleles *06, *12, *16 and *jba had a frequency above 5% and occupied 69.9% of the breed’s allele fund overall. The commonest allele was BoLA-DRB3.2*16 (44.1%). In total, we found 40 genotypes. Considering the significant dominance of variant *16, as expected, 5 genotypes with its inclusion occurred: *16/*16, *12/*16, *06/*16, *16/*24 and *jba/*16. It was present in the genotype of two out three studied animals. Parameters of heterozygosity, effective number of alleles, Shannon and Pielou indices indicate that Ukrainian Gray cattle are characterized by lowest level of genetic variability and biodiversity according to the BoLA-DRB3 gene compared with other breeds. Due to significant dominance of allele *16, the breed has no inbred motifs. We noted deviation toward increase in homozygosity without deviations from the norm of the distribution according to Hardy-Weinberg equilibrium. The obtained results will be used for genetic-populational programs with the purpose of improving the genetic potential of cattle breeds in terms of economically beneficial traits and diseases of cattle.

Mastitis in cows is an important problem in the milk industry. Major histocompatibility complex (MHC), also called bovine lymphocyte antigen (BoLA), is attracting attention due to its association with host immunity. The BoLA system has some equally operating genes that provide antigen presentation by MHC system molecules followed by an immune response to pathogens. Exon 2 of the BoLA-DRB3 gene is the most important and highly polymorphic. Alleles that had a close connection with mastitis have been detected and are considered DNA markers. These play a decisive role in the breeding of cattle to create herds resistant to diseases. Polymorphism of the BoLA-DRB3 gene (exon 2) of the White-Headed Ukrainian cattle breed was studied by PCR-RFLP to search for DNA markers associated with mastitis. In the general sample and group of resistance cows, 28 alleles were found, and in the group of animals prone to mastitis -21 BoLA-DRB3.2 alleles. The most common variant was BoLA-DRB3.2*24 (12.3%). It also dominated among cows susceptible to mastitis (25.9%). Allele *22 (13%) maximally showed the animal's resistance to the disease. Based on relative risk, it has been reliably established that there are at least two mastitis-sensitive alleles: variant BoLA-DRB3.2*22 (p<0.05) associated with resistance, and *24 -with a susceptibility to the disease (p<0.001). It was also established that the genotype DRB3.2*11/*24 (p<0.01) can be used as a DNA marker of mastitis resistance after additional verification. The obtained results will be useful in the formation of herds of dairy cows resistant to mastitis.

Molecular genetic markers can detect polymorphism at the DNA level. This feature determines the possibility of their widespread use in genetics and breeding. Alleles of the BoLA-DRB3 gene (exon 2) can act as such markers if a statically significant association between the disease and the allele is established. The presence of such DNA markers in the genotype of animals makes it possible to judge the likelihood of disease in postnatal ontogenesis immediately after the birth of a heifer, based on which we can conclude about the conditions of further use of the animal in the main herd. According to the results of studying the polymorphism of the BoLA-DRB3 gene in cows of the Ukrainian black and white dairy breed resistant and susceptible to necrobacteriosis, four "informative" alleles were revealed. Two of them *03 and *22 are associated with resistance, and the other two - *16 and *23 with susceptibility to necrobacteriosis. The presence of these alleles in the genotype of the animal is determined by testing performed by PCR-RFLP. The method is time consuming, labor intensive and costly. To simplify it, the following technique is proposed. Restriction fragments of alleles *03, *16, *22 and *23 for endocluases RsaI, XhoII and HaeIII have the following DNA patterns: bbb, jbd, mba and nba. Due to the peculiarity of the restriction fragments, which is that endonuclease XhoII reveals in these alleles only one pattern b with length of 284 bp, the process of determining informative alleles can be simplified. Isolation of DNA from blood samples and amplification of a fragment of the BoLA-DRB3.2 gene with a size of 284 bp is carried out according to the established technique. Next, the restriction of the fragment by endonuclease XhoII and sampling having a pattern b. Selected samples are treated with RsaI endonuclease and only those with patterns b, j, m and n remain. The next step is to restrict the selected samples with HaeIII endonuclease and select heifers with bbb (*03) and nba (*23) genotypes. After the first restriction, blood samples without pattern b are eliminated from the experimental sample; after the second – two alleles with patterns RsaI + XhoII jb (*16) and mb (*22) are unambiguously determined, after the third – genotypes bbb and nba, which correspond to alleles *03 and *23. In total, only 75% of blood samples are typed, which reduces the material consumption, time and cost of work to identify heifers genetically susceptible (resistant) to necrobacteriosis.