The purpose of the master's thesis was to select cndidate genes that would help to develop a genetic test that would allow to detect alleles associated with aggressive behavior in the parent dogs of a planned litter. The genetic test would enable the selection of mating combinations, which would result in good police and military working dogs as well as good service and sports dogs. Several genes and variants were tested, which, according to the literature, are associated with more aggressive dog behavior. We found that the number of dogs with at least one locus containing alleles, which were suggested to be associated with aggressive behavior, was distributed pretty equally, both among representatives of different breeds (Malinois and German Shepherd) as among dogs used for different purposes. We found that the homozygous genotype for the presence of adenosine repeats (polyA) in the intron of the gene SLC6A3 is more common in Malinois. Since such a variant of the gene is associated with more aggressive behavior, it would be reasonable to introduce genetic testing for the presence of adenosine repeats, which would help in proposing the mating combinations of dogs, depending on the purpose for which the dogs will be bred.