Genetic testing has becoming more common among physicians. Until recently gynecologists and oncologists were more interested in genetic testing, but nowadays genetic testing has spread to the sphere of activities of dermatologists, cosmetologists, nutritionists, as well as introduced into practice a new specialty — a doctor of preventive medicine. At the end of the institute, the idea of medical genetics was exclusively in the aspect of hereditary (often chromosomal) diseases. This knowledge is not enough to understand the results of genetic testing. This review presents the basic concepts of medical genetics needed to provide minimal understanding of how genetic tests work. Material carrier of genetic information is DNA, the gene is a section of DNA, consisting of their sequences of nucleotides. DNA molecules are packaged in chromosomes, existing in pairs in all eukaryotes. For this reason, when talking about genotype, we are talking about alleles of genes (genes located in identical loci of paired chromosomes) and the body can be homozygous or heterozygous, depending on its existing alleles. Despite the presence of DNA polymerase (correcting errors in DNA replication), mutations occur, although quite rarely. Separately, genetic polymorphisms — their main difference from mutations — in the frequency of occurrence — at least 1% of each allele. One of the great interests in medicine are single-nucleotide polymorphisms that influence the development of multifactorial diseases. Evaluating the results of genetic tests can predict the risks of multifactorial disease, as well as reduce these risks by conducting preventive measures.