Genetic markers in various forms of their manifestation, both at the phenotypic and at the biochemical or molecular levels, are a tool for establishing genetic linkage, including when building genetic maps and/or assessing genetic diversity, including the “genotype-environment” interaction [1]. In general, today there are three main areas of application of genetic markers. First, markers can provide a new, expanded and enhan-ced picture of the genetic diversity that exists between and within species. This infor-mation is of particular interest for the management of plant genetic resources (PGR) and for the implementation of the rational use of PGR in breeding programs. Secondly, markers make it possible to construct genetic maps that allow one to localize and iden-tify loci of quantitative and/or qualitative traits, as well as to establish the effects of the action or interaction of loci of these traits. And finally, thirdly, markers make it possible to establish „marker-trait“ associations and carry out associative mapping, which gives an undoubted advantage due to the effective screening of natural and artificial breeding populations that are of practical interest for breeding.  Along with classes and categories, genetic markers can be divided into three types depending on the nature of the genetic linkage to the target gene or chromosomal locus. If there are identified markers that are physically located next to the gene of interest to the researcher or even directly in the desired gene, it becomes possible to carry out the so-called marker-assisted selection (MAS), i.e. select identified marker variants (alle-les) in order to select the necessary unidentified variants of the desired gene. Based on the relationship of a genetic marker with the QTL or gene of interest to the researcher, three main types or varieties of polymorphic genetic markers can be distinguished [3]. (I) Direct markers (direct, D) mark loci encoding various kinds of functional genes or mutations. Such markers, if molecular, are located directly in the desired gene. In this case, we can speak of gene assisted selection (GAS). This is the most favorable situ-ation for MAS, since the inheritance of marker alleles is directly related to the alleles of the desired gene or QTL. However, this type of marker is the rarest (in the physical sense, it is not common enough in the genome) and therefore it is the most difficult to find. An exception is morphological and biochemical markers, and only if they are genes of direct interest to the researcher, and not genetic markers linked to other genes that the researcher needs. (II) Markers of disequilibrium linkage (linkage disequili-brium, LD). They mark chromosomal loci or genes that are in a population in linkage disequilibrium with a functional gene or mutation (morphological markers), the bio-chemical product of a marker gene (biochemical markers), or with a DNA marker (mo-lecular markers). In fact, linkage disequilibrium is the tendency of some combinations of alleles to be inherited together. In a population, LD can be found if the markers and desired genes are physically very close to each other and/or when lines or breeds have crossed in recent generations. A special case is quasi-coupling. It can also be attributed to non-equilibrium linkage and is equally applied in breeding practice. Selection using an LD marker variety may be referred to as LD marker assisted selection (LD-MAS). (III)    Linkage equilibrium markers (LE). They mark loci that are in a population in equilibrium linkage with a functional gene or mutation (morphological markers), the biochemical product of a marker gene (biochemical markers), or with a DNA marker (molecular markers). In this case, linkage is a random event and the genetic marker is not in linkage disequilibrium with the gene of interest to the researcher in the entire population. Selection using this variety of markers can be called LE marker assisted selection (LE-MAS). This is the most difficult situation for application in genetics and practical breeding. At the same time, LE markers can be easily identified based on genome-wide analysis. For this, certain breeding crosses or specially created families (lines) that are strictly related to each other, for example, mapping populations, are usu-ally used. Such a genome scan requires only saturated marker maps with an interval be-tween molecular markers from 15 to 50 cM, depending on the marker informativity and the economic cost of genotyping [4]. This is usually sufficient to identify the majority of QTLs with a medium or main effect of the action. However, a distance of 15–50 cm is insufficient for the marker and the desired gene to be in linkage disequilibrium. LD markers must be located in the vicinity of the gene of interest for a valid, population-wide, linkage disequilibrium between the marker and an existing QTL or gene (usually 1 to 5 cM, up to a maximum of 10 cM in plants, depending on the LD limits, which, in in turn, depend on the structure of the population and its history). LD markers can be identified through the use of candidate genes or precise mapping techniques [2]. D-    markers (i.e., polymorphisms determined by functional genes or mutations) are the most difficult to find, because their causality or conditioning is difficult to verify or establish. As a result, only a limited number of examples of the detection of D markers are known to date, with the exception of traits encoded by a single gene. As a rule, such traits are genetically very simple. For example, many traits of disease resistance in plants, or traits related to timing of flowering, are controlled by one or two to three genes. At the same time, most economically valuable traits are quantitative. They are determined by a genetic complex that includes not one or three, but many genes (the so-called QTLs), which, moreover, is often influenced by the environment. If a certain molecular D-marker is used to screen genetic samples that contain a gene labeled with this D-marker that the researcher needs, then due to the degeneracy of the genetic code, mutations and/or recombination, such a molecular D-marker may “not work” on the DNA of the desired sample with the desired gene. The use of LD markers is more pre-ferable, since they do not have a rigid “binding” to the genes or QTLs they mark, which are numerous and, moreover, mutually replaceable and complementary. Simply put, if during the screening of genetic breeding material any LD marker turns out to be unsu-itable or “does not work” like a D-marker, then it can be easily replaced or replenished with another LD marker or markers located nearby, which cannot be done in case of a molecular D-marker. All three varieties of markers differ not only in the methods of their determination, but also in how and where they are used in genetic breeding programs. It should be noted that the above types of markers are predominantly molecular markers. However, phenotypic and biochemical genetic markers can also act as the indicated types of mar-kers, but only in those cases if they meet the above requirements for this type of mar-kers. While LD markers and, to a lesser extent, D markers allow genotypic selection in the entire population due to the strong relationship between genotype and phenotype, LE markers from line to line must belong to different phases of genetic linkage between markers and QTLs. or genes (table). Thus, the ease and possibility of using markers in genetic breeding research is inversely proportional to the ease of their detection, and this correlation increases from D markers to LD markers and further to LE markers.