###### Abstract
SCA can be used to produce plants with more stable growth and development, which affects yield uniformity. The ability to predict the value and relevance of heterosis and SCA effects could shorten the breeding process; in some cases even by several years. This would translate into obvious economic benefits, among others. One possibility for such a potential prediction of heterosis and SCA effects is selection of parental forms for crosses. We can perform this selection based on the diversity of parental forms: phenotypic or genetic [10; 11; 12; 13; 14]. Phenotypic variation depends on: the measure of its estimation used, environmental conditions, selection/choice of observed traits, and the number of traits analyzed. Environmental conditions appear to be the factor that most significantly determines the responses of genotypes to observed traits. [PERSON] et al. [15], studying winter wheat cultivars infected with _Fusarium graminearum_, observed very different responses of two pairs of cultivars, Success and Rywalka, as well as Fineja and Soraja, in two localities: Cerekwica and Sitaniec. Genotype is independent of environmental conditions, hence when assessing genetic diversity, this problem is eliminated. Genetic diversity, on the other hand, depends on the number of markers from which it is estimated. Recently, with the very large number of markers available, this problem seems less significant [16; 17]. In addition, values of genetic similarity/diversity depend on the measure used to estimate it. Relationships between phenotypic and genetic diversity of eighteen parental lines of F1 hybrids of winter oilseed rape based on the CMS ogura hybridization system were examined by [PERSON] et al. [18]. The conclusion of this study was that genetic divergence is not the same as phenotypic divergence [18]. Considering the above, it may be interesting to answer the question: does the diversity of parental forms affect the values of the heterosis effect and the SCA effect? If there is such an influence, then another question can be posed: what kind of influence is it? To answer these two questions, publicly available data were analyzed in the context of the influence of diversity of parental forms on the heterosis effect and specific combining ability. Accordingly, the research hypothesis was that phenotypic and genetic variations in parental forms affect the values of heterosis and specific combining ability effects of hybrids obtained from crossing these parental forms. Studies to date tend to focus on analyzing a single population and evaluating breeding parameters such as general combining ability, SCA, and heterosis [19]. Less often, phenotypic or genetic variation of parental forms is analyzed in the context of evaluating hybrids obtained by crossing these parental forms [20; 21; 22; 23; 24]. Analyses of the determination of the differentiation of parental forms in terms of the values of breeding parameters of hybrids are not usually undertaken. In order to fill the gap in the literature on the subject, we investigated whether such an influence exists and, if so, what direction it takes. The aim of this study was to analyze the effect of the diversity of parental forms on the values of breeding parameters, i.e., the heterosis effect and the SCA effect. The variation of parental forms was considered at two levels: phenotypic (based on Mahalanobis distance) and genetic (based on observations of molecular markers).