In this article, we discuss the features of the genetic determination of a continuous quantitative trait, the mass of newborn offspring in populations of the domestic pig. We defined several components that determine the phenotypic trait, such as the maternal effect, complete dominance, interaction of the parental alleles in the genotype of the offspring, and the uncertainty of phenotypic realization of genotype. We found that a phenotypic trait of high genetic determinacy can also have a maximum range in phenotypic realization, in which case each genotype encountered in the population can realize within the entire range of possible phenotypes.