Mixed models have been extended to make them applicable to description of inheritance of quantitative traits in pedigrees obtained via interbreed or interpopulation hybridization. Additional assumptions specify definite relationships between the parameters that characterize the inheritance of the trait in two parental populations. It is assumed that (1) parental populations differ from each other only in the allelic frequencies in the major gene locus and polygenic loci and (2) the patterns of phenotype formation based on an individual genotype and the transmission of genes to the next generation are the same in both parental populations, as well as in their hybrids. Based on these assumptions, the likelihood function for a hybrid pedigree is derived, and the heterosis effect is formalized. The results of this study offer new possibilities for estimating the contribution of the major gene effect to interpopulation and interbreed differences in the expression of quantitative traits.