Heterosis describes the superior performance of heterozygous hybrids compared to their genetically diverse parental inbred lines. Heterosis is extensively exploited in agriculture for more than 100 years. Nevertheless, the underlying molecular basis remains largely enigmatic. Heterosis is not only manifested in adult plants in traits like plant height or yield but can already be observed during embryo and early seedling development. The classical dominance model of heterosis explains the superiority of hybrid plants by the complementation of deleterious parental alleles by superior alleles of the second parent. Genes active in one inbred line and the hybrid but inactive in another represent an extreme instance of allelic diversity defined as single parent expression. Thus far we were able to demonstrate that a large set of such genes consistently display expression complementation in a diverse set of maize hybrids in different tissues and along different developmental stages of primary roots. As a consequence of expression complementation, hundreds of genes are additionally active in hybrids.