Some interspecific hybrids and allopolyploids show morphological, physiological and molecular traits biased to one progenitor, which is referred to as genome asymmetry or genome dominance, in spite of the inclusion of the genomes of the two or more progenitors in the same nucleus. Besides the occurrence of the expression dominance of the rRNA genes with the B>A>C hierarchy in three cultivated Brassica allotetraploids, the genome asymmetry for their phenotypic features is also obvious, for some traits of Brassica oleracea are strongly expressed and are dominant over those of B. nigra and B. rapa, while those of B. nigra are dominant over those of B. rapa, with the C>B>A hierarchy. As to the phenotypic performance in the intergeneric hybrids and allopolyploids of Brassica species with several relatives including Raphanus sativus, Orychophragmus violaceus and Isatis indigotica, Raphanus sativus and Orychophragmus violaceus are dominant, particularly with their basal serrated leaves being persistently expressed. Notably, these dominant traits are controlled by one or several chromosomes. The phenotypic dominance is possibly related to the structure and nature of the parental genome and gene(s) involved and the interaction between divergent genomes, but the regulation mechanisms remain largely to be elucidated.