Addition of each chromosome of Brassica oleracea to the chromosome complement of B. rapa results in a series of nine monosomic alien addition lines. Heneen et al. report the development and use of such a series for mapping genes, determining chromosome homoeology, and for assigning genetic linkage groups to specific chromosomes. Seven B. oleracea chromosomes carry major or minor genes controlling seed colour either maternally or through the embryo. Chromosome homoeology prevails largely between chromosomes occupying similar karyotypic positions. The two species are the progenitors of oilseed rape, B. napus, and the results have potential applications in marker-assisted selection and breeding for yellow seeds.