20 S1 S1Plant cellular organization and genetic structure: an overview S1.1 The unit of organization of living things S1.2 Levels of eukaryotic organization S1.3 The plant cell and tissue S1.4 Plant genome S1.5 Chromosomes and nuclear division S1.6 Genetic linkage and its implications S1.7 Chromosome mapping S1.8 Penetrance and expressivity