The actions of chromosomes during mitosis, meiosis and fertilization, as discovered in 19th C. Germany, are brought to life using entertaining animation and state-of-the-art microscopic footage. These microscopic findings, together with the rediscovery of Mendel's long lost research in 1900, gave birth to the Chromosomal Theory of Heredity. We next visit Thomas Hunt Morgan's 'fly room' lab at Columbia University, where in the early 20th C. attention first turned to the gene. Sex chromosomes, X-linked recessive traits, linkages, cross-overs and gene mapping are also entertainingly explained. The eugenics movement coincided with this era and it is recounted using rare archival photos and footage.