Students should be able to describe the development of our understanding of genetics including the work of Mendel, and understand why the importance of Mendel's discovery was not recognised until after his death. • In the mid-19th century Gregor Mendel carried out breeding experiments on plants. One of his observations was that the inheritance of each characteristic is determined by 'units' that are passed on to descendants unchanged. • In the late 19th century behaviour of chromosomes during cell division was observed. • In the early 20th century it was observed that chromosomes and Mendel's 'units' behaved in similar ways. This led to the idea that the 'units', now called genes, were located on chromosomes. • In the mid-20th century the structure of DNA was determined and the mechanism of gene function worked out. • This scientific work by many scientists led to the gene theory being developed.