Chromosomes were duplicated during cell division, and genes were duplicated as well and thus transmitted from one cell to the next, and from one organism to the next. Chromosomal abnormalities precipitated abnormalities in the growth and development of sea urchins, and so abnormal genes must have been responsible for this dysfunction. In 1915, Morgan proposed a crucial advance to Mendel’s theory of inheritance: genes were borne on chromosomes. It was the transmission of chromosomes during cell division that allowed genes to move from a cell to its progeny.
