Cuenot (1905), a frech geneticist, studied the inheritance of mouse body colour and discovered lethality in mice. He noted that the yellow body colour is controlled by a dominant gene but the yellow mice are never true breeding. When a cross is made between two yellow mice, yellow and black offsprings are produced in a ratio of 2:1. A back cross of yellow to black revealed that the yelllow mice are always heterozygous for the yellow gene and that individuals homozygous for the yellow gene cannot be found. This is because yellow has a dominant phenotypic efffect and in homozygous condition the gene is lethal and the mice bearing such genes die in vitro.