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On the basis of facts very similar to these Mendel developed his laws of inheritance, and the early geneticists attempted to apply these laws very widely so as to account for and predict the inheritance of a large number of physical characteristics. But soon it was found that the characteristics did not always appear in the offspring in exactly the proportions that were predicted. Three reasons have emerged to account for this fact. In the first place the genes themselves may sometimes change their character (mutate) in an unexpected and unpredictable way, with the result that they adopt a form different from that which, on theory, they ought to have, and thus influence the development of a particular characteristic in a different way. This mutation, which at first was completely inexplicable, is beginning to come under human control. It is at present possible to influence the

mutation rate by subjecting chromosomes to X-ray, or other forms of short wave, bombardment. In this way genes have been turned from the dominant to a recessive form and also from a recessive back to the dominant. Furthermore it is found that the mutation rate seems to increase if the external environmental conditions are made very much less favourable to the organism.