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THE NEW INDIVIDUAL 



Part IV 



in many plants and animals, so frequently in some that they are known as 

 mutating species. In recent years, a thousand or more have been found in 

 fruit flies. Many times that number were examined without discovering a 

 structure suspected of being a mutation. And when some new feature was 

 found, the fly had to be bred and several generations produced in order to 

 show whether or not the new feature was inherited. Fortunately, fruit flies 

 mature and breed quickly. Their lifetime in days is about the same as the 

 human lifetime in years. In 1927, H. J. Muller discovered that if fruit flies 

 were exposed to x-rays, the mutations would occur about 150 times more 

 often than naturally; later treatment with radium increased them to 200 times 

 (Fig. 20.20). The effect of the radiation suggested that mutations might 

 be induced by cosmic rays. Fruit flies were taken to mountain tops where 

 such radiation is more intense and mutations were speeded up. In later ex- 

 periments, mutations were produced by certain extremes of temperature, by 

 chemical substances, and by other influences inside and outside the flies. 

 Almost every type of mutation found in nature has been induced in them 

 experimentally, and some once believed to be unique results of experiments 

 have been discovered in wild flies. Changes in the genes have gone on through 

 millions of years of evolution as they are continuing quietly now. 



Frequency of Natural Mutations. Mutations in any one gene are rare, 

 estimated about one in 50,000 generations. The rate varies in different genes. 

 It is also estimated that a mutated gene occurs in every ten human sperms 

 and eggs. This seeming contradiction disappears when it is remembered that 



Fig. 20.19. Mutations for lack of pigment. Albino twins, without pigment in 

 hair, eyes and skin, a recessive mutation in a pair of identical twins. (From Rife, 

 Schonfeld, and Humstead in Journal of Heredity.) 



