Frequency of Gene Mutations 235 



Somatic mutations, whether they are bud sports in which the 

 whole bud is mutant tissue or are chimeras, are produced by the 

 same causes that produce mutations in general. A small per- 

 centage may be the result of recombination resulting from so- 

 matic crossing over (fruit flies with small patches of yellow and 

 singed tissue in an otherwise normal body are a form of chi- 

 mera) ; a very large number may arise from chromosomal aber- 

 rations of various kinds; many are due to gene mutations. 



Somatic mutations in animals are lost with the death of the 

 individual in which they occur, but bud mutations in many plants 

 may be preserved indefinitely by means of vegetative reproduc- 

 tion and sometimes by seeds produced by the mutated branch. 

 An excellent example is the bud mutation of the peach which 

 produces the smooth-skin type known as the nectarine. The 

 nectarine behaves as a recessive when crossed with the peach; 

 but peach trees occasionally produce nectarine bud sports, and 

 occasional bud mutations on a nectarine tree result in branches 

 which bear peaches. 



In some plants, propagation is carried out chiefly by bud- 

 ding or grafting. In such plants, bud sports are propagated by 

 inserting buds from the mutated branch into other trees which 

 are used as a stock. Bud mutations from the Washington navel 

 orange are perpetuated in this way. A variety that often arises 

 as a bud mutation is the Thomson strain, but these trees fre- 

 quently produce one or more fruits of the Washington strain, 

 showing that bud mutations in these varieties are produced in 

 both directions. 



The occurrence of the same somatic mutation at various stages 

 in the development of a bud or shoot is also illustrated by the 

 navel orange. If the mutation occurs early in the development 

 of the bud, the whole limb will be of a mutant nature and will 

 bear the mutant type of fruit. If, however, the mutation occurs 

 in the cells that produce merely a single fruit, just one fruit on 

 the limb will be of the mutant type. 



Frequency of Gene Mutations 



The problem of the frequency of gene mutations is not an 

 easy one to solve. With wild populations it is complicated by 

 natural selection, for harmful mutations would tend to be elim- 

 inated in competition with the nonmutant, better-adapted genes. 



