234 The Nature of Gene Mutations 



the zygote, the whole animal or plant sporophyte would have 

 the mutant gene and would show that mutant character if it was 

 a dominant mutation. If, however, the mutation occurred in 

 one of the two cells that result from the first division of the 

 zygote, only half the organism would have the new gene. If the 

 mutation occurred late in somatic development, only those cells 

 derived by division from the mutant cell would have the mu- 

 tant gene; one or more patches of mutant tissue would be found 

 in the body of the individual, if it was a dominant mutation or 

 if it was a mutation in the X chromosome or Z chromosome of 

 the heterogametic sex. The number of patches would depend 

 upon the number of cells of the immature organism in which the 

 mutation occurred and the size of the patches would depend 

 upon the stage of development of the individual in which the 

 mutation arose. 



Although mutations have been observed at various stages of 

 development, they seem to occur with greater frequency at some 

 times than at others. The time of greatest frequency is just 

 before or during meiosis, apparently in both plants and animals. 

 Evidence is based on the fact that dominant mutations and 

 mutations that occur in the X or Z chromosomes (that is, muta- 

 tions that can be detected in the immediate offspring) appear 

 generally in only one individual. If the mutation had occurred 

 early in germinal tissue, a much larger number of mutated 

 gametes and therefore of mutant phenotypes would be expected. 



Bud Mutations 



One class of somatic mutations in plants is of considerable 

 importance from an agricultural and horticultural point of view. 

 This type, known as a bud mutation or bud sport, occurs in the 

 meristematic tissue of a bud. If the mutation occurs in the earli- 

 est stages of bud development, all or practically all the cells of the 

 bud will be mutant in nature; and when the bud develops into a 

 shoot, all the cells of the shoot will be of the mutant type. If 

 the mutation occurs later in development, only some of the cells 

 will be mutant. The bud will therefore be part mutant and 

 part nonmutant, and the shoot which arises from such a bud will 

 likewise be made up of two kinds of tissue, mutant and non- 

 mutant. A shoot which is thus a mixture of two or more geno- 

 typically different tissues is a chimera. 



