PLANT GENETICS 21 



in partial shade in the greenhouse under temperatures of 30 to 

 35C. only white flowers are produced. If those same plants 

 are brought into another greenhouse with temperatures of 15 to 

 20C. the flowers which then develop are the normal red color. 

 It is pointed out that what this red primula inherits is not a red 

 flower color but the ability to produce a certain flower color 

 under certain conditions of the environment. Non-inherited 

 variations have no value as a means of producing new varieties or 

 strains. Such variations are, however, of importance to the 

 breeder. For example, a small shriveled seed of wheat has the 

 same inherited characters as a large, plump seed of the same 

 pure line, nevertheless, the seedling produced by the shriveled 

 seed may get an unfavorable start. Familiar examples of non- 

 heritable variations are differences in height of plants, within a 

 variety, which are dependent on differences in food supply, 

 moisture, or sunlight. 



Inherited variations may be placed in two classes: (1) muta- 

 tions, (2) new combinations. 



Mutations are due to a sudden change in the hereditary factors 

 of an organism, or to the loss of a genetic factor. In some cases 

 mutations result from abnormal chromosome behavior during 

 the process of cell division. Before we can discuss profitably 

 the reason why mutations occur it will be necessary to know 

 much more about the nature of hereditary factors than we now 

 do. Mutations are sometimes of much value to the breeder. 

 Examples of mutations of economic importance will be found 

 under a discussion of the breeding of various crops. When a 

 desirable mutation occurs it can be utilized as a means of 

 producing a new race. As there is no known means of artificially 

 inducing mutations, the breeder can not depend on them as a 

 means of producing improved varieties. 



New combinations result from crossing varieties which contain 

 different hereditary factors. The first generation of a cross 

 between homozygous parents which differ in a certain character 

 may resemble either the one or the other parent or may be inter- 

 mediated, but all FI plants will be of like habit. F 2 plants, how- 

 ever, are of different kinds, due to the segregation of hereditary 

 factors in the germ cells of the FI plants. New combinations of 

 factors may occur and thus new individuals may be produced 

 which have some of the characters of the one parent combined 

 with some characters of the other. In some cases characters 



