20 BREEDING CROP PLANTS 



The Inheritance Factors. The inherited character is con- 

 sidered to be the result of certain definite factors which are 

 located in the chromosome. Moreover, a factor is considered to 

 be located at a certain definite place in the chromosome. After 

 the rediscovery of Mendel's law in 1900, numerous crosses were 

 studied. In many cases the inheritance of each differential 

 character in which the parents differed was easily explained by 

 the hypothesis that one parent contained a genetic factor for the 

 development of the character and that the other parent lacked 

 this factor. This led to the erroneous conception that many 

 characters were dependent on a single factor for their develop- 

 ment. That this is not so may be easily seen if one considers 

 that each character is a part of the physiological complex which 

 goes to make up an organism. Thus, many genetic factors play 

 a part in the development of the character. When a cross shows 

 that two parents differ by only a single factor this does not mean 

 that only a single factor is necessary for the development of a 

 character. It does mean, however, that a single factor of in- 

 heritance may cause a profound change in the expression of the 

 character of an organism. Some crosses show that many factors 

 play a part in the development of a single character. The present 

 view is that a character is usually the result of the interaction 

 of several factors. When a plant breeds true for a particular 

 character each gamete produced contains all factors necessary 

 for the development of the character. Before considering the 

 results of certain crosses it. will be desirable to review briefly 

 the subject of variation. 



Variability of Characters. It is commonly recognized that 

 no two plants or animals are exactly alike. These differences are 

 called variations. Various means of classifying variations have 

 been used. From the standpoint of the plant breeder variations 

 are of two kinds: (1) non-heritable, (2) heritable. 



Non-heritable variations are those which are solely due to 

 some difference or differences in the environmental conditions 

 under which the plants develop, while heritable variations are due 

 to some difference or differences in the hereditary characters 

 of the organisms. 



Several illustrations may help to make clear what is meant 

 by non-heritable variations. Baur (1914) cites races of Primula 

 sinensis which under normal conditions breed constanlly true 

 for red and white flowers respectively. If the red race is placed 



