CHAPTER II 

 PLANT GENETICS 1 



Since the rediscovery of Mendel's law in 1900 there has been 

 an intensive study of the laws of inheritance through experimental 

 breeding and other means. This has resulted in the develop- 

 ment of a new biological science which is called Genetics. A 

 knowledge of the principles of this science is a necessity if the 

 student of crop breeding is to pursue his work in the most logical 

 manner. The writers, therefore, believe that a study of genetics 

 should precede plant breeding. There are, however, many 

 people interested in crop improvement who have not had an 

 opportunity to pursue an intensive study of genetics. For this 

 reason it seems advisable to present genetic principles in some 

 detail. 



Methods of Studying Inheritance of Characters. The charac- 

 ters of a plant are those qualities which serve to identify it. 

 They are the means whereby one variety is differentiated from 

 another. The production of a variety with only desirable 

 characters is the main aim of the breeder. It is a commonly 

 accepted fact among geneticists that Mendel's law may be 

 used to explain the inheritance of nearly all plant and animal 

 characters. The character is considered to be the end result of 

 the interaction of certain inherited factors which are located 

 in the germ cells; these factors under favorable environmental 

 conditions cause the production of the character. Thus environ- 

 ment and heredity both play important roles in character 

 development. The laws of inheritance have been developed 

 mainly by controlled crosses between parents of known in- 

 heritance. By correlating the facts of character transmission 

 from parent to offspring with known facts of cytology, an 

 idea of the mechanism of heredity has been obtained. Before 

 presenting a description of the factorial scheme which has been 



1 In preparing this chapter other works on genetics have been freely used. 

 BABCOCK and CLAUSEN (1918) and EAST and JONES (1919) have been par- 

 ticularly helpful. 



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