'Hi 



(iKNETICS IN RELATION TO AGRICULTURE 



fore, greatly matter what methods are adopted so long as they represent 

 clearly and adequately the operation of these principles. It is necessary 

 for the student to familiarize himself with at least some of the more 

 widely employed methods, remembering that with the basic principles 

 clearly in mind, it should be possible to interpret very easily the methods 

 employed in the presentation of any body of Mendelian data. 



For ordinary work it is well to have a definite system of interpreting 

 problems. In the following treatment only cases involving the category of 

 independently segregating pairs of factors are considered, but it should be 



possible to extend the system without great 

 difficulty to other categories as will be pointed 

 out in those sections dealing with such 

 categories. 



When an individual is heterozygous for 

 one pair of factors two types of gametes are 

 possible. If the factors involved are repre- 

 sented by A and a, the genotypic constitution 

 of such an individual is A a, and the two 

 types of gametes are A and a. If an in- 

 dividual is heterozygous for two pairs of 

 factors, its genotypic formula will be AaBb, 

 and there are then possible four different kinds 

 of gametes, namely AB, Ab, aB and ab. As 

 the number of heterozygous factors increases, 

 therefore, the number of possible combinations 

 of factors increases geometrically so that it is 

 necessary to adopt a method of writing 

 down these possible combinations. The 

 method of dichotomy may be used in such 

 cases, and the diagram, Fig. 47, explains its operation without further 

 comment. 



With as few as five pairs of factors, therefore, there are possible no 

 less than thirty-two different kinds of gametes as follows: 



Fio. 47. Method of writing 

 all possible combinations of 

 any number of pairs of factors. 



ABODE 



ABCDe 



ABCdE 



ABCde 



ABcDE 



ABcDe 



ABcdE 



ABcde 



AbCDE 



AbCDe 



AbCdE 



AbCde 



AbcDE 



AbcDe 



AbcdE 



Abode 



aBCDE 



aBCDe 



aBCdE 



aBCde 



aBcDE 



aBcDe 



aBcdE 



aBcde 



abCDE 



abCDe 



abCdE 



abCde 



abcDE 



abcDe 



abcdE 



abode 



By following this method consistently it is possible to write out readily 

 the possible combinations of any series of factors. Fortunately or un- 



