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THE FACTOR HYPOTHESIS 149 



been ascertained within the last decade that some char- 

 acters require more than a single germinal determiner 

 to bring them to somatic expression. The converse 

 is also true, namely, that certain single determiners 

 may control more than one character. For instance, 

 the determiner for gray hair in rats also produces a 

 lighter color on the belly. 



The idea of compound germinal determiners for a 

 single character has been termed the factor hypothesis 

 of heredity. 



Hereditary germinal factors, that may sometimes 

 need to combine in order to produce a visible somatic 

 unit character, are known as genes ( Johannsen). 



2. DIFFERENT KINDS OF GENES 



There are various kinds of genes that bring about the 

 visible expression of unit characters in various ways. 

 An attempt to tabulate the kinds of genes is herewith 

 given. 



SINGLE 



Alternative 



Allelomorphic 

 Presence or absence 

 PLURAL 



Cumulative 

 Modifying 



Complementary 

 Supplementary 

 Lethal. 



When genes are derived from two parents, as in all 

 cases of sexual reproduction, they are always in pairs, 



