150 GENETICS 



that is, one from each parent, and in the production of 

 a unit character they may act in single or in several 

 pairs. 



If a single pair, the genes may be interpreted accord- 

 ing to either the allelomorphic or the presence-or- 

 absence hypothesis. In the first instance it is either 

 one thing or an alternative that produces the charac- 

 ter. For example, as in the case of the pea-vine, it is 

 either tallness or dwarfness. In the second instance, 

 the determiner of the character either is present or 

 it is not, and the resulting unit character is dependent 

 upon which of these two possibilities obtains. That is, 

 applied to the illustration just given, if the hereditary 

 factor or gene for tallness is present the pea-vine will 

 be tall but if there is no gene for tallness the plant 

 will be a dwarf. This condition is expressed by the term f 

 alternative genes and the operation of alternative 

 genes follows in the typical Mendelian fashion described 

 in Chapter V. 



Under various kinds of plural determiners which in- 

 volve more than one pair of genes, cumulative genes 

 are those that are all alike in their separate effects 

 but which, acting together, alter the degree of expres- 

 sion that is given to the unit character. These will be 

 more fully described in Chapter VIII upon "Blending ' 

 Inheritance." 



Modifying genes are those germinal factors that are 

 without effect alone but which in conjunction with other 

 factors produce an alteration of those factors. They 

 may be (1) Complementary, when a factor is added 

 to a dissimilar factor in order that a particular charac- 



