CHAPTER IX 

 TYPES OF FACTOR INTERACTIONS 



In the present chapter it is proposed to illustrate some of the various 

 types of relations which exist between different pairs of factors so that 

 the student may come to appreciate some of the more complex features 

 of factor interaction. 



The System of Aleurone Color Factors in Maize. It has already 

 been pointed out that some particular cases of Mendelian phenomena 

 depend for their explanation on the presence of different factors which 

 give similar results. These are to be considered as cases of Mendelian 

 factor interactions as specifically as those more fully discussed in this 

 chapter. Such a condition may be illustrated very satisfactorily by 

 the four-factor system for aleurone color which is known to exist in maize. 

 The factors involved in this system and their actions are as follows: 



C a factor for chromogen base. C is necessary for the production of 

 any aleurone coloration in maize. Its allelomorph c constantly gives 

 white grains. 



R a factor which when present with C gives a red aleurone color. Its 

 allelomorph, r, constantly gives white. 



P a factor which when present with C and R gives purple aleurone 

 color. 



W a dominant factor for white aleurone color. When it is present 

 the grains will be white no matter what other factors may be present. 



With this series of factors, the following homozygous races are ob- 

 tainable and have the phenotypic expression here indicated: 



1. WWCCRRPP white. 9. wwCCRRPP purple. 



2. WWCCRRpp white. 10. wwCCRRppred. 



3. WWCCrrPP white. 11. wwCCrrPP white. 



4. WWCCrrpp white. 12. wwCCrrpp white. 



5. WWccRRPP white. 13. wwccRRPP white. 



6. WWccRRpp white. 14. wwccRRpp white. 



7. W WccrrPP white. 15. wwccrrPP white. 



8. WWccrrpp white. 16. wwccrrpp white. 



Of the sixteen pure breeding forms, fourteen are whites, and although 

 indistinguishable phenotypically these whites may be separated by 

 proper breeding tests. Some of the relations existing between the 

 different genotypes have already been dealt with, but the student may 



163 



