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 givesaredaleuronecolor. Its 
allelomorph, 7, 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. WWCCRERpp—white. 10. wwCCRRpp—red. 
3. WWCCrrPP—white. 11. wwCCrrPP—white. 
4. WWCCrrpp—white. 12. wwCCrrpp—white. 
5. WWecRRPP—white. 13. wwccRRPP—white. 
6. WWecRRpp—white. 14. wweck Rpp—white. 
7. WWecrrPP—white. 15. wweerrPP—white. 
8. WWeerrpp—white. 16. wweerrpp—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 
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