THE NATURE AND EXPRESSION OF MENDELIAN FACTORS 143 
board. As in the previous instance these ratios are merely modi- 
fications of the typical Mendelian dihybrid and trihybrid ratios due to 
the fact that many of the classes are white and hence are merged into 
one. 
It should be apparent from the discussion in this chapter that many 
complex relations exist as respects the nature and expression of factors. 
Only some of the best, established and most conspicuous cases have been 
discussed and some of these in rather incomplete fashion, but the material 
presented is sufficient to establish several facts concerning factors, 
namely that some factors have very minute, others very far reaching 
effects, that factors may affect many characters in the individual, that 
factors may vary in their expression in individuals, that sometimes this 
variability in factor expression is dependent upon definite environmental 
conditions and sometimes on obscure or unknown causes, and that at 
times different factors may have similar somatic expressions. It is 
dificult to treat such various matters in any systematic fashion, con- 
sequently this chapter must be regarded merely as an introduction to 
the general topic of factor interactions. 
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