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 

 difficult 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. 



