CHAPTER XXXI 

 THE CHARACTER EXPRESSION OF GENES 



The Differential Genes or Factors. — In a preced- 

 ing chapter it was shown that no character is ever the 

 result of the activity of one single gene or factor alone. 

 (Page 357). Nevertheless it is true that whether a par- 

 ticular character of a plant or animal appears or not often 

 depends on the activity of one or a relatively few genes, 

 which may, therefore, be called differential genes or fac- 

 tors. All the examples which have been used thus far 

 in this book have depended for their final expression on 

 some one factor. The real situation may possibly be made 

 clearer by the relations existing in the vinegar fly in re- 

 spect to eye-color. About twenty-five different modifica- 

 tions of eye-color in this fly have already been studied. 

 Many genes, of course, interact to produce the natural 

 wild type with red eye. The other colors are due to a 

 change in some single gene for each but not always the 

 same gene or even one in the same chromosome. 



The Relative Position of Genes. — It has already 

 been shown that the two differential genes responsible 

 for a pair of contrasting characters are carried in the 

 same chromosome-pair, the one in one chromosome and 

 the other in its mate, and so show segregation. Moreover, 

 the genes are arranged in linear order so that correspond- 

 ing genes lie at exactly the same relative positions in their 

 respective chromosomes. This, however, fails to explain 

 fully the hereditary behavior of many characters, as, for 

 example, eye-color in vinegar flies. A red-eyed fly (Fig. 

 113) crossed to either a white-eyed or a cherry-eyed fly 

 shows the ordinary segregation in the second hybrid gen- 



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