CHAPTER XI 



MODIFICATION OF UNIT CHARACTERS 



The severest blow at Mendelism is the modification 

 of unit characters. The three theses involved in 

 Mendel's law are unit characters, dominance, and segre- 

 gation. It has been seen how dominance may fail with- 

 out affecting the essential feature of Mendel's law. We 

 have just considered how segregation might occur else- 

 where than at the reduction division and yet not inter- 

 fere seriously with Mendel's law. Unit characters, 

 however, cannot be treated in this way, for they are 

 the very foundation of Mendel's law. In the very 

 nature of things they remain as units, separate in 

 inheritance; if they are observed splitting up or varying 

 in any way they would no longer be units, and the 

 foundation of Mendelism would be weakened. 



Before taking up the real cases of modification of 

 unit characters, another unusual phenomenon concern- 

 ing unit characters may be considered first, a phenome- 

 non which is quite in accord with Mendelism as explained 

 today. It is introduced here because at first thought 

 it might seem to strike at the essential nature of unit 

 characters; in fact, it has been so used. 



Castle (i), working with fancy mice, found that on 

 -raTe""occasions a unit character might disappear com- 

 pletely in cross-breeding. It might be claimed that this 

 result is due to the introduction of an inhibiting factor, 

 or to the separation of complementary factors. Castle 



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