134 GENETICS 



ent part in the case of diverse defects. Then each chromo- 

 some would carry a dominant normal part corresponding 

 to the recessive defective part of the other. Thus, if the 

 defect that produces eosin eye is in the left half of the 

 chromosome, while that which produces vermilion eye is in 

 the right half, then when the two defective chromosomes 

 are together, there is present a dominant normal left half 

 and a dominant normal right half, so that the individual 

 is normal (figure 31, A). 



According to this suggestion therefore : ( i ) The chro- 

 mosome is composed of diverse parts and a certain part 

 may become modified or defective, while the rest remains 

 normal. (2) If one chromosome contains a dominant nor- 

 mal part corresponding to the recessive defective part in 

 the other, the individual produced is normal, so far as 

 this defect goes. (3) Hence when two chromosomes having 

 recessive defects in different parts are brought together, the 

 individual is a dominant normal. 



This suggestion we must examine further to determine 

 whether it agrees with the rest of the facts. 



An important consequence is to be noted at once. A hun- 

 dred or more diverse recessive characters are known that 

 are due to modifications of X-chromosomes in Drosophila. 

 The general rule is that when any two of these diversely 

 modified X's are brought together in the same cell, the in- 

 dividual produced is normal (certain exceptions will be 

 dealt with later). This would require, according to the sug- 

 gestion that we are considering, that the X-chromosome is 

 composed of a hundred or more diverse parts. Can this be 

 accepted? There is further evidence on this point, set forth 

 in the following. 



Certain other discoveries were made as to the mating 

 of parents having X's that are diversely modified. We know 

 that when two recessive parents having the same modifica- 

 tions in the X's are mated, the offspring all show the same 



