CHROMOSOMES AND GENES 



49 



from their pupa-cases and examined, more eyeless flies 

 are detected. The eyeless fly has come from an egg that 

 did not carry chromosome-IV and was fertilized by a 

 sperm with chromosome-IV carrying the eyeless gene. 



Haplo-lV 



Eyeless 

 Diplo-lV 



Germ-cells 



Normal D 



Eyeless 



Diagram of a cross between a normal-eyed, haplo-IV fly, and an 

 eyeless fly with two fourth chromosomes, each carrying a gene for 

 eyeless. The fourth chromosome carrying the eyeless gene is here 

 represented by an open circle, that for normal eyes by a black dot. 



As shown in the diagram (Fig. 31), half of the flies should 

 be eyeless, but most of these do not pass beyond the pupal 

 stage, which means that the eyeless gene itself has a 

 weakening effect on the individual, and that when to this 

 is added the effects due to the absence of one of the fourth 



