ARE RECESSIVES LOSSES OF GENES? 



79 



As far as this evidence goes, notch might be interpreted 

 as a recessive lethal gene that acts as a dominant wing 

 modifier in the hybrid. It was, however, later shown by 

 Bridges and by Mohr that a much greater length of the 

 X-chromosome is involved in the notch mutation than in 



Notch O 



N -n 



XX t 4 



normal cf 



X^Y 



Gametes 



Fi 



JVotch 



Fig. 45. 

 Diagram to illustrate a cross of a Notch female, X N X n by a normal 

 male, X n Y. The X-chromosome with Notch is XN; the other X, 

 carries the normal allelomorph, X n . 



an ordinary ' ' point mutation ' ' ; for when recessive genes 

 are present in the region of notch in one X-chromosome, 

 and notch in the other X-chromosome, the recessive 

 characters appear in such an individual as though a cer- 

 tain region of the notch chromosome were absent or at 

 any rate inactive (Fig. 46a). The result is practically the 

 same as though an absence had actually arisen. In some 

 of the notch mutants the "lost" region extends over 

 about 3.8 units (from the left of white to the right of 



