48 



THE THEORY OF THE GENE 



a normal sperm, the fertilized egg contains only one of 

 the fourth chromosomes. It develops into a fly ('4iaplo- 

 IV") that shows in many parts of its body slight differ- 

 ences from the normal fly (Fig. 29). 



Fig. 30. 

 Characters in the fourth linkage group of D. nielanogaster. To the 

 left, bent wings; to the right (above), four heads showing "eye- 

 less," one in dorsal, three in side view; below, and to the right, 

 shaven. 



The result shows that specific effects are produced 

 when one of these chromosomes is absent, even in the 

 presence of the other fourth chromosome. 



There are three mutant elements or genes in this chro- 

 mosome, namely, eyeless, bent, sliaven (Fig. 30). All three 

 are recessives. If a haplo-IV female is mated to a diploid 

 eyeless male with two fourth chromosomes (each ripe 

 sperm with one) some of the offspring that hatch are 

 eyeless, and if the pupae that do not hatch are removed 



