100 



THE THEORY OF THE GENE 



simulans (Fig. 58). Careful scrutiny shows tliem to be 

 different in many ways. They cross with difficulty and the 

 hybrids produced are completely sterile. 



Forty-two mutant types are now known in D. simulans. 

 These fall into three linkage groups. 



Fig. 58. 



Drosophila melanogaster to the left, and D. simulans to the right; 



both males. 



Twenty-three of these recessive mutant genes in simu- 

 lans are recessive in the hybrid, and 65 recessive mutant 

 genes of melanogaster have also been shown to be reces- 

 sive in the hybrid. This result means that each species 

 carries the standard or wild type gene of each of the 

 recessive genes of the other species. 



Sixteen dominant genes have also been tested. All but 

 one produced nearly the same effect in the hybrid that 

 they produce within their own species. This means that 



