28 



PHYSICAL BASIS OF HEREDITY 



sooty. These and other tests show that in the F^ hybrid 

 segregation of the same kind as in the preceding cases has 

 taken place, but the results are obscured by the wide 

 variability of the hybrid flies. In other words, evidence 

 can be obtained that the segregation of the genes has been 

 clean cut, even although this is obscured by the character 

 of the heterozygous flies. 



Fia. 4. — Male and'female vinegar fly (DTOsophila melanogaster). 



In the preceding illustrations the character difference 

 between the two races is supposed to show itself in the 

 same environment. It has been found in a few other 

 cases that the dominance of one character over the other 

 may depend on the environment. For example, in the 

 normal vinegar fly the black bands of the abdomen show 

 great regularity (Fig. 4), but in a mutant race called 

 "abnormal abdomen" (Fig. 5) the bands may be irregu- 

 larly broken up, or even absent. In cultures with abund- 

 ance of fresh food and moisture, all the individuals have 

 very irregular bands, but as the culture gets old, and the 



