192 MULTIPLE FACTORS 



tors. For instance, bar, a first chromosome factor, 

 acts in much the same way as the ordinary first chro- 

 mosome intensifier. The sex factor also intensifies 

 truncate, i.e., truncate appears more readily in the 

 females than in the males and may, therefore, be called 

 partially ''sex limited." Especially noteworthy is 

 the fact that while recessive in the normal gray it is 

 generally dominant in an individual either homozyg- 

 ous or heterozygous for black. 



This latter circumstance made it possible to study 

 truncate as a dominant in heterozygous condition. 

 As will appear later, this simphfied the problem 

 greatly, especially in determining whether or not 

 (1) the factors for truncate are stable; (2) whether 

 they are contaminated by their allelomorphs. 



A truncate male containing factors for truncate in 

 both its second and third chromosomes was mated 

 to a normal winged female containing in its second 

 chromosomes the factor for black, and in its third 

 chromosomes the factor for pink. The male offspring 

 of this mating will, therefore, have the formula 



:; — , , , , — ^ — ^-r- They will not contain Ti, as 

 long black long pmk 



males derive all sex-linked factors from their mother. 



An Fi male was then backcrossed to black pink 



females. Since there is no crossing over in the male, 



all the gray red offspring of this backcross will be 



genetically identical, and like their father — unless 



the factors for truncate are unstable, or contaminated 



by their normal allelomorphs. The gray reds were 



not all alike in appearance, however, some being 



