244 ■ MULTIPLE FACTORS 



whose modes of transmission were known ; the latter 



factors were again used as 'identifying marks" in 



following the distribution of the factors for truncate. 



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 



To gray T2 red ^, -n ^ ^ • n^ 



1 f-j — ] — ] ^-j— Ihey will not contam 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 offspring of this backcross containing the 

 ''identifying characters" gray and red 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 

 truncate, though most were long. Males of these 

 two classes were then tested by mating them in- 

 dividually, again to black pink females. From the 

 result of these matings it was clearly shown that the 

 longs and the truncates produced almost exactly 

 the same proportion of truncate, proving that they 

 had been alike genetically. 



It will be observed that these test matings of 

 heterozygous gray red males to black pink females 

 were of exactly the same type as the cross of Fi; 

 consequently gray red males having the same 



