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 
T. gray T, red 
long black long pink. 
males derive all sex-linked factors from their mother. 
An F, 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 
They will not contain T,, as 
