LINKAGE 57 
sult is concerned, as his Y chromosome does not 
contain any factors dominant to yellow and white, 
even though it came from the gray red male. 
The reciprocal cross also offers certain points of 
interest. When a gray red female is mated to a 
yellow white male both sons and daughters are gray 
red. The daughters get a gray red chromosome 
from the mother and these factors dominate the 
factors derived from the father. The sons (Fj) get 
their single X chromosome from their mother and 
show her colors (gray and red). 
If these gray red F, females are back crossed 
to a yellow white male they give the same numerical 
result that this test gave in the reciprocal cross, viz., 
four classes of offspring with 1 per cent. of crossing 
over. 
The F, males behave in all crosses exactly as do 
wild males, which is to be expected, since their single 
X chromosome is derived from the wild type mother. 
It will not be necessary to consider in detail the 
same cross when the two factors enter from different 
parents; they will now keep apart exactly to the 
same degree that they kept together before. This 
is illustrated for the backcross as follows: 
Non-crossovers Crossovers 
Yellow red Gray white Yellow white Gray red 
49.5 per cent. 49.5 per cent. 0.5 per cent. 0.5 per cent. 
99 per cent. 1 per cent. 
As pointed out in the discussion of the black vestigial 
cross, this fact is very important, for it serves to 
