LINKAGE 



57 



suit 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 (Fi) get 

 their single X chromosome from their mother and 

 show her colors (gray and red). 



If these gray red Fi 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 Fi 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 



