56 LINKAGE 



the mother. The explanation of this result is obvious ; 

 for the son gets his single X chromosome from his 

 mother, and should therefore have the characters 

 that go with this chromosome. His Y chromosome, 

 derived from the father, does not influence the result 

 at all. The daughters, however, get one X chromo- 

 some from the mother (yellow white) and the other 

 from the father (gray red). The factors for gray 

 and red dominating give gray red daughters. 



The composition of these Fi females can be tested 

 by breeding to the double recessive male (yellow 

 white) since this does not carry any dominant factors 

 which will obscure what factors are received by the 

 F 2 females from their mothers. But the Fi males 

 are themselves yellow white, so that the FI females 

 may be mated to their brothers. In fact, the out- 

 come is the same, whether a yellow white male from 

 stock or a yellow white Fi brother is bred to the FI 

 female. The F 2 offspring of such crosses give the 

 following classes and ratios: 



Non-crossovers Crossovers 



Yellow white Gray red Yellow red Gray white 



49.5 per cent. 49.5 per cent. 0.5 per cent. 0.5 per cent. 



99 per cent. 1 per cent. 



This F 2 result reveals the kinds of eggs produced by 

 the FI female (since a double recessive father was 

 used). Crossing over takes place between yellow 

 and white in only 1 per cent, of cases. 



There is no way of testing linkage in the Fi male, 

 which is like a homozygous individual so far as the re- 



