50 LINKAGE 



factors as were present in Pi unless an interchange 

 has taken place between the two chromosomes. The 

 best way to find out whether such an interchange 

 has taken place is to mate the Fi males and females 

 to the double recessive type, black vestigial, because 

 black and vestigial being recessive factors will not 

 obscure the factors that are carried by the gametes 

 of the Fi to be tested. When the Fi male is back- 

 crossed to a black vestigial female, Fig. 21 (second 

 line), only two classes of offspring are produced. 

 Half of the flies are black vestigial and half are gray 

 long. This must mean that there has been no cross- 

 ing over in the hybrid Fi male; for he produces only 

 two kinds of gametes and these are of the kind that 

 combined to produce him. In other words, the 

 chromosomes received from his parents have re- 

 mained intact. 



If we test the Fi female, by back-crossing to a 

 black vestigial male, the result is different. If such a 

 female is bred to the double recessive male, black 

 vestigial, four kinds of offspring result, as follows: 



■ Non-crossovers Crossovers 



Black, vestigial Gray, long Black, long Gray, vestigial 



41.5 per cent. 41.5 per cent. 8.5 per cent. 8.5 per cent. 



83 per cent. 17 per cent. 



Of these four classes the first two correspond to the 

 combinations which the Fi received from its parents, 

 namely, black vestigial and gray long ; the other two 

 are classes that would be expected if crossing over had 



