238 C. B. BRIDGES AND A. H. STURTEVANT. 



is in our case more crossing over in the females than in the 

 combination studied by Morgan. 



We have seen that when black and curved enter FI separately, 

 one class, namely, the double recessive, does not appear in F 2 , 

 for the reason that a double recessive gamete is not formed in 

 the FI male through lack of crossing over. But if black and 

 curved enter from the same parent then half the gametes of the 

 FI male are doubly recessive, and, therefore, give in a back cross 

 test the amount of crossing over in the female. The other half 

 of the gametes of the male are the double dominant class and in 

 consequence half of the flies fall into a single double dominant 

 class. If, then, there is crossing over in the female in the ratio 

 of one crossover gamete to n of the original combination, the Fa 

 will consist of n : I : I : n zygotes from the doubly recessive 

 sperm, and a like total, that is 2n -\- 2, from the doubly dominant 

 half of the sperm. The F 2 proportions expected are therefore 

 3 + 2 : i : i : n and the ratio of each single recessive class (i) 

 to the double recessive class (n) gives the gametic ratio directly. 

 The results of such a cross as that described appear below : 



Wild X black curved. 



948 74 63 191 



The occurrence of black and of curved flies in this experiment 

 demonstrates that crossing over takes place, and from the evi- 

 dence of the preceding experiments, we conclude that it must have 

 been in the females. That this is the correct interpretation is 

 shown by direct tests of such females. As stated above, the 

 percentage of crossing over can be calculated directly from tests 



1 In all counts reported here 5 signifies that the record includes only the offspring 

 of a single female. For reasons which will appear in future publications from this 

 laboratory it has seemed advisable to include only the first cultures obtained from 

 any females. When later cultures are available but have been omitted, the record 

 will be marked with an asterisk. 



