182 



THE SECOND-CHROMOSOME GROUP 



is often not convenient or possible to have complementary crosses of 

 equal weight; but whatever is done in that direction, however little, 

 is of advantage, and even a partially balanced result is to be preferred 

 to one from only one type of cross. With improvements in culture 

 methods, inviability effects have been very much reduced everywhere. 

 Also, with the great increase in the number of mutations, there is now 

 provided an abundance of forms which show only negligible inviability. 

 Our regular work utilizes only these viable forms, and except for very 

 special purposes those mutants which show more than a slight invia- 

 bility are avoided. 



Table 34. — B. C. offspring given hj Fi wild-type daughters, from the out- cross of 

 a purple male to a vestigial female, when hack-crossed to purple vestigial males. 



The first back-crosses of purple-vestigial had shown a marked 

 inviability for vestigial and a slight amount for purple. The new 

 back-crosses showed practically no inviability for purple and a very 

 moderate amount for vestigial, but still enough to repay the added 

 labor required by the balancing cross. As in the first back-cross test 

 of the female, the linkage shown was fairly strong. Since the linkage 

 shown by second broods proved to be different from that of firsts, only 

 first broods will be considered for the moment. The ''coupling" 

 experiment (table 33) gave a total of 2,839 first-brood flies, of which 

 305 or 10.7 per cent were cross-overs. The ''repulsion" first broods 

 (table 34) gave a total of 2,335 flies, of which 303 or 13.0 per cent were 

 cross-overs. When the first brood data from both these experiments 

 are combined, so that the inviability is balanced, the cross-over value 

 is 1 1.8 (608 cross-overs in a total of 5,174.) 



These two component cross-over values differed slightly from each 

 other and from the value (9.1) obtained in the original experiment. 

 It may be questioned whether the difference in the cross-over values 



