CAUSE OF HYBRID VIGOR 185 



Now if a female fly with dachs legs, gray body, purple 

 eyes and normal wings (dBpV), be crossed with a male 

 having long legs, black body, red eyes and vestigial wings 

 (DbPv), the resulting progeny will have the usual wild- 

 type characters, long legs, gray body, red eyes and normal 

 wings, and will be considerably more vigorous than either 

 parent. If these factors segregated independently, one 

 would expect to find one gamete out of every sixteen to be 

 of the constitution DBPV, and would obtain one F 2 indi- 

 vidual homozygous for this combination of the four domi- 

 nants out of every two hundred and fifty-six. As a mat- 

 ter of fact, owing to the linkage relations found, only one 

 gamete of this kind is produced in two thousand and then 

 only in the female. It is, therefore, impossible to obtain 

 the type sought in the F 2 generation. But males of the 

 all-dominant type will appear in F 2 , and the pure strain 

 may be established in F 3 . The word "may" is used as a 

 sort of forlorn hope, however. There is a possibility of 

 establishing the homozygous dominant strain in F 3 , but 

 when one realizes that in F 2 only one such male and one 

 heterozygous female similar in appearance to hundreds of 

 her sisters will be produced in every four thousand pro- 

 geny, the difficulties in the situation are emphasized. 



The frequency of the linkage breaks is large and the 

 number of factors small in this illustration. When it is 

 remembered that in other organisms there are ten, twenty, 

 or even forty chromosome pairs to be considered, with 

 possibly dozens of factor differences, instead of four in 

 each chromosome, some idea may be obtained of the real 

 difficulties involved in producing individuals of maximum 

 vigor unaffected by inbreeding. Practically speaking, it 

 is impossible unless dealing with a small number of 



