238 GENETICS 



long; gray-vestigial; black-long and black-vestigial. 

 The actual experiment, however, shows but two classes 

 of offspring, viz., gray-long and black- vestigial, like the 

 two grandparents (Fig. 69). In other words, gray- 

 body and long-wings entering the cross from one parent 

 stay linked together as do also black-body and vestigial 

 wings. The method of crossing the hybrid back to 

 the recessive is the common procedure in order to bring 

 out what is latent in the hybrid, for the recessive, since 

 it does not dominate or conceal anything, allows what- 

 ever is present in the hybrid being tested to appear. 

 The Mendelian practice of crossing the FI hybrids 

 together tends to conceal linkage and perhaps has 

 prevented its earlier recognition. 



The reciprocal cross is shown in Figure 70. In 

 this case likewise, whatever goes in together comes 

 out together and no new combinations appear. 



4. CROSS-OVER 



When a gray-bodied long-winged female hybrid, such 

 as is produced by crossing gray-long and black-vesti- 

 gial together in the preceding experiment, is crossed 

 back to a recessive black-vestigial male, there are pro- 

 duced four kinds of offspring, gray-long and black- 

 vestigial like the grandparents and two new combina- 

 tions, gray-vestigial and black-long. These four types 

 of F 2 are what would be expected upon free assort- 

 ment of all the gametes and they should all occur in 

 equal numbers or in the proportion of 1 : 1 : 1 : 1. See 

 Figure 68. Instead, as an actual result of extensive 



