OF MUTANT CHARACTERS. 



187 



triple without the chance of failure entailed by method actually used. 

 A stock of black purple vestigial was made from the triple reeessives 

 that hatched in Fg (March 1913). 



In carrying out the triple back-cross, the principle of balancing the 

 inviability by complementary crosses was applied. To comi)letely 

 balance a three-locus experiment requires four types of crosses, so that 

 every class may appear in each of the four cross-over categories, 

 namely, (0) non-cross-overs, (1) cross-overs in the first region, that 



Table 41. — Pi, black purple X vestigial; B. C. o/ Fi 9 9 singly. . 



Table 42. — Summary of the four types of black purple vestigial back-cross, 



with inviability balanced. 



between black and purple, (2) cross-overs in the second region, that 

 between purple and vestigial, and (1, 2) double cross-overs, the simul- 

 taneous occurrences of crossing-over in both regions. Thus, the cross 

 of black purple vestigial by wild and the back-cross of the Fi wild-type 

 daughters by the triple-recessive male gave one of the four types of 

 crosses (table 38). The other types of experiment carried out were 

 black by purple vestigial (table 39), black vestigial by purple (table 

 40), and black purple by vestigial (table 41). In order that these 

 four crosses should balance closely, the same number of cultures (6) 



