288 CALVIN B. BRIDGES 



10 is approximated, though not very closely. On this basis, the 

 order of these genes is black purple vestigial, and not black 

 vestigial purple. 



A THREE-POINT BLACK CROSS, BLACK PURPLE CURVED, WITH 

 BALANCED INVIABILITY 



Most of the linkage experiments up to this time had involved 

 only two loci, as the three just cited, namely, purple vestigial, 

 black vestigial, and black purple. It was now realized that a 

 more complex type of experiment involving all three loci at once 

 would yield returns whose value far outweighed the greater 

 labor entailed. Thus, a multiple back cross for black purple 

 vestigial would give linkage data upon all three crossover values 

 simultaneously, and these values would be strictly comparable, 

 since there would be no possibility of discrepancies due to 

 different conditions of culture or parentage. Accordingly, the 

 simple black purple back cross was done on a scale only large 

 enough to decide between two possible values and thus show 

 what was the order of the three loci. A knowledge of this order 

 is of great advantage in synthesizing the multiple recessive. It 

 was found, as already stated, that black and vestigial are the 

 two farthest apart and the mating was accordingly arranged so 

 that a crossover anywhere within this whole distance would 

 give the required triple form. That is, black purple and purple 

 vestigial were mated together and the resulting purple offspring 



( I were inbred. The F2 black purples and purple 



\+ pr vgy 



vestigials were crossed together in several mass cultures, and 



in F3 some triples occurred, showing that some of both kinds of 



F2 flies used had come from crossovers eggs. A better method 



would have been to back cross the Fi female by a black vestigial 



male. In this case every black vestigial crossover would be 



(b pr vg\ 

 , j, and these inbred 



would give the pure triple without the chance of failure that 

 the method, actually used, ran. A stock of black purple ves- 

 tigial was made from the triple recessives that hatched in Fs 

 (March, 1913). 



