360 



CALVIN B. BRIDGES 



ebony. He made tests of the eosin stock, and found some 

 individuals that carried Cm, from which he secured the desired 

 stock. 



In making the tests of the last experiment (table 18) the out- 

 crosses of cream III were made to eosin dichaete in order to 

 establish by a three-point linkage experiment the location of 

 cream III in the third chromosome. The preceding experiments 

 (tables 15, 16, and 17) had already given 404 crossovers in a 

 total of 2887 flies, or 14 per cent, in cases where the mothers were 

 heterozygous for Cni- In other experiments (by Sturtevant and 

 Muller) in which ebony has been run with various third chromo- 



TABLE 18 



The B. C. offspring from the outcross of a cream III ebomj female from ^77 to an 



eosin dichaete male, and the Fi eosin dichaete females backcrossed singly 



to cream III ebony males 



some genes in heterozygous C„i condition, the amount of crossing 

 over of sepia with ebony has been 25 ±, of pink with ebony 2 

 and of ebony and rough 0.0 + . That is, there is some crossing 

 over in the region to the left of ebony, nearly all of which is in 

 the sepia pink region, but almost none to the right of ebony 

 (ebony rough 0.0 +). It seemed probable from the crossover 

 value 14.0 that cream III lies to the left (i.e., near the sepia end) 

 and at a position between sepia and pink. A comparison of the 

 high value for cream III ebony (41.0) with the known homo- 

 zygous Cm values (Muller) for sepia ebony (48+) and for pink 

 ebony (46+) led to this same conclusion though less definitely. 

 The mutant dichaete was well adapted for this test because of 



