244 



THE SECOND-CHROMOSOME GROUP 



The F2 ratio of 442 :217 : 207 :0 is a very close approximation to a 

 2:1:1:0 ratio and proves that the gene for cream is in the second 

 chromosome (cream II). 



A similar experiment in which cream was crossed to eosin-ebony 

 (ebony being a third-chromosome mutant, see Stiirtevant, 1914) gave 

 a typical 9:3:3:1 ratio (table 92), which agrees with the fact that 

 the cream gene is not in the third chromosome. 



Table 92. — F^, from the cross of cream II cf hy eosin ebony 9 . 



In order to find the locus of cream within the second chromosome 

 it would have been necessary to run two linkage experiments in which 

 all the flies were eosin; thus, one of these might have been cream II 

 by eosin black and a back-cross of the Fi female to black cream males, 

 and the other a similar back-cross in which curved was used in place 

 of black. The amount of crossing-over between black and curved was 

 known to be about 27 per cent. The two values black cream and 

 curved cream which would be found by two such experiments (both 

 values might, of course, be found from a single suitably devised experi- 

 ment) would enable the locus of cream to be calculated with consid- 

 erable accuracy. While much is to be learned of the mechanism of 

 crossing-over from a study of the relative distributions of loci within 

 various regions of the chromosome, yet in the case of cream II it was 

 thought that the compensation would not be worth the effort. Any 

 further use of cream II in other linkage experiments would involve 

 the " eosinization " of all the stocks used. In the case of certain of the 

 later creams, an approximate location of the gene within the chromo- 

 some has been made, but such location was made easier by the dis- 

 covery of certain dominant mutations which were not available at 

 the time the work on cream II was finished. 



TREFOIL (//). 



(Plate 5, fig. 6; plate 8, fig. 1.) 



ORIGIN AND STOCK OF TREFOIL. 



The character trefoil was found by Morgan about November 1913, 

 and a pure-breeding stock was secured without difficulty. 



DESCRIPTION OF TREFOIL. 

 The character trefoil is quite variable in the intensity of pigmenta- 

 tion, as is the case with all of the thorax pattern characters. The 



