130 



THE SECOND-CHROMOSOME GROUP 



hea,\y deposit of pigment in the lips of a spiracle which is situated 

 a little behind and below the juncture of the base of the wing with 

 the thorax (fig. 73 6). There is also present upon the side of the 

 sciitellum and of the thorax both above and below the wing an added 

 faint tinge of pigment or areola which seems to persist after olive has 

 been eliminated from speck. However, speck is rarely seen without 

 a decided olive color, because the gene for olive is very closely linked 

 to that for speck. Since there is always the chance of crossing-over 

 between speck and olive, and since certain other genes give roughly 

 similar pigmentation, it is well to disregard this pigmentation and 

 classify by means of the speck alone, which is in itself a sufficient 

 index of the speck gene. 



INHERITANCE AND CHROMOSOME OF SPECK. 



During the fall and winter of 1912 considerable breeding-work with 

 speck was carried out. The results of these experiments, particu- 

 larly of the F2 from the crosses of speck olive by black, contained the 

 answer to the questions as to which chromosome carried the genes for 

 speck and for olive, but the records were not carefully examined until 

 May 1913. Meanwhile, Sturtevant had begun to work with speck 

 (December 1912) and had shown that it was a complete and invari- 

 able recessive and behaved with regularity in inheritance. 



Table 2. — Pi, speck olive 9 X wild cT; Fi wild- 

 type 9+^1 wild-type cf. 



By means of the 2: 1 : 1 : ratio in the F2 of the cross of curved to 

 speck he showed that speck was a member of the second-chromosome 

 group. (Jan. 13, 1913). 



In the crosses carried out by Miss Wallace, speck was found to be 

 readily classified and fully viable. Speck olive females were crossed 

 to wild males and 10 F2 pair cultures raised (table 2). The reciprocal 

 cross (speck d" X wild 9 ) was also made to the extent of 5 F2 cultures 



