248 



Sturtevant. 



Table IX. 



Another case of variation in the strength of linkage has recently 

 been met with in chromosome II. As recorded in table XI, the loci 

 Vg and Sp ordinarily give about 36 "/o of crossing over. However, when 

 the vestigal speck stock which was used in the experiments there re- 

 corded was crossed to a wild stock obtained for me from Liverpool, 

 Nova Scotia by Miss E. M. Wallace, and an Fi female was backcrossed 

 to vestigial speck males, 99 flies were produced without a cross-over 

 among them. Two normal daughters of this female mated to their 

 vestigial speck brothers gave a total of 4 cross-overs among 256 fUes. 

 The same strong linkage has also appeared in the succeeding generations. 

 The peculiarity has been transferred to females heterozygous for numerous 

 other combinations of second chromosomal genes, and in every case the 

 strength of linkage has been greatly increased. The result is due to 

 a peculiarity of the second chromosome itself, as shown by the method 

 of its inheritance. The case is still under investigation, and there are 

 many details still to be worked out. For this reason the data are 

 reserved until a more complete case can be made out^). 



The data on double crossing over which have already been dis- 

 cussed show another interesting point in this connection. "V\lien these 

 data are tabulated, as in table X, so as to show the observed crossing 

 over between each pair of loci considered separately, and these values 

 are compared with those recorded in tables I and VI for the same loci, 



■*) Tlie terms coupling and repulsion are used here merely as convenient words to 

 denote whether the two dominants lie in the same or in different chromosomes. 



-) A preliminary report on this race was read before the American Society of 

 Naturalists, at Philadelphia, Dec. 31, 1913. 



