134 THE SECOND-CHROMOSOME GROUP 



finished, very large amounts of additional accurate data have been 

 collected upon the cross-over relations of speck with other second- 

 chromosome genes. These data appear in the tables of the sections 

 following this (see especially star). 



Only one of these later experiments had as the main object the more 

 exact determination of the locus of speck. This experiment (table 7) 

 was a triple back-cross for the three loci purple, curved, and speck 

 and gave a curved speck cross-over value of 27.9 per cent on the basis 

 of the 952 flies, of which 266 were cross-overs between curved and speck. 

 The base of reference in the determination and mapping of the locus 

 of speck is curved, which is the nearest locus accurately mapped in 

 relation to black, the primary base of the entire second chromosome. 



For the sake of convenience a summary of all the cross-over data in 

 which speck is one of the loci involved is given in table 8. In calcu- 

 lating the locus of any mutant one must consider not only this direct- 

 linkage data, but also the whole mass of data on the other loci of the 

 same chromosome, and especially the information upon the amount 

 of double crossing-over and coincidence in the various regions. By 

 this method speck has been mapped at a locus 31.6 units to the right of 

 curved, which is its immediate base of reference, or, referring back to 

 star as the zero-point, speck is at 105.1. 



VALUATION OF SPECK. 



Speck is at present one of the most generally useful and used of the 

 second-chromosome mutants, first, because of the perfect accuracy, 

 ease, and speed with which the recessive character is separable from 

 wild-type; secondly, because it can be used in experiments with any of 

 the other second-chromosome characters (including black) without 

 masking effects or confusion in the classification; thirdly, on account 

 of the value of the position of its gene near the right-hand end of the 

 second chromosome, speck being by far the most workable mutant in 

 that general region; and finally, because its viability, its productivity, 

 and its fertility are above reproach, and it is singularly free from such 

 bad habits as getting drowned, or stuck in the food, or refusing to be 

 emptied from the culture bottle, etc., which alienate the affections of 

 the experimenter from certain other mutants. Speck is to be com- 

 mended for students' use, but care should be taken that the character 

 is clearly recognized. 



LITERATURE OF SPECK. 



The more important papers referring to speck are: Morgan, 1910, 

 describing its origin and giving the irregular breeding results already 

 commented upon above; Sturtevant, 1915, giving the data of table 6, 

 by means of which the locus of speck was first worked out; and Mailer, 

 1916, speck being one of the mutants used in the progeny test of the 

 linkage of second-chromosome genes. 



