272 THE SECOND-CHROMOSOME GROUP 



From the first two of these values it appears that streak is very 

 nearly midway between star and dachs, and that star is about 32.1 

 units to the left of dachs, which agrees well with the position as cal- 

 culated from the star dachs data, with allowance for double crossing- 

 over. The coincidence for star streak dachs was only 9.8, which is 

 very low indeed. This means that double-crossing over in the left 

 end of the chromosome, as we had earlier found to be the case in the 

 right end, is very much lower than it is in the middle of the chromosome. 

 This would seem to be connected in some way with the fact of median 

 attachment of the spindle fiber, and further analysis of the problem 

 should throw much light on synapsis and crossing-over. 



The dachs purple cross-over value of 19.1 is lower than expected, 

 and leads to the mapping of dachs somewhat nearer to black than 

 formerly. 



The calculation of the other linkage values — those of which streak 

 is not one of the loci concerned — can be made on the total number of 

 flies (1,027), since these values are established by the classification of 

 fully separable characters. 



A summary of all available linkage data involving star is given in 



table 122. 



VALUATION OF STAR. 



Star is now the most used second-chromosome mutant. Its via- 

 bility (heterozygote) is on a par with that of the wild fly. Its position 

 is ideal. It interferes with the classification of only one other second- 

 chromosome character — morula — which loses in usefulness because of 

 this conflict with star. The separability of star from the wild-type is 

 not as clear and sharp as desirable. There is danger of overlooking 

 some of the star flies among the wild-type. This difficulty is not 

 general, but is much more pronounced in certain cultures, wherefore 

 it seems likely that most of the suppression is due to modifiers. Two 

 modifiers that markedly increase the separabiUty of star are known, 

 one of which is in the third chromosome (the stock of the double form * 

 being known as S^) and the other of which is sex-Unked (S^ stock). 



There is another danger in the use of star that must be constantly 

 guarded against. Mutations of the moruloid type are very frequent 

 and confusion has resulted from the presence of such forms in experi- 

 ments supposed to contain only star. When the presence of such a 

 mimic is once recognized steps can be taken to eliminate it and thereby 

 remove the difficulty. One such, "pitted," apparently arose in the 

 star dichaete stock, and through the extensive use of this stock be- 

 came spread widely through our experiments and other stocks derived 

 from these experiments. Star can therefore be used successfully only 

 by those thoroughly familiar with it and under favorable conditions 

 of illumination and magnification, which fact prevents its general 

 use by students. 



