168 



THE SECOND-CHROMOSOME GROUP 



cross-overs appeared in a total of 7,419 flies (Bridges and Sturtevant, 

 1914). In the male there was no crossing-over whatever, while in 

 the female there was about 25 per cent. 



The direction along the II chromosome from black to curved was 

 called "to the right," and curved was therefore mapped at a locus 25 

 units to the right of black. 



The position of curved has been more accurately determined by 

 use of intermediate loci than was possible from the rather long black 

 curved interval. Thus the purple curved stock, made by Bridges in 

 preparation for the triple back-cross black purple curved, was used by 

 Mr. W. S. Adkins to run extensive purple curved back-cross tests. 

 These crosses, the data for which are not yet available, gave about 18 

 per cent of crossing-over between purple and curved, which agrees 

 with the result of the black purple curved back-crosses. Much data 

 has since been collected on this cross-over value, largely incidental to 

 the work on age variation by Bridges and temperature variation by 

 Plough. A total of 51,136 flies included 10,205 cross-overs between 

 purple and curved ; the observed percentage of crossing-over was thus 

 19.9. It is possible that there is considerable double crossing-over 

 within this region, since it is in the middle of the chromosome, where 

 double crossing-over in relation to map-distance has been found to be 

 extraordinarily high. If a coincidence of 70 is assumed, the corrected 

 purple-curved value becomes 21.4 and the locus of purple is 27.6 units 

 to the right of bla-ck. 



The locus of curved was also referred to vestigial by Sturtevant, who 

 ran vestigial curved and black vestigial curved back-crosses. This 

 method has great advantages in mapping the locus of curved, since 

 vestigial is itself accurately mapped in relation to purple and is an 

 intermediate base between purple and curved. Only the not-vestigial 

 back-cross flies can be used in the calculation, since curved vestigials 

 can not be distinguished from the simple vestigial class. Hence the 

 vestigial curved data are too meager as yet (1,720 flies) to be used 

 as the main basis of the location of curved. 



A sumLmary of the cross-over data involving curved and other second- 

 chromosome loci is given in table 25. 



VALUATION OF CURVED. 



Curved is in all respects a mutant character of first rank, both for 

 student use and in special experiments. Its separabiUty from the 

 wild-type is both eagy and accurate, even without experience. Its 

 viability is excellent. It causes no trouble through liability to drown- 

 ing or miring, which might have been expected on account of the 

 strongly divergent wings. Its locus is the outpost of the central body 

 of well-mapped genes and it is therefore the base of reference for speck 

 and for all genes near the right end of the chromosome. 



