OF MUTANT CHARACTERS. 



127 



Special attention has been given in the treat- 

 ment of the data to bring out the genetic methods 

 employed, and to trace the development of such 

 methods. The part each mutant has played in 

 this development of methods and principles lias 

 been given fully, often at the expense of repeti- 

 tion. An effort has been made to evaluate each 

 mutant with respect to its usefulness as a working 

 tool. Some are practically useless, while certain 

 others, from the possession of excellent charac- 

 teristics or favorable location in the chromosome, 

 etc., must be considered in every carefully planned 

 experiment. By such methods of presentation it is 

 hoped to make available not simply a body of data 

 but also a working familiarity with the material. 



From the various published and hitherto un- 

 published materials on crossing-over in the second- 

 chromosome a summary is given of total data 

 available on amount of crossing-over between va- 

 rious loci (table 140). The cross-over values cal- 

 culated from these data are still further summar- 

 ized and presented in graphic form in the map of 

 the second chromosome which appears as fig. 72. 



In the construction of this map it was necessary 

 to correct some of these values by aid of informa- 

 tion gained from a study of the amount of double 

 crossing-over and coincidence in the various re- 

 gions of the second chromosome. Details of how 

 this was done will be found in the last section, 

 which deals with the construction and use of 

 this map. The coincidences, and consequently 

 the corrections, are at present only quite rough 

 approximations, and the same is true of the meth- 

 ods of weighting employed in that section. It 

 should be borne in mind that the map is a com- 

 posite picture in which differences in the data from 

 different sources are no longer apparent. Ref- 

 erence to these separate data will show, however, 

 a very surprising uniformity, especially in view 

 of the many conditions now known to be able to 

 cause the amount of crossing-over to vary. 



A glance at the map shows at the right end (bot- 

 tom) an unusually dense cluster of genes. WTien 



Text-figure 72. — Map of the second chromosome, giving the 

 locations with reference to star, and the symbols of the 

 mutants whose loci are known. 



0-- Stiir ( S 1 



15 V - SU-eaJc(3;4) 



2.5-- Creamb(Crbt 



Ufl.O 



3S5-- S<j<iat(Sj) 



4o 5 

 46 T 

 ■+8.5 



Aptci-6us(a.p> 

 Trefoadjl 



-- Piu-plc(p 1 



GSO 

 66 5 



105.1^ 

 105.S- 

 106 I ? 



106 3 



Ti-unca»*(T> 



L)aclu(dl, 



dacha-IeUuil(di) 



2 5--Lethal tlaC 1, 



. .Vestigial (vrfi,9trap(vg*|. 

 --Tclescopc^tsl * 



73 5- - Curved (c) 



96 2 - - Plexus ( Pj^ I 



98 o.. Frmged UV' 

 98.VT.\rC(ln 



Blistered (bg) 



/Speck (Sp) 

 BaIlooii(ba.l 

 " Oliw ( i>i ) 

 'Morula (nij-l. 

 lunitBd T 



FlO. 72. 



