2O2 MILDRED HOGE RICHARDS. 



FI. Genes for these two characters are located quite far apart 

 in the third chromosome. 



Scarlet X sooty 



F! Wild-type 



FI 9 back crossed to scarlet sooty cf gave the following: 



f Scarlet 1230 



F 2 



non cross overs , 



I Sooty 1511 



f Wild-type 695 

 cross overs -I 



L Scarlet sooty 623 



Percentage of crossing over was 32.4 



Dichaete X scarlet. Dichsete is a dominant character, non- 

 viable in the homozygous condition. The dichaete to which 

 scarlet was crossed was therefore heterozygous. The Fi dichaete 

 females were back crossed to scarlet males and the percentage of 

 crossing over thus immediately obtained in the F 2 . 



f Scarlet 781 

 Non cross over classes { .. , 



L Dichaete 952 



f Scarlet dichaete 23 

 Cross over classes s , TT ., . 



[Wild-type 45 



Percentage of crossing over is therefore 3.2. 



Scarlet is thus placed at a locus of 3.2 from dichaete, and 32.4 

 from sooty. 



According to unpublished work from the laboratory at Colum- 

 bia University, the approximate loci for these third chromosome 

 genes are the following: 



Dichaete n 

 Pink and peach 16 

 Sooty 34 



Since such a large per cent, of cross over was obtained for scarlet 

 sooty (32.4), scarlet is probably at the left of dichaete (on the 

 sepia side), sepia being at o. Since it is 3.2 from dichaete, it is 

 at the approximate locus of 7.8, (n 3.2). 



Since dichaete is 23 to the left of sooty, (34 - n), we should 

 expect the value of scarlet sooty to be 3.2 + 23 or 26.2, instead 

 of 32.4. The larger value obtained in this work may have been 

 due of one of several causes. Genetic factors which influence 

 the amount of crossing over have been found by Sturtevant 

 and Muller for the third chromosome group. Plough has also 



