50 Linkage Group V. 



interrupted class and the remainder to the interrupted approximated 

 class. A calculation of the totals thus obtained, using all of the 

 flies, gives 16.9 per cent crossing-over. Using only the interrupted 

 flies it gives 17.5 per cent crossing-over. When all of the data in 

 both parts of the experiment are used, a value of 17 per cent is 

 obtained. The agreement between these various values is sufficient 

 to indicate that 17 per cent is probably not far from the correct 

 value. 



Branched and ruffled. — RufSed has thus far only been tested with 

 branched. Four back-crosses (experiment 69), in which the mutant 

 genes were in opposite chromosomes, gave 200 non-cross-over flies 

 and 198 cross-overs. This gives a value of approximately 50 per 

 cent, showing that ruffled is remote from branched. 



DISCUSSION. 



On the basis of the experiments involving two genes at a time, 

 the following (approximate) values are obtained: branched-fused 



18 per cent, branched-interrupted 2.4 per cent, fused-approximated 

 35 per cent, interrupted-approximated 17 per cent, f used-interrupted 



19 per cent, branched-ruffled 50 per cent. Since interrupted is 

 placed here with reference to both fused and approximated, it is 

 possible to arrange the three genes in a series, in which interrupted 

 comes about midway between the other two. Since branched and 

 interrupted appear to be closely linked, and give nearly the same 

 cross-over value with fused, branched should fall near interrupted. 



It should be possible to check these values by means of three-point 

 crosses, but we have not been able to do so, thus far. Since branched 

 and interrupted are both irregular, it is unsafe to use them in the 

 same experiment, and it is unsafe to use fused with interrupted 

 and approximated for reasons mentioned above. This leaves only 

 the combination branched, fused, approximated with which to make 

 the test. And in the results obtained from this (experiment 70) 

 the two smallest cross-over classes are so nearly equal that it is impos- 

 sible to determine which represents the double cross-overs. The 

 order should be either fused-branched-approximated, or branched- 

 f used-approximated, with a value of about 19 per cent for the first 

 region and 38 per cent for the second, in either case. If the former 

 order is correct, the fused approximated value (60 per cent) is much 

 greater than that (35 per cent) obtained when these two characters 

 were used alone. On the other hand this agrees better with the 

 two-point experiments involving interrupted, because it places 

 branched near interrupted and between the other two. 



The location of the genes on a map may be postponed until the 

 loci are determined more accurately. For this reason the characters 

 are merely fisted in figure 8 below a map of 57 units length, which 

 is the length indicated by experiment 70. 



