226 



THE SECOND-CHROMOSOME GROUP 



back-cross (table 73). This second back-cross gave linkage results 

 which differed only very slightly from those of the first, and in such a 

 way that by combining the two sets of data the deviations of one tend 

 to balance those of the other and more nearly correct values can be 

 calculated. 



Table 73. — Pi, streak purple 9 X curved cf ; B.C., Fi streak 9 X 



curved cf of stock. 



purple 



= 99.8 



) 



The combined data gave 1,807 individuals, of which 931 were non- 

 cross-overs, 501 cross-overs between streak and purple, 244 cross-overs 

 between purple and curved, and 131 double cross-overs. The cross- 

 over values calculated from this distribution are streak purple 35.0 

 per cent, purple curved 20.7 per cent, and streak curved, 41.2 per cent. 



The coincidence from the first back-cross was 98.0 and from the 

 second 102.0. The coincidence from the combined data was 99.8 



/ 1807X 131X100 



V 632X375 



For a section of this great length and involving this particular region 

 the net result was that the occurrence of a cross-over in one section was 

 without effect upon the occurrence of a cross-over in the other section. 

 The locus of streak as calculated from the above data, making 

 allowance for the probable amount of double crossing-over, was 40.6 

 to the left of purple or 34.7 units to the left of black, which was the 

 locus farthest to the left of those previously determined. This great 

 gap was almost immediately filled by the mapping of dachs at 17.9 

 units to the left of black, or almost exactly midway between the two. 

 This position of dachs offered a new base of reference for streak and 

 one much more dependable than the remote purple. The data on 

 which a direct determination of the streak dachs distance was made is 

 included in the section on star, for by means of a quadruple back-cross 

 involving star, streak, dachs, and purple the loci of both star and streak 

 were linked up with the portions already mapped. The streak | 

 dachs interval was found by this experiment to be about 16.2 units. 



