STURTEVANT 



71 



Table 2 



Factors Concerned 



BCO ... 



BO 



BP 



BR 



BM 



COP .... 

 COR ... 

 COM ... 



OP 



OR 



OM .... 



CR 



CM 



B(C, O) 

 (C, 0)P 

 (C, 0)R 

 (C, 0)M 



PR 



PM 



Proportion 



of 

 Cross-overs 



193 



16287 



2 



373 

 1464 

 4551 

 115 

 324 

 260 

 693 

 224 

 748 



1643 

 4749 

 76 

 161 

 247 

 836 

 183 

 538 

 218 

 404 

 236 

 829 

 112 

 333 

 214 

 21736 



471 

 1584 

 2062 

 6116 

 406 

 898 

 17 

 573 

 109 

 405 



Per Cent 



of 



Cross-overs 



1.2 



0.5 

 32.2 

 35.5 

 37.6 

 30.0 

 34.6 

 47.2 

 29.4 

 34.0 

 54.0 

 28.5 

 33.6 



l.O . 

 29.7 . 

 33.7 -■ 

 45.2 ~ 



3.0 

 26.9 



For this reason I shall, in so far as 

 possible, use the per cent of cross-overs 

 between adjacent points in mapping 

 out the distances between the various 



factors. Thus, B (C, O), (C, O) P, 

 PR, and PM form the basis of the dia- 

 gram. The figures on the diagram rep- 

 resent calculated distances from B. 



Of course there is no knowing 

 whether or not these distances as 

 drawn represent the actual relative 

 spacial distances apart of the factors. 

 Thus the distance CP may in reality 

 be shorter than the distance BC, but 

 what we do know is that a break is far 

 more likely to come between C and P 

 than between B and C. Hence, either 

 CP is a long space, or else it is for 

 some reason a weak one. The point I 

 wish to make here is that we have no 

 means of knowing that the chromo- 

 somes are of uniform strength, and if 

 there are strong or weak places, then 

 that will prevent our diagram from 

 representing actual relative distances— 

 but, I think, will not detract from its 

 value as a diagram. 



Just how far our theory stands the 

 test is shown by Table 3, giving ob- 

 served per cent of cross-overs, and 

 distances as calculated from the fig- 

 ures given in the diagram of the 

 chromosome. Table 3 includes all 

 pairs of factors given in Table 2 but 

 not used in the preparation of the 

 diagram. 



It will be noticed at once that the 

 long distances, BM, and (C, 0)M, 

 give smaller per cent of cross-overs, 

 than the calculation calls for. This 

 is a point which was to be expected, 

 and will be discussed later. For the 

 present we may dismiss it with the 

 statement that it is probably due to 

 the occurrence of two breaks in the 

 same chromosome, or 'double cross- 

 ing over.' But in the case of the 

 shorter distances the correspondence 

 with expectation is perhaps as close as 

 was to be expected with the small 

 numbers that are available. Thus, BP 

 is 3.2 less than BR, the difference ex- 

 pected being 3.0. (C, 0)R is less than 



