SEX-LINKED FACTORS IN DROSOPHILA 49 



THE LINEAR ARRANGEMENT OF THE FACTORS 



Table 2 shows the proportion of cross-overs in those cases 

 which have been worked out. The detailed results of the crosses 

 involved are given at the end of this paper. The 16287 cases 

 for B and CO are from Dexter ('12). Inasmuch as C and O are 

 completely linked I have added the numbers for C, for O, and 

 ^ for C and taken together, gi\'ing the total results in the lines 

 beginning (C, O) P, B (C, 0), etc., and have used these figures, 

 instead of the individual C, O, or CO results, in my calculations. 

 The fractions in the column marked 'proportion of cross-overs' 

 represent the number of cross-overs (numerator) to total avail- 

 able gametes (denominator). 



As will be explained later, one is more likely to obtain accurate 

 figures for distances if those distances are short, i.e., if the asso- 



o 



B C P R M 



00 ID 30.7 337 576 



Diagram 1 



elation is strong. For this reason I shall, in so far as possible, 

 use the percent of cross-overs between adjacent points in mapping 

 out the distances between the various factors. Thus, B (C, O), 

 (C, 0) P, PR, and PM form the basis of diagram 1. The figures 

 -on the diagram represent 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 chromosomes 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 observed per cent of cross-overs, and distances as calcu- 



THE JOURNAL OF EXPERIMENTAL ZOOLOGy, VOL. 14, NO. 1 



