No. 595] THE MECHANISM OF CROSSING-OVER 423 



Total Single, Double, and Triple Crossing-over 



In the case of this chromosome, too, the law of linear 

 linkage is graphically illustrated by the characteristic 

 "sectional" mode of interchange between the groups. 

 The non-cross-overs here constitute only 32.5 per cent, of 

 the population, whereas the single cross-overs make up 

 51.1 per cent., the double cross-overs 15.2 per cent., and 

 the triple cross-overs 1.3 per cent. In making a map of 

 this chromosome, the chances of error are greater than in 

 the preceding case, since not so many flies have been ob- 

 tained. Nevertheless, the values correspond very closely 

 with estimates of the results obtained in other work, al- 

 though figures exactly representing the sum total of other 

 work are not just now available for comparison. 



Let us now construct a curve showing the frequency 

 with which, in the experiment with the first chromosome, 

 points various distances apart showed coincidence of 

 crossing-over. Suppose that in this curve the horizontal 

 line represents the distance apart of the two coincident 

 cross-overs, and the vertical line the per cent, of cases in 

 which double crossings-over at such distances occur. For 

 example, if it were known that double crossing-over for a 

 distance anywhere between 15 and 16 units occurred in .2 

 Per cent, of all cases the height of the curve above the 

 figures 15 and 16 would be made .2 vertical units. Now, 

 each case of double crossing-over that actually happens 



