64 LINKAGE 



sable and bar should be somewhat protected from 

 crossing over. The usual amount of crossing over 

 between sable and bar is 14 per cent., but in those 

 cases in which crossing over between vermilion and 

 sable occurs, this value becomes reduced to somewhat 

 less than 4 per cent. In this same fashion a region 

 just to the left of sable is protected, but this protec- 

 tion decreases with the distance from the vermihon 

 sable region. The fact that one crossing over makes 

 less likely another crossing over in a nearby region, 

 or in a sense interferes with a second crossing over 

 nearby, is called interference. It is found that in- 

 terference decreases with increase of distance until, 

 in group I, it vanishes at a distance of about 46; 

 i.e., a crossing over at one point does not affect the 

 chance of crossing over at another point 46 units 

 away. Weinstein finds, however, that at a still 

 greater distance interference reappears, so that 

 there is a modal distance between the two breaks in 

 double crossing over, possibly due to the threads 

 bending in loops that tend to have a certain length. 

 In the chromosome maps the distance taken as a 

 unit is that within which 1 per cent, of crossing 

 over occurs. Thus yellow and white are 1.5 units 

 apart in the frontispiece, since there is 1.5 per cent, 

 of crossing over between them. White and bifid 

 give 5.5 per cent., hence are placed 5.5 units apart, 

 and since yellow and bifid give 7 per cent., bifid must 

 be placed on the other side of white from yellow. 

 The other factors have been plotted similarly, 

 each locus being determined, as far as possible, by 



