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 vermilion 

 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. As has been shown, 

 interference decreases with increase of distance.^ 



In the construction of the chromosome maps shown 

 in the frontispiece the distance taken as a unit is 

 that within which 1 per cent, of crossing over will 

 occur. Thus, yellow and white are placed one unit 

 apart, since there is 1 per cent, of crossing over be- 

 tween yellow and white. White and bifid give 5 per 

 cent, of crossing over, hence they are placed five units 

 apart; and since yellow and bifid give 6 per cent., 

 bifid must be on the other side of white from yellow. 

 In a similar way the relative positions of the other 

 factors have been plotted, the position of any factor 

 on the map being determined, as far as possible, by 



1 If it should be found that crossing over takes place at a stage when 

 the chromosomes actually are tightly twisted, there is no evident mech- 

 anism which would tend to prevent crossing over from taking place at 

 two points near together, unless in this case we should suppose that 

 crossing over results from a breaking of the threads at some point due to 

 the strain of very tight twisting, and that a break at one point relieves 

 the strain in the vicinity, thus tending to prevent another crossing over 

 nearby. 



