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. It is found that in- 
terference decreases with increase of distance until, 
in group I, it vanishes at a distance of about 46; 
1.€., @ 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 
