LINKAGE RELATIONS IN MENDELISM 119 
of chromatin have occurred which affect the relations of the factors to 
each other, so that this type of separation after synapsis gives the non- 
cross-over gametes (wmB’X) and (WMb'X). Types B and C involve 
single breaks in the chromosomes followed by chromatin interchange in 
reunion. They are the single cross-overs and give the cross-over gametes 
i P 
Ww 
m 
B 
er, 
ae — 
Pee Lie a) 
ie : 
m m M 
U d 
ome ee Bae a 
46.3 28.15 15.45 10.1 
Fig. 55.—Diagram to show crossing-over in a three-factor experiment. 
(wMb/X) and (WmB’X) and (wmb’X) and (WMB’X) respectively. 
Finally in type D the chromosomes have broken and exchanged material 
at two points. This type is called double crossing-over and results in 
the production of gametes of the genetic constitutions (wMB’X) and 
(Wmb’X). In. this last case, although chromatin interchange has 
occurred between the two chromosomes, the relations between the loci 
W and B’ remain unchanged. 
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