LINKAGE RELATIONS IN MEN DELI SM 



123 



there is still some evidence of interference. Based purely on the laws 

 of chance, if crossing-over occurs between W and M in 33.1 per cent, of 

 cases and between M and B' in 20.5 per cent., then the chance of coinci- 

 dent crossing-over is equal to the product of the independent chances of 

 crossing-over. This gives a value of 6.8 per cent, which is slightly greater 

 than the value 5.05 per cent, calculated from the experimental data. 



A three-point experiment involving shorter distances, however, gives 

 a clearer idea as to the extent of interference. Morgan and Bridges 



2 w 



35 A 

 5.5 IK 



13 .C, 



29-V 

 31-M 



37- S 



4&SR 

 4S.7F 



SO B' 



FIG. 56. Diagram showing types of double crossing-over in females of Drosophila 

 heterozygous for twelve sex-linked factors. The figures below indicate the number of times 

 the type occurred in 712 cases. (The loci indicated in the "map" at the left are only approx- 

 imately correct according to recent data of Morgan and Bridges, but they are sufficiently 

 accurate for the purpose of this diagram.) 



have reported such an experiment involving the loci for vermilion, sable, 

 and bar with the results given in Table XXVI. From this table the total 

 percentage of crossing-over between vermilion and sable is 9.8 per cent, 

 and between sable and bar 13.8 per cent. The expected percentage of 

 double crossing-over for these values obtained by taking 9.8 per cent, of 

 13.8 per cent, would be 1.35 per cent. The observed amount of double 

 crossing-over, 0.25 per cent., is only about one-fifth of this value. 



That interference is normally to be expected from the method of 

 chromatin interchange in synapsis may be seen clearly by a consideration 

 of Fig. 57. Thus if the chromosomes have a modal length in loop 

 twisting about each other in synapsis, then a crossing-over at point B 



