LINKAGE RELATIONS IN MENDELISM 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 
007 f QO q ras 
2w \ 
35 A 
5.5 B, 
LE 
13 .C 
Ws 
29-V | \ N . 
3I-M \ NF minh 
H a. wink 
37-S \ \ | \ ‘ 
a a 
o : | I . 
50 B UJ | | S oe 
2 1 2 7 1 8 2 1 
Fic. 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 XX VI. 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 
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