LINKAGE RELATIONS IN MENDELISM 121 
The most striking confirmation of the hypothesis of linear arrange- 
ment is found in the case of “deficiency” in the Y-chromosome, which 
was investigated by Bridges (see p. 155) and in which the location of 
forked spines within the deficient region ‘was detected and proved as a 
result of deliberate search among those genes which had previously been 
mapped closest to bar!” 
The Mode of Interchange in Crossing-over.—F actor interchange 
conceivably might take place by interchange of isolated factors here and 
there along adjacent threads or it might follow as a consequence of inter- 
change of relatively large sections of chromatin between chromosomes. 
The sectional mode of chromatin interchange appears to have more cyto- 
logical evidence in its support and Plough’s recent studies on the effect 
of temperature on crossing-over corroborate Muller and Bridges’ inference 
that crossing-over takes place in the fine thread stage of synapsis, which 
would be the most favorable stage for sectional interchange. But breed- 
ing investigations of themselves clearly establish this hypothesis. Thus 
Muller made up females which contained twelve sex-linked mutant 
factors. These females received from one parent the factors for yellow 
body color, white eye color, abnormal abdomen, bifid wings, vermilion 
eye color, miniature wings, sable body color, rudimentary wings, forked 
spines, and from the other parent the mutant factors cherry eye color, 
club wings and bar eyes. Using the system of writing the genetic 
formulz which has been followed in this text, these females were of the 
genetic constitution. 
(ywA’b; Cumsrft/X)(Ywea’B.aVMSRFB’X). 
Muller found in tests of 712 individuals arising from gametes from such 
females, that the proportions of crossing-over between factor loci in the 
formation of gametes occurred according to the figures given in Table 
XXV. The results show that in this experiment there was no crossing- 
over in 54.4 per cent. of cases; single crossing-over in 41.7 per cent., and 
double crossing-over in 4.2 per cent. No example of triple crossing-over 
was found among these flies, but a few such cases have been observed. 
The values agree satisfactorily with those calculated from the three-point 
experiments involving the loci W, M, and B’ in this same chromosome. 
If we consider the double cross-overs which were obtained in this 
experiment we find abundant evidence in support of the sectional mode 
of chromatin interchange. It is difficult to visualize the relations from 
the numerical data, consequently Fig. 56 has been prepared to illustrate 
diagrammatically the types of double crossing-over obtained in these 
experiments. In all but one case the points of crossing-over are far re- 
moved from each other, and even in the exceptional case the distance 
between the points of crossing-over may have been as great as ninetecn 
units distance. 
Digitized by Microsoft® 
