LINKAGE RELATIONS IN MEN DELI SM 121 



The most striking confirmation of the hypothesis of linear arrange- 

 ment is found in the case of "deficiency" in the X-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. Factor 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 

 formulae which has been followed in this text, these females were of the 

 genetic constitution. 



(ywA 'bi dvmsrfb'X) ( Ywa'B e ci VMSRFB'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 nineteen 

 units distance. 



