LINKAGE RELATIONS IN MEN DELI SM 



115 



gray normal and of the genetic constitution (Bc v )(bC v ). When these are 

 interbred to obtain the F z generation the results are shown in the checker- 

 board in Fig. 53. According to this checkerboard, the Fz will consist of 

 flies in the ratio 



2 gray normal : 1 gray curved : 1 black normal. 



No black curved flies are obtained in this cross in F z , and it is of interest to 

 note that no matter what the amount of crossing-over in the female the 

 flies in F 2 will always be in the ratio 2:1:1. In F z black curved flies 

 may be obtained from a certain percentage of matings of black normal 

 or of gray curved flies. 



The failure of the double recessive class to appear in F z has been much 

 used by Morgan in determining the factor group to which new mutations 



(Bc v ) 



(bC.) 



11.5 (BC,) 



38.5 (Be,) 



38.5 (bC,) 



11.5 (be,) 



FIG. 53. Fz obtained by crossing gray curved and black normal flies. 



belong. For this purpose black-pink flies are crossed with the new mutant 

 type. Since black lies in the second group and pink in the third, if the 

 new factor belongs to either of these groups it will fail to show the corre- 

 sponding double recessive form in F%. Whether it belongs to the sex- 

 linked group is of course readily determined from the sex relations ob- 

 tained in such an experiment, and if the test shows that the factor in 

 question belongs to none of these three groups, by exclusion it must be- 

 long to the fourth. 



Linear Arrangement of Factors. It was an old idea of Roux brought 

 forward to explain the division of the chromatin while in the form of a long 

 thin thread that the individual elements of the chromatin are arranged 

 in a linear series in the chromosomes. Later Janssens developed the 

 idea that in synapsis homologous chromosomes twist about each other 



