LINKAGE RELATIONS IN MENDELISM 115 
gray normal and of the genetic constitution (Be,)(bC,). When these are 
interbred to obtain the F', generation the results are shown in the checker- 
board in Fig. 53. According to this checkerboard, the /’, 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 /’2, and it 1s of interest to 
note that no matter what the amount of crossing-over in the female the 
flies in F, will always be in the ratio 2:1:1. In F3 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 fF’; has been much 
used by Morgan in determining the factor group to which new mutations 
(Bey) (bC.) 
11.5 | 11.5 
11.5 (BC) (BC,) (Bev) (BC>) (bC») 
gray normal gray normal 
38.5 88.5 
38.5 (Bev) (Bey) (Bev) (Bey) (bCy) 
gray curved gray normal 
38.5 38.5 
38.5 (bC») (bCy) (Ber) (bC.) (Cv) 
gray normal black normal 
bear 11.5 
11.5 (be,) (bey) (Bev) (bey) (bCo) 
gray curved black normal 
Fig. 53.—F 2 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 Fz. 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 along 
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 
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