326 INHERITED LINKAGE VARIATIONS 



stocks derived from it (ebony, spread, eosin), greatly decreases cross- 

 ing-over in the right-hand end of the third chromosome when it is 

 present in heterozygous form; but this result disappears in flies homo- 

 zygous for C Ur . Moreover, the gene is itself located in the region 

 in which it produces its greatest effect. The following sample experi- 

 ment will illustrate its action. 



Certain experiments carried out by Dr. C. B. Bridges, in investi- 

 gating cream III, led to the hypothesis that the eosin stock was im- 

 pure for Cm. Accordingly two males from this eosin stock were 

 mated individually to sepia spineless sooty rough females, and daugh- 

 ters were back-crossed to sepia spineless sooty rough males, with the 

 results shown in table 23. The values for sepia spineless are not given, 

 because sepia was not easily classifiable in the eosin males produced. 



There are clearly two quite distinct types of results here. In 9 of 

 the cultures there is less than 1 per cent crossing-over between spine- 

 less and rough; in the other 4 there is about 25 to 30 per cent crossing- 

 over between these loci. 1 The results are due to the presence of 

 C HI in those females that gave the low result, and its absence in those 

 that gave the high one. That the difference was due to the nature 

 of the third chromosomes derived from the fathers was shown by 

 testing the crossing-over in wild-type daughters of these females. 

 In every case such daughters gave approximately the same results 

 as their respective mothers. Daughters of all but 2615 and 2621 

 were so tested. 



In females homozygous for C m the crossing-over between s s and 

 e rises to about 40 per cent (j-fTr = 41.6 per cent, in one experiment 

 selected at random), as against about 12 per cent in the absence of 

 Cm, and less than 1 per cent when it is present in heterozygous form. 

 This result is in agreement with Muller's (1916) conclusion that 

 homozygous C HI results in the production of more crossing-over than 

 occurs in "normal" females. 



The Cm experiments are still in pro- TABLE 24. 



gress, and will be reported in detail in 

 connection with the other third-chromo- 

 some data accumulated in this labora- 

 tory. From the above account, how- 

 ever, it may be seen that the parallel 

 between C n r and Cm is very close. The effect of each upon the region 

 in which it lies is shown in table 24. The s s e values are only 

 approximately correct. 



1 It will be observed that both males from eosin stock were heterozygous for Cm There 

 was later found to be a lethal near the Cm. This, in connection with other results obtained with 

 the eosin stock, suggests that it was a "balanced lethal" stock for the third chromosome (see 

 Muller, 1917). This stock has now died out, so that it is no longer possible to test such a 

 hypothesis. 



Usual result 



Heterozygous C . 

 Homozygous C . . 



ii-p T 8 P 



46.5 



2.9 



41.5 



in-s, e 



12.0 



0.5 



40.0 



