STURTEVANT 



have found this tv^pe hard to breed 

 and have therefore not obtained large 

 numbers of offspring. It may be re- 

 called that Zeleny has obtained both 

 bar and round from such females, but 

 not in experiments in which forked 

 and fused were present. In table 3, 

 showinsf the data I have obtained, only 

 males are recorded, for the same rea- 

 son as in table 2, and also because the 

 females could not be classified for 

 fused. 



Table 3 



fBB 

 BBf, 



9 XfBB S 



129 



In the case of double-bar over 

 round, bar should be produced by any 

 crossover between the two bars of the 

 double-bar chromosome. This event 

 might seem less unlikely to occur than 

 the t\'pe of unequal crossing over in- 

 voked in the preceding experiments; 

 and both of the chromosomes resulting 

 from such crossing over should yield 

 bar, whereas in the preceding cases, a 

 given crossing over must always have 

 yielded chromosomes bearing two dif- 

 ferent kinds of bar allelomorphs. It is 

 accordingly in agreement with the 

 hypothesis that tables 4 and 5 show a 

 higher percentage of mutation than 

 do tables 1 and 2. Two non-dis- 

 junctional individuals (a /f „ .3 and a 



+ + + X , u • J r 



9 ) have been omitted from 

 / dB f„ 



table 5. 



No mutations are to be expected 



from females that are heterozygous 



Table 4 



BB _ r r ^ 



J^ 9 Xffu$ 



for bar and for round (■~), since cross- 

 ing over can not produce any new 

 combination. The results of the tests 

 are shown in tables 6 and 7, and 

 are in agreement with this expecta- 

 tion. One non-disjunctional female 



\~f~R~f — ) ^^^^ ^^^° produced in this 



series.'* 



4 This series also produced one female that 

 was wild-type in appearance. Such a female 



L. V. Morgan (1922) has described 

 a race of D. Jiielajiogaster in which the 

 two X chromosomes of the female are 

 attached to each other, so that such a 



would be either a double crossover — which 

 is not at all probable in such a short chro- 

 mosome section, or a non-crossover rever- 

 sion of bar. This female was mated to an 

 unrelated bar male, and produced 124 bar- 

 over-round daughters and 96 round-eyed 

 sons; but none of the sons showed either 

 forked or fused. This result must mean that 

 the exceptional female was due to con- 

 tamination. 



