128 



eight reversions, and also both of the 

 double-bars, occurred in gametes that 

 came from crossing over between 

 forked and fused, though the total 

 forked fused crossovers constituted 

 only 2.4 percent of the population. 

 The one exceptional case, a wild-type 

 male, is not above suspicion of having 

 arisen through contamination rather 

 than reversion of bar. That he was 

 really round-eyed was proven by tests. 

 Only 4 other exceptions to the rule 

 that mutation in this locus is accom- 

 panied by forked-fused crossing over 

 have been met with in the work here 

 reported. These will be discussed sepa- 

 rately later. 



On the basis of these results we may 

 formulate the working hypothesis that 

 both reversion and the production of 

 double-bar are due to unequal crossing 

 over. If we suppose that, in a female 



+B+ . . , 



, crossmg over occurs m such 

 T " lu 



a way that the respective points of 

 interchange lie to the left of the bar 

 locus in one chromosome, but to the 

 right of it in the other one, there will 

 result chromosomes of the constitu- 

 tion jBB+ and -t-f„ (or f-|- and 

 + BBfu). The hypothesis is that re- 

 verted round is simply no-bar, and that 

 double-bar is BB,— this being the rea- 

 son for abandoning Zeleny's name, 

 ultra-bar. 



This hypothesis makes reverted 

 round and double-bar complementary 

 crossovers, and they should accord- 

 ingly be produced with equal fre- 

 quency. Table 1 agrees with Zeleny's 

 more extensive data in showing that 

 round is apparently far more frequent 

 than double-bar; but such a result was 

 to be expected for two reasons. Dou- 

 ble-bar is not as viable as round, so that 

 fewer of the double-bar mutant indi- 

 viduals would be expected to survive; 

 and double-bar is not always clearly 

 distinguishable from bar, so that some 



STURTEVANT 



mutant individuals are probably over- 

 looked, while it is not likely that any 

 reversion is overlooked through diffi- 

 culty of classification. 



The double-bar over bar experi- 

 ments reported by Sturtevant and 

 Morgan (1923) can be interpreted in 

 the same way: the reversion is here 

 due to unequal crossing over just as in 

 homozygous bar. In the earlier cul- 

 tures of the experiments previously 

 reported, only the reversions were 

 classified for forked and for fused. 

 Two of the reversions were in such 

 incompletely classified cultures. Ta- 

 ble 2, including all the double-bar over 

 bar data for which complete counts 

 are available, contains one of the pre- 

 viously reported reversions and one 

 new one. This table includes only the 

 male offspring, since the BB derived 

 from the fathers rendered the classi- 

 fication of the females uncertain. 



The process of unequal crossing 

 over might be expected to give rise to 

 triple-bar from the females that are 

 double-bar over bar. No individual 

 that could be so identified was ob- 

 tained, though several specimens with 

 very small eyes were tested. All those 

 that were fertile proved to be double- 

 bar. Apparently triple-bar is either 

 inviable or sterile. This problem will 

 be discussed again below. 



Table 2 



9 XfBBf^ $ 



fBBf, 



My own experiments with homo- 

 zygous double-bar have not yielded 

 any mutations, probably because I 



