STURTEVANT 



genes (deficiencies and duplications), 

 and what have been called "point- 

 mutations" or "frene-mutations." Prob- 

 ably this last type includes quite 

 diverse processes. It is therefore im- 

 portant to collect information as to the 

 nature of specific examples of muta- 

 tion. For this purpose it will com- 

 monly be necessary to work with a 

 frequently recurring mutation. Only 

 one frequently mutating gene has 

 hitherto been discovered in Drosoph- 

 ila, namely, bar. Crossing over has 

 proved to be the key to the mutation 

 behavior of bar, as will be shown in the 

 present paper. The case appears not 

 to be, strictly speaking, a point-muta- 

 tion after all, but a new kind of sec- 

 tion mutation, in which the section 

 concerned is so short as to include 

 only a single known gene, and in 

 which unequal crossing over furnishes 

 the mechanism for bringing about the 

 new types. 



HISTORICAL 



In 1914 Tice (1914) found a single 

 male of Drosophila melanogaster that 

 had narrow eyes (see figures 1 and 2). 

 The new type, called bar (or "barred" 

 in the earlier literature), was found to 

 depend on a sex-linked gene located at 

 57.0 in the X chromosome. It was fur- 

 ther found that the bar character is 

 dominant, in the sense that females 

 carrying one bar gene have eyes dis- 

 tinctly different from the wild-type or 

 "round" eye (figure 3). Because of 

 this dominance the type has been ex- 

 tensively used in linkage experiments. 

 A^Iay (1917) reported that the bar gene 

 occasionally reverts to normal (figures 

 4 and 5)— a process that has more re- 

 cently been extensively studied by 

 Zeleiiy (1919, 1920, 1921). Zeleny 

 found that the frequency of reversion 

 is variable, but in many stocks is such 

 that about 1 in 1600 offspring from a 

 pure bar stock receives a not-bar, or 



125 



round, allelomorph. Zeleny also con- 

 cluded that the reversion probably 

 occurs chiefly (or perhaps exclusively) 

 in females. His argument, based on 

 the sex ratio found among reverted 

 individuals, is not as convincing as the 

 direct tests that will be described in 

 this paper, and which verify his con- 

 clusion. Zeleny also found that homo- 

 zygous bar gives rise to a new and 

 more extreme allelomorph of bar, that 

 he has called "ultra-bar." For reasons 

 that will be developed in this paper, 

 I prefer to call it "double-bar." The 

 eyes of double-bar are distinctly 

 smaller than those of bar (figure 6). 

 Zelenv' has shown that the type is more 

 strongly dominant over round than is 

 bar, and also that double-bar is largely 

 dominant over bar. 



Zeleny likewise found that homo- 

 zygous double-bar stocks revert to 

 round with a frequency not very dif- 

 ferent from that of homozygous bar 

 stocks, and that double-bar occasion- 

 ally mutates to bar; that is, it can go all 

 the way back to round at one step, or 

 it can give bar, which, in turn, is 

 capable of reverting to round. Zeleny 

 has argued that the three types, round, 

 bar, and double-bar, have the same 

 characteristic properties, regardless of 

 their origin. The round eye of re- 

 verted bar is indistinguishable from 

 wdld-type; bar derived from double- 

 bar does not differ from the original 

 bar, etc. This point will be considered 

 in more detail in a later section. 



Sturtevant and Morgan (1923) 

 showed that double-bar over bar ^ also 

 gives rise to round-eyed individuals. 

 They reported three reversions from 

 this combination and three from ho- 



1 In this paper the constitution of hetero- 

 zygotes will be expressed as above in order 

 to avoid circumlocution or indefiniteness. 

 "Double-bar over bar" is to be understood 

 as: "carrying double-bar in one X chromo- 

 some and bar in the other X chromosome." 



