STURTEVANT 



Mutations are known in which there 

 appeared to be no crossing over in the 

 region concerned,— both in females 

 and in males where crossing over does 

 not normally occur at all. The previ- 

 ously cited cases of mutations in the 

 X chromosomes of males are examples. 

 These may seem to furnish conclusive 

 evidence that mutation need not be 

 accompanied by crossing over. There 

 is, however, one possibility that needs 

 to be considered in this connection. 



Recent results (not yet published, 

 but soon to appear) obtained with 

 triploid females (Bridges and Ander- 

 son) and with females having unlike 

 attached X's (Anderson, L. V. Mor- 

 gan, and Sturtevant) have shown that 

 crossing over must normally occur 

 when the homologous chromosomes 

 are doubled: that is, in a "four-strand 

 stage" (in diploid females). These re- 

 sults show also that crossing over may 

 occur between only two of the strands 

 at a given level. Now, if it be supposed 

 that sister strands may cross over with 

 each other, there will result chromo- 

 somes in which no rearrangement of 

 mutant genes has occurred, since sister 

 strands come from the division of one 

 chromosome and will be identical in 

 the genes that they carry. Yet it is con- 

 ceivable that such crossing over might 

 be unequal, and in such a case might 

 lead to the production of a new muta- 

 tion that did not appear to be due to 

 crossing over. 



The data presented in this paper 

 show that such an event must be ex- 

 tremely rare in the case of bar, since 

 no clear case was found of bar muta- 

 tion (in a female) unaccompanied by 

 evident crossing over between forked 

 and fused. The few^ exceptional cases 

 may be accounted for in this way; but, 

 as pointed out when they were de- 

 scribed, it seems at least equally prob- 

 able that all of them are due to experi- 

 mental errors. We must conclude that 



145 



sister strands do not cross over with 

 each other; or, if they do, that the 

 crossing over is rarely, if ever, un- 

 equal. 



It is therefore unlikely that apparent 

 non-crossover mutations in other loci 

 are to be referred to crossing over be- 

 tween sister strands. 



"presence and absence" and 

 quantitative view of mutation 



It will be observed that the hypoth- 

 esis advocated in this paper makes 

 bar, double-bar and round by rever- 

 sion (or infrabar, double-infrabar and 

 round by reversion) represent quan- 

 titative variations of the same sub- 

 stance. In the case of bar and round, 

 the hypothesis is the same as the origi- 

 nal and most special type of quantita- 

 tive view, the "presence and absence" 

 hypothesis. But the present scheme 

 differs from the earlier ones in that it 

 is based on definite evidence for the 

 occurrence of unequal crossing over. 

 That is, the mechanism whereby the 

 quantitative differences are brought 

 about is an essential part of the hy- 

 pothesis. In the preceding section we 

 have seen that there is definite evi- 

 dence to show that unequal crossing 

 over is not usual in the production of 

 new mutant types. It is especially note- 

 worthy that this evidence was derived 

 in part from the white locus of Dro- 

 sophila and the variegated locus of 

 maize,— two of the best-known ex- 

 amples of loci that have produced 

 large series of multiple allelomorphs. 

 It is clear, therefore, that the bar case 

 does not furnish support to the idea 

 that mutations in general are quantita- 

 tive in nature. Even with respect to 

 multiple allelomorphs, where the 

 quantitative view has often been urged, 

 it is obvious that, at least in the cases 

 of white and variegated, the bar evi- 

 dence does not in any way support 

 that view. 



