STURTEVANT 



147 



the notch deficiency may arise at 

 stages in the life cycle at which cross- 

 ing over and bar reversion do not 

 normally occur, and, in the third case, 

 there is definite evidence that crossing 

 over did not occur. While it may still 

 be supposed that unequal crossing over 

 will sometimes give rise to section de- 

 ficiencies, the evidence indicates that 

 the three best-known examples of sec- 

 tion deficiencies in Drosophila have 

 not arisen in that way. 



UNEQUAL CROSSING OVER AND 

 THE EXACT NATURE OF SYNAPSIS 



The data on crossing over have all 

 indicated consistently that when two 

 chromosomes cross over they do so 

 at exactly corresponding levels. The 

 case of bar is the first one in which 

 any inequality of crossing-over levels 

 has been detected; and we have seen 

 in the preceding sections that an analy- 

 sis of other possible instances of such 

 an occurrence makes it probable that 

 they must be explained in some other 

 way. The case of bar is clearly quite 

 exceptional. But it does serve to sug- 

 gest that the exact correspondence of 

 crossover levels, that is so constant, is 

 not to be referred to a property com- 

 mon to all the genes. For unequal 

 crossing over occurs in females that are 

 homozygous for bar or for infrabar, 

 and in such females these loci are alike 

 in the two X chromosomes that cross 

 over unequally. It is- difficult to imag- 

 ine how the chromosomes can pair so 

 extremely exactly as they must do, un- 

 less in some way like genes come to lie 

 side by side. But the present case indi- 

 cates that this interpretation will have 

 to be applied with some caution. 



2. This result can be explained if it 

 is supposed that such crossovers are 

 unequal, so that one daughter chromo- 

 some gets two representatives of the 

 bar locus while the other receives 

 none. 



3. Only one mutation in this locus 

 has been shown to have occurred in 

 the germ track of a male. This one 

 gave rise (from bar) to a new and less 

 extreme allelomorph called infrabar. 



4. Infrabar does not appear to rep- 

 resent a quantitative change in the bar 

 gene. 



5. When, by unequal crossin£r over, 

 bar and infrabar come to lie in the 

 same chromosome, they maintain their 

 separate identities, and may be re- 

 covered again. 



6. In such double forms the two 

 elements also maintain their sequence 

 in the same linear series as the rest of 

 the genes. It is thus possible to obtain 

 bar-infrabar and also infrabar-bar. 

 These two types look alike, but can be 

 distinguished by their origin and by 

 the usual tests for determining the se- 

 quence of genes. 



7. Facet counts are given for all the 

 possible combinations of the following 

 members of the bar series: round, in- 

 frabar, bar, double-infrabar, bar-infra- 

 bar, double-bar. 



8. Analysis of these data shows that 

 tw^o genes lying in the same chromo- 

 some are more effective on develop- 

 ment than are the same two genes 

 when they lie in different chromo- 

 somes. 



9. A general survey makes it seem 

 improbable that many mutations in 

 other loci are to be explained as due 

 to unequal crossing over. 



SUMMARY 



1. Sixteen different kinds of changes 

 at the bar locus are shown to occur 

 exclusively, or nearly so, in eggs that 

 undergo crossing over at or near the 

 bar locus. 



LITERATURE CITED 



Bridges, C. B. 1917 Deficiency. Genetics 2: 

 445-465. 



1919 Vermilion deficiency. Jojir. Gen. 

 Physiol. 1: 645-656. 



1921 Triploid intersexes in Drosophila 

 melanogaster. Science, n.s. 54: 252-254. 



