164 



Sturtevant and Morgan "* and Sturte- 

 vant ^ found that these two-way 

 changes were the result of a novel type 

 of "unequal" crossing-over, by which 

 the two genes originally present in the 

 two parental chromosomes both 

 emerged in the same chromosome 

 (Bar-double) while the other resultant 

 chromosome was without Bar (Bar- 

 reverted). The change from Bar to 

 Bar-double was considered to be a 

 single gene duplication, while the con- 

 verse change, from Bar to Bar-re- 

 verted, corresponds to a one-gene 

 deficiency. Since the Bar-reverted type 

 proved to be indistinguishable from 

 the normal unmutated wild-type, the 

 gene present in Bar and lost in Bar- 

 reverted must have itself corresponded 

 to a new addition or one-gene duplica- 

 tion.^ 



Sturtevant ^ found the unexpected 

 relation that two Bar genes in the 

 same chromosome (BB/B-I-) gave a 

 greater reduction in the size of the 

 Bar eye than did two Bar genes in 

 opposite chromosomes (B/B), an in- 

 tensification of action which he formu- 

 lated as a "position effect." Dobzhan- 

 sky '^ interpreted his allelic Baroid 

 mutant as a position effect due to the 

 substitution of material at or near the 

 Bar-locus (in the normal X) by ma- 

 terial translocated from the right limb 

 of chromosome 2, and the reduction 

 in the Bar eye to the interaction be- 

 tween a gene in the X chromosome 

 and the duplication. 



A chance to clear up some of the 

 puzzles as to the origin and behavior 

 of Bar was offered by the salivary 

 chromosomes. Study of the banding in 



* Sturtevant, A. H., and Morgan, T. H., 



Science 57:746-7, 1923. 



•^Sturtevant, A. H., Genetics 10:117-47, 

 1925, ( see p. 67 this vol.). 



« Wright, S., A?ner. Nat. 63:479-80, 1929. 



^ Dobzhansky, Th., Genetics 17:369-92, 

 1932. 



BRIDGES 



a stock of Bar (forked Bar) showed 

 that an extra, short section of bands is 

 present in excess of the normal com- 

 plement, forming a duplication. The 

 insertion point of this duplication is in 

 the bulbous "turnip" segment, not far 

 from the basal end of the X.^ 



The exact point of the insertion is 

 ambiguous, for a reason which will 

 appear below. The normal X in this 

 region (see revised map below) 



BAR- REVERTED 



NORMAL 



BAR-DOUBLE 



shows in sub-section 16A a heavy 

 band, which in well-stretched chro- 

 mosomes, or with certain fixations, is 

 a clear doublet, usually with the halves 

 united in a capsule, but occasionally 

 completely separate. This is followed 

 by a very faint dotted line, which can 

 be seen only in the most favorable 

 conditions. Next follows a fairly weak 

 line which is distinctly "dotted" in 

 texture, with the separate dots loosely 

 connected across the width of the 

 chromosome. Next follows closely a 

 still fainter, diffuse, continuous-tex- 

 tured doublet, with the doubleness 

 generally appearing as mere broaden- 

 ing. The last line of sub-section 16A 

 is again a very faint dotted singlet. 

 Sub-section 16B starts with a sharply 

 discontinuous line of fairly heavy dots 

 or vesicles and is a line very easy to 

 recognize. The greatest width of the 

 bulbous segment 16A is at the two 



8 Bridges, C. B., Jour. Hered. 26:60-4, 1935. 



