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/•//) SIOLOGICAL GENETICS 



in Round (Wild type but also irregularly arranged. The 

 strangest difference, however, is the reaction to temperature. 

 Whereas in the Bar and Ultrabar alleles facel aumber decreases 

 with rise of temperature, it increases in [nfrabar (Luce, 1931). 

 Special dominance features of Infrabar will soon he mentioned. 



We shall consider first the effects of different quantities of 

 Infrabar, which have been obtained in a parallel series to Bar, 

 according to Sturtevant (1925) (Table 15). 



We see a perfectly parallel series to the one recorded for Bar, 

 again showing that the Infrabar allele also acts in different dosages 

 proportional to its quantity. 



BAR- REVERTED 



NORMAL 



BAR-DOUBLE 



Fig. 27. — Salivary chromosomes of Drosophila at the Bar-locus. (From Bridges, 



1936, Science 83.) 



These two sets of facts in which the actually known different 

 doses of the alleles and the normal allelomorphs fit into an orderly 

 arrangement suggest that the Wild type, the Bar, and the 

 Infrabar alleles are all different quantities of what was supposed 

 to be a gene. The first two of these conclusions drawn by 

 Goldschmidt (1927c) have now been proved correct. Bridges 

 (1936) and also Muller et al. (1936) have shown that the Bar 

 mutation is an internal duplication of a small section of the 

 X-chromosome (Fig. 27). Two such sections then represent 

 Bar. and three Ultrabar or Double bar. This new fact makes it 

 possible to analyze the case further. A series of increasing 

 combinations of B + and B represents, then, a series of two to six 

 doses of this chromosome section. If we assume, for reasons 



