Chromosomes and Genes 121 



The subsequent history of the Bar case bears out this statement, 

 and so does the welter of other facts concerning position effect. In 

 1932 Dobzhansky found a translocation with one break near the 

 genetic locus of Bar. This rearrangement had a phenotypic effect 

 resembling Bar, and acted like an allele of the latter. Muller and 

 Altenburg (1930) had aheady noticed that most translocations in 

 Drosophila had some phenot)'pic effect like lethality, sterility, or 

 changed phenotype. But Dobzhansky 's baroid translocation was the 

 first discovery of what we now consider a genuine "position effect," 

 which should be called "rearrangement effect" if a change of termi- 

 nology were still possible. This was clearly different from Sturtevant's 

 position of two Bar genes in one or two chromosomes, since no mu- 

 tant "Bar genes" were involved. The solution of the discrepancy in 

 the causation of the two phenomena, both of which were called po- 

 sition effects, became possible when Bridges (1936) and Muller 

 (1936) found that the "mutant" Bar is the phenotypic effect of a 

 duplication of a number of bands (visible in the salivary gland 

 chi-omosomes ) in the region where the Bar effect is locaHzed geneti- 

 cally ( illustrations can be found in all textbooks ) . Bar had these bands 

 twice in tandem, and double Bar had them three times. At first sight 

 this could mean that the duplicated section contained a Bar gene 

 which in one dose (or two homozygous) had no effect; but in two 

 doses, namely, four homozygous, had the Bar effect; and in six doses, 

 the double Bar effect. Expressed in Mendelian formulation this would 

 mean, symbolizing the old Bar gene by B and the group of bands 

 which are present in triplicate, B^: 



OLD FORMULATION CYTOLOGICAL FORMULATION 



1. B/B = Bar homozygous 1. B*^ I B'' = normal 



Bb 

 Bb 



Bb 



= Bar homozygous 

 = Bar heterozygous 



5. Bb 

 Bb 



Bb 



B'' = double Bar homozygous 

 B'' 



Bb 



= double Bar heterozygous, 



with position effect (see 3) 



