THE N ATI RE OF THE GENE 305 



be only variations of the same phenomenon. All the work was 

 done with Drosophila. 



1. A first type may be described as follows. At the locus of a 

 known mutation, a break in the chromosome is produced, and a 

 translocation between this chromosome and another chromosome 

 occurs. The new chromosome arrangement is accompanied by a 

 phenotypic effect, described in ordinary genetic language as a muta- 

 tion, w r hich behaves as does an allele to a known mutation at the 

 same locus with the same type of phenotypic effect. Dobzhansky 

 (1932) analyzed such a translocation. The X-chromosome was 

 broken at the Bar locus, and the left end was exchanged for the 

 right end of the second chromosome. The result was an effect 

 of the type of a Bar allele and was called Baroid. In this case, it 

 could be fairly well proved that nothing else had happened 

 and that the Baroid phenotype is a result of a new arrangement 

 at the Bar locus, i.e., a position effect. (Here the term has a 

 different meaning from the one in the original Bar case : there it 

 described the actual facts; here it implies a definite interpretation. 

 But as the term is widely accepted, it may as well serve for 

 descriptive purposes.) 



Since Dobzhansky's discovery was made, other such breaks 

 at the Bar locus have been reported with a corresponding effect. 

 In some cases, it is stated that the break is only near the Bar locus 

 (Offermann 1935). We shall return to this point later. Dubinin 

 and Volotov (1935) produced numerous breaks at the Bar locus 

 by X-raying, many of which — but not all — produced the Bar 

 effect ; and Dubinin (1936) described another case of Bar mutation 

 similar to Baroid, produced by a translocation between chromo- 

 somes I and II w r ith the break at the Bar locus. Altogether they 

 produced 34 mutations of the Bar type of which 10 proved to be 

 breaks at the Bar locus, 1 a deficiency at that locus, and 6 near by. 

 In the case of 7 incomplete reversions thus produced, 5 had a 

 break at some distance from Bar. 



2. The second type of position effect is of the same order as the 

 first; i.e., a break at a definite locus occurs, and the phenotypic 

 effect of an allele of this locus is produced. But in this group the 

 rearrangement takes place within the same chromosome, usually 

 as an inversion. It had been known for a long time that some 

 of the alleles of the scute series were accompanied by chromosome 

 breaks, also alleles of yellow and a few others (Serebrowsky and 



