442 G. W. DELUZ HAMLETT. 



rough interval is even lower than in the experiments here reported. 

 These differences may be due to a differential viability in the 

 two cases; there may also be differences in genes influencing 

 crossing over, but such differences in ratios do not affect the 

 principal conclusion of this paper. The important point is that 

 the effect of the translocation is the same whether it is hetero- 

 zygous or homozygous. 



SUMMARY. 



1. The chromosomal abnormality in Drosophila, known as 

 translocation I., is located at approximately 76.9 on the third 

 chromosome. 



2. The presence of translocation does not affect the crossover 

 values for the first 45 units in the left-hand end of the chromo- 

 some. To the right of this point, the per cent, of crossing over 

 progressively decreases as the locus of translocation is approached, 

 reaching its minimum of less than one third normal value in the 

 region between ebony and rough. 



3. The same results are obtained no matter whether the trans- 

 location is homozygous or heterozygous. 



4. The fact that the results are the same in both heterozygous 

 and homozygous flies shows that the attached fragment is not 

 interpolated in the third chromosome, but is attached to its side. 

 The resulting chromosome is thus of a type which has hitherto 

 not been reported: that is, one in which the genes are not in a 

 single linear series. 



BIBLIOGRAPHY. 

 Bridges, C. B. 



'23 The Translocation of a Section of Chromosome II. upon Chromosome III. 



in Drosophila. Anat. Record, 24, 426. 

 Bridges, C. B., and Morgan, T. H. 



'23 The Third-chromosome Group of Mutant Characters of Drosophlia melano- 



gaster. Carnegie Institution of Washington, Publication No. 327. 

 Morgan, T. H. 



'19 The Physical Basis of Heredity. 



