A GENE FOR THE FOURTH CHROMOSOME OF 

 DROSOPHILA 



HERMANN J. MULLER 



From the Zoological Deparitnent of Columbia University 



Drasophila ampelophila contains two pairs of long 'autosomes' 

 (chromosomes other than sex-chromosomes) and one pair of 

 minute 'autosomes' ; there are, in addition, a pair of long sex-chro- 

 mosomes alike in the female ('X's) but unlike in the male ('X' and 

 'Y')."^ Correspondingly, breeding tests of the numerous mu- 

 tants of Drosophila have revealed a great group of genes (contain- 

 ing over thirty members) which are sex-linked in such a way that 

 they are distributed, in the reduction division, to exactly the same 

 cells as is the X-chromosome, and two other great groups (with 

 about twenty members each) which are not sex-linked and so 

 have been considered to follow in their distribution two of the 

 pairs of autosomes (presumably the long ones). The genes in 

 these three groups are therefore said to lie in Chromosomes I, II 

 and III, respectively. The members of any group of genes are 

 linked with one another ; in the female the linkage is partial and is 

 of the linear type to be expected on the basis of the chiasmatype 

 theory of Janssens and Morgan; in the male the linkage is complete 

 (no crossing-over). All the members of one group assort inde- 

 pendently of the inembers of either of the other groups, in both 

 sexes, as would be expected on the generally accepted postulate of 

 the random assortment of non-homologous chromosomes in the 

 reduction division. 



1 Two lines of recent cytological investigation, the one followed by Mr. C. W. 

 Metz, the other by Mr. C. B. Bridges, have given evidence (not yet fully published) 

 which goes far towards refuting the view suggested by Stevens that the so-called 

 'X' chromosome of Drosophila actually consists of a short autosome with the real 

 X-chromosome joined to it and that the 'Y' chromosome represents this short auto- 

 some without any 'X' attached. 



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