HYBRIDITY IN DROSOPHILA 



179 



possible (Painter, Bridges and many others) to identify the structural 

 differences such as in some cases exist within natural populations, 

 or in other cases distinguish races and species, or in others again are 

 produced by irradiation. Where a deletion has occurred in the 

 middle of a chromosome, the chromomeres of the opposite chromo- 

 some having no partner are buckled. WTiere an inversion has 

 occurred a reversed loop is formed. \\'here an interchange has 

 occurred the chromosomes change partners. All these structures 

 are strictly comparable with those observed at meiosis in a structural 

 hybrid. The various kinds of differences have been referred to in 

 the proper place. They have shown that the same kind of structural 



Fig. 61. — An error of pairing in a homozygous D. pseiido-obscura 

 due to the torsion setting up non-homologous pairing within a 

 chromosome. The result is internal interlocking. Right-hand 

 relational and relic coihng. x 1000 (Roller, 1935). 



differentiation has taken place in Drosophila as had been inferred 

 from the study of meiosis in other organisms. In the hybrid 

 Drosophila melanogaster $ X /). dmiilans ^ the chromosomes fail 

 to pair in certain places although they seem to be structurally 

 similar. This might be due to invisible differences, but in view of 

 the behaviour found in D. sinmlans it is more likely to be due to 

 the pairing being genotypically controlled, and in the hybrid partially 

 inhibited (Patau, 1935). 



Secondly, b}^ comparing these differences with the crossing-over 

 properties and phenotypic expression of the heterozygotes and 

 homozygotes of the changed types it has been possible for Muller 

 and his collaborators to introduce a new criterion of the gene : to 

 find out that the unit of structural change, the smallest piece into 



