THE GENETIC NATURE OF TAXONOMIC DIFFERENCES 265 



Better evidence of inversion, translocation, etc., is found in those 

 species where definite genes or sections of the cross-over map can be 

 identified. Several examples of this are now available in Drosophila, 

 and a beginning has been made with the comparison of maize and 

 related species. The Drosophila comparisons depend on the identifi- 

 cation of homologous genes in different species, and a certain caution 

 is necessary in this respect, since we know (p. 186) that in each species 

 there are probably several loci capable of producing similar phenotype 



il II H^ II ^ 



C 





\S II s\ I, 



<. .. A 



" e « . II 





w 



»5= £« -^^J" 



tt II II ..2t vv^. 



Fig. 117. Diagram of Mitotic Metaphase Figures in Drosophila Species.— 



The figures are of females, the X chromosome being at the bottom of each figure. 



(From Metz and Moses.) 



effects. When, however, the Fi hybrids can be obtained, the allelo- 

 morphism of two genes can be tested by examining the double hetero- 

 zygote, which will be wild-type in appearance if the genes are not 

 allelomorphs. Homologous loci have been identified in this way in 

 Drosophila melanogaster and O. simulans, and the cross-over maps can 

 therefore be compared with some certainty.^ In general the order of 

 the genes (but not their distances) is the same in both species, but 

 there is one large inversion in the third chromosomes. This can also 

 be clearly seen in the salivary glands chromosomes of the hybrid, 

 which show a characteristic inversion loop. Other minor differences 

 are also present in the saUvary chromosomes, and pairing may fail even 

 when no definite structural diflferences are visible.^ Relative inversions 

 between species can also be discovered, though not so precisely located, 



1 Sturtevant 1929a. Cf. Sturtevant and Tan 1937. ^ Patau 1935. 



