THE GENETIC NATURE OF TAXONOMIC DIFFERENCES 



269 



More markedly unbalanced forms have been synthesized in Datura.^ 

 Here the secondary trisomic (cf. p. 172) sugarloaf has an extra chromo- 

 some which consists of two of chromosome ends known as 2 united 

 together. By various segmental interchanges, two other types have been 

 produced which also differ from normal in having two extra doses of 

 end 2 which, however, are not attached to each other but to other 

 chromosomes. They are true breeding, and all show much the same 

 effect of their unbalance; the differences between them are presumably 

 due either to the interchanges not being always at exactly the same 



^t Jilt. 



Fig. 1 20. The Artificial Reduction of Chromosome Number in Drosophila. — 



To the left is a diagram of the normal chromosome complement of a male 

 D. melanogaster, showing the steps by which the new race is built up; first a 

 translocation of the IVth on to the Y, and then a translocation of part of the Y 

 with the IVth to the X. At the right is the chromosome set of a male of the new 

 race; the original X is white, the Y is dotted, and the IVth lined. 



(After Dubinin.) 



place or to different position effects according to the regions to which 

 the extra ends are attached. 



The examples which have been given above show that related groups 

 of organisms may differ by any of the possible forms of structural 

 change, though changes leading to unbalance, such as deficiencies and 

 duplications, are rare. It is not always possible to correlate the magni- 

 tude of the cytological changes with the taxonomic interval between 

 the groups. Thus the different races of D. psendo-obscura, which are 

 all classified in the same species, have greater chromosomal differences 

 than do the separate species D. melanogaster and D. simulans. Some- 

 times, however, the cytological and phenotypic differences run more or 

 less parallel; this is true, for instance, in the great group of Crepis 

 (Compositae) species, in which the chromosomes show a fairly compli- 



^ Cf. Blakeslee 1932. 



