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AN INTRODUCTION TO MODERN GENETICS 



last chromosome may be curled round so that the two a's are also 

 paired, giving a ring. Examples of this are common in the species of 

 Oenothera (p. no), and a similar phenomenon has been found in a 

 few other cases such as in Campanula,^ Pisum,^ and Datura.^ 



Fig. 119. Comparative Chromosome Maps of Drosophila pseudo-obscura and 

 D. miranda. — Regions with the same gene arrangements are white; inverted 

 sections, cross-hatched ; translocations, stippled ; and sections of which homologues 

 are not detectable in the other species, black. (From Dobzhansky.) 



In some cases unbalanced forms, arising by translocation, may be 

 viable and take part in evolution. An artificial case has been described 

 in D. melanogaster,^ in which one of the IV's was translocated to the Y, 

 and then this part of the F to the X; the male has a supernumerary 

 piece of the Y, but this has very Uttle effect, and similarly the female 

 has two extra pieces of Y. The chromosome number has been reduced 

 from n = 4 to n = ^, 



^ Gairdner and Darlington 193 1. 



2 Pellew and Sansome 193 1, Sansome 1932. 



3 Blakeslee 1929, Blakeslee, Bergner, and Avery 1937. 

 * Dubinin 1934, 1936. 



