Heredity -515 



DIPLOID (14) 





TRIPLOID (21) 



TETRAPLOID (28) 



PENTAPLOID (35) 



HEXAPLOID (42) 



OCTAPLOID (56) 



Fig. 26-30. Chromosome groups of different species of roses. Some of these 

 forms (those with odd chromosome numbers, at least) are probably hybrids. 

 (After Tackholm.) 



lished in such species. In fact quite a num- 

 ber of species in closely related plant groups 

 have chromosome numbers that are multiples 

 of a common factor (Fig. 26-30). In any 

 tetraploid stock, subsequent aberrations oc- 

 curring over a period of many years may re- 

 sult in the production of other stocks that 

 are hexaploid (6n), octaploid (8n), and so 

 forth. 



Other types of aberration, which also can 

 be important in the origin of species, display 

 quite a variety of forms (Fig. 26-31). Defi- 

 ciencies, because of the loss of one chromo- 

 some, or any fragment of a chromosome; 

 duplications, as a result of the presence of 

 an extra chromosome or any of its pieces; 



Fig. 26-31. Types of aberrations. In 

 each case the normal chromosome 

 is shown on the left, the abnormal 

 one on the right. 



translocations, owing to the attachment of a 

 piece to some other chromosome; and inver- 

 sions, resulting from the rejoining, by the 

 wrong ends, of two pieces of a single chro- 

 mosome — all have important genetic conse- 

 quences that have been studied very inten- 

 sively in Drosophila. Irradiation of animals 

 and plants with x-rays greatly increases the 

 occurrence of such aberrations, since the 

 x-ray treatment tends to produce a fragmen- 

 tation of the chromosomes. Careful studies 

 of the chromosomes of several closely related 

 species have shown that the difference be- 

 tween two species may, in some cases, arise 

 suddenly by chromosomal aberration. In one 

 case, for example, a single inversion occur- 



DEFICIENCY 



DUPLICATION 



TRANSLOCATION 



INVERSION 



