Changes Involving Genomes and Chromosomes 



139 



♦ 



f 



IN 2N 3N 



FIGURE 18-2. Ploidv in Datura (A^ = 12) (sil/joiiettes). 



4N 



Once a phenotype is proved to result from 

 a mutation, the basis for the genetic change 

 has still to be determined. Let us start our 

 study of the basis and character of mutation 

 with some examples taken from plants. 



In the evening primrose, Oenothera, a 

 giant type called gigas is found to be a 

 mutant. Other Oenothera, like most sexually 

 reproducing species, are diploid, having 

 two sets of chromosomes, or genomes — one 

 genome having been contributed by each 

 of the gametes. In the gigas type, cyto- 

 logical examination shows that there are 

 three genomes, so that such individuals are 

 triploid. Studies of other groups of diploid 

 plants have revealed related types which prove 

 to have four genomes and so are tetraploid, 



others may have six sets (hexaploids) or eight 

 (octaploids). Triploidy has been found even 

 in human beings. The occurrence of any 

 ploidy greater than diploidy is called poly- 

 ploidy, although this term may also be used 

 for multiples of the haploid number when 

 monoploidy is the normal condition. 



Different forms of the Jimson weed. Datura, 

 are found to carry different ploidies.^ Some 

 are haploid, others diploid, triploid, or tetra- 

 ploid. The appearance of the flowers each 

 type produces is shown in Figure 18-2 line B, 

 and their respective seed capsules are shown 

 above in line A of the Figure. Note that the 

 flower size increases with ploidy. The seed 

 capsules illustrated are those which might 

 ^ Based upon work of A. F. Blakeslee and J. Belling. 



