Changes Involving Unbroken Chromosomes 



pure lines cannot be obtained because self- 

 fertilization does not occur, detection of 

 mutations is facilitated by knowing the pre- 

 existing genotypes. 



Although we have seen how a phenotypc 

 is proved to be the result of a mutation, we 

 have not determined the basis for the genetic 

 change involved. The change could con- 

 ceivably encompass as much as an entire 

 genome, or as little as the genetic material 

 in a single gene locus. The latter type of 

 change may not be detectable cytologically. 

 Although a cytological study has not been 

 made, genetic studies indicate that the 

 chromosomal change associated with the 

 occurrence of Hexaptera is submicroscopic. 

 Let us now look at mutants known to be as- 

 sociated with a gross visible change in chro- 

 mosome composition, as detected by either 

 genetical or cytological methods, or both, 

 and leave for later the consideration of muta- 

 tions involving submicroscopic changes in 

 chromosomes. 



Heteroploidy 



In the evening primrose, Oenothera, a giant 

 type called gigas is found to be a mutant. 

 Other Oenothera, like most sexually repro- 

 ducing species, are diploid, having two sets 

 of chromosomes — one genome contributed 

 by each of the gametes. In the gigas type, 

 cytological examination shows that there are 

 three genomes; that is, the individuals are 

 triploid. Studies of other groups of diploid 

 plants reveal related types which prove to 

 have four genomes (tetraploids), others may 

 have six sets {hexaploids) or eight (octa- 

 ploids). A chromosomal composition made 

 up of an abnormal number of normal chro- 

 mosome sets is said to be heteroploid. The 

 occurrence of extra whole genomes is called 

 polyploidy, a term which is applicable for 

 multiples of the haploid number when mono- 

 ploidy is the normal condition. Note that 

 changes in genome number preserve the 



151 





figure 11-2. Ploidy in Datura (N = 12) 

 (silhouettes). 



same ratios that chromosomes (and genes) 

 have to each other under normal conditions. 

 Such changes are said to be euploid (right- 

 fold). 



1. Autopolyploidy 



Different forms of the Jimson weed. 

 Datura, carry different numbers of whole 

 genomes, or ploidies. 1 Some are haploid, 

 others diploid, triploid, or tetraploid. The 

 flowers that each of these types produces are 

 shown in Figure 11-2, line B (the respective 

 seed capsules are shown in line A). Note 

 that flower size increases with ploidy. The 

 seed capsules illustrated are those which 

 might have been obtained had the individual 

 under test been fertilized by pollen from a 

 diploid — the differences in size being due 

 partly to the number of seeds that have set 

 or developed. 



Triploid and tetraploid embryos are found 



1 The following is based upon work of A. F. 

 Blakeslee and J. Bellinc 



