Changes Involving Genomes and Chromosomes 



143 



derived from different species. These cases 

 represent examples of amphiploidy, or allo- 

 polyploidy, in which two or more genomes 

 have come from each of the different species. 

 Cultivated wheat is an amphiploid. Amphi- 

 ploids often show a combination of char- 

 acteristics of their different parent species, 

 just as you would expect. This type of poly- 

 ploidy is discussed in more detail in Chap- 

 ter 29. 



Changes in genome number represent the 

 class of mutational events which involves the 

 largest amounts of genetic material. While 

 many plants are polyploid, this type of mu- 

 tation does not occur many times in succes- 

 sion, for chromosome number would become 

 unwieldy in nuclear division. It should be 

 noted also that certain other classes of muta- 

 tion, like those involving a single locus, would 

 have a greater difficulty expressing them- 

 selves in polyploids than they would have in 

 haploids or diploids where there is no other, 

 or just one other, homologous locus capable 

 of masking the mutant effect. 



Changes in genome number preserve the 

 same ratios that genes or chromosomes have 

 to each other in the normal diploid. Such 

 changes are said to be euploid ("right-fold"). 



The next category of mutations to be dis- 

 cussed in the present Chapter involves the 

 addition or subtraction, not of whole chro- 

 mosome sets, but of single whole chromo- 

 somes. Such mutations upset the normal 

 balance referred to and produce aneuploid 

 ("not right-fold") genetic (chromosomal) 

 constitutions. By what mechanisms can 

 single whole chromosomes be added to or 

 subtracted from a genome? We have already 

 discussed two ways in previous Chapters, in 

 the phenomena of chromosomal nondis- 

 junction in normal diploids (Chapter 12) and 

 of chromosomal segregation in autopoly- 

 ploids (triploids in Chapter 14). 



You recall that nondisjunction in the germ 

 line of Drosophila can produce offspring, 

 otherwise diploid, that are XO, XXX, and 

 XXY. Nondisjunction of the small fourth 

 chromosome can lead to the production of 

 individuals with one fourth chromosome 

 (haplo-IV individuals) or three (triplo-IV 

 individuals), as pictured in Figure 18-6. Even 

 though addition or subtraction of a chromo- 

 some IV makes visible phenotypic changes 

 from the normal diploid condition, as you 

 can see by referring to the Figure, both 

 changes are viable. This is not true for 



FIGURE 18-6. Haplo-IV (left) and triplo-IV {right) females of D. melanogaster. 

 The haplo-IV is smaller than the wild-tvpe female shown in Fig. 1 2-1. {Drawn by 

 E. M. Wallace.) 



