get associations of 4, 3 + 1, 2 + 2, 2 + 1 + 1, and 1 + 1 + 1 + 1, of 

 which the first two are most likely. The unwieldiness of the first type of 

 association and the random distribution at anaphase of the unassociated 

 chromosome in the second leads to the production of unbalanced gam- 

 etes which tend to be inviable, and thus a true aiitotetraploid may range 

 from being partially to almost completely sterile. 



/ A ^A^ 5- \/ 



• *^<^^*^^v^v» 



Figure 8-2. Diagram of Cross Between Triploid Commercial Banana 

 (Variety Gross Michel, 2n = 33) and Naturally Occurring Diploid {Miisa 

 acuminata, Variety Zebrina) to Give Tetraploid Seedling (2n = 44). The 

 tetraploid contains three sets of Gros Michel and one set of M. acuminata 

 chromosomes. (From Wilson, G. B., 1946. "Cytological Studies in the 

 Musae. I, II, III," Genetics, 31, Fig. 3, p. 249; Fig. A, p. 478; Fig. A, p. 484.) 



If a diploid containing two more or less nonhomologous chromosome 

 sets, AB, has its chromosome number doubled, it will give rise to an 

 allotetraploid of constitution A ABB. At meiosis in such an organism, 

 A chromosomes will tend to pair with A and B with B to give pairs 

 similar to a normal diploid and thus a mechanically fertile individual. 

 The extreme case is known as an amphidiploid and is cytologically and 

 often genetically difficult to distinguish from a diploid except in com- 

 parison with its putative parents. 



Polyploids are sometimes classified on the basis of origin as primary 



176 / CHAPTER 8 



