polyploid may therefore be defined as an organism with more than two 

 complete sets of chromosomes. Regardless of apparent complexity, the 

 origin of any polyploid traces back either to failure of anaphase separa- 

 tion in mitosis or meiosis, or to stimulation of a cell already potentially 

 polyploid through endomitosis. 



Polyploids are generally classified according to whether the original 

 stock was a cytological homozygote or a cytological heterozygote. The 



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Figure 8-1. (a) Photomicrograph of Normal Diploid Metaphase in Divid- 

 ing Meristematic Cell of Allium, (b) Photomicrograph of Tetraploid Meta- 

 phase in Allium Root Tip Cell Induced by Cyclochlorohexane. (Fig. (a) 

 from Wilson. G. B., Hawthorne, M. E.. and Tsou. T. M., 1951. "Spontaneous 

 and Induced Variations in Mitosis," J. Heredity, 42, Fig. 12 A, p. 188. Fig. (b) 

 from Wilson, G. B., Tsou, T. M., and Hyppio, P., 1952. "Variations in 

 Mitosis. II. The Interrelation of Some Basic Deviations," J. Heredity, 43, 

 Fig. 71, p. 212.) 



former gives rise to an antopolyploid in which all the chromosome sets 

 are homologous and the latter to an allopolyploid in which, ideally, no 

 chromosome has more than one homologue. There are, of course, many 

 theoretical intermediates and complex combinations of these two types 

 (Stebbins, 1950) and in practice most cases are intermediate. 



The following illustration may serve to show the essential differences 

 between the two classes. If a diploid containing two Jwnwlogous chro- 

 mosome sets, AA, has its chromosome number doubled, it will give rise 

 to an organism having four sets of matching chromosomes, AAAA. 

 Pairing at meiosis will therefore tend to be in groups of four (see Figure 

 8-2). In practice such perfect association may not occur, and we may 



CYTOGENETICS / 175 



