66 



AN INTRODUCTION TO MODERN GENETICS 



Polyploids fall into two groups, those with an even multiple of the 

 basic number, in which the formation of gametes with half the somatic 

 number of chromosomes is theoretically possible; and those with an 

 uneven multiple in which a regular reduction division is impossible. 



Fig. 26. Polyploidy and Cell Size.— Cell volume (in 100,000 cu. /a) in polyploids 

 of the mosses Physcomitrella patens (dotted), Funaria hygrometrica (plain line), and 

 in allopolyploid hybrids between them (dashes). 



(From Wettstein.) 



The term "polysomics" or "aneuploids" is reserved for organisms 

 with one or a few chromosomes more or less than a multiple of the 

 basic number; e.g. 4jc + i is a pentasomic tetraploid, 6x — 2 a. doubly 

 pentasomic hexaploid. 



The possibility also arises that the different basic sets of chromo- 

 somes may not be identically alike. A tetraploid, for example, may arise 

 by the doubling of the chromosome number in a hybrid between the 

 species A and B (p. 254) and the A and B sets of chromosomes will be 

 to some extent different from one another, the difference depending on 



