plex a process as in nonpolyploid species. Finally, a partial reason for 

 difference between plants and animals with respect to polyploid incidence 

 may be purely statistical. In most animals there are comparatively few 

 cells which are capable of producing a new organism, namely, those 

 directly in the germ line, while in plants almost any cell not too differen- 

 tiated to divide may produce another organism either direcdy by vege- 

 tative means or indirectly through production of a flowering branch. 

 Therefore, given the same frequency of suitable mitotic accidents, a 

 plant is much more likely to produce, and perhaps to maintain, a poly- 

 ploid strain than is an animal. This reasoning does not explain, however, 

 why there should be marked differences between families in incidence 

 of polyploidy. It is probable that all these explanations have some 

 validity. 



Aneuploidy 



An organism which contains one or more incomplete chromosome 

 sets is known as an aneuploid. Such a condition may arise either from 

 union of two gametes at least one of which is unbalanced in chromosome 

 number, or through perpetuation of a cell which has gained or lost one 

 or more chromosomes through failure to divide properly at anaphase of 

 mitosis. As a rule, aneuploids tend to be considerably less viable than 

 the corresponding euploids and accordingly are rather rare in nature. 

 Nevertheless, they do occasionally become established and under certain 

 circumstances may be of some evolutionary significance. Aneuploids 

 near the diploid level are, as a rule, less likely to survive than those near 

 a polyploid level, that is, 2n plus 1 as compared with 4n plus 1 . Presum- 

 ably this difference is correlated in part with the difference in per cent of 

 genie imbalance in the two cases. If an extra chromosome is added to- 

 a diploid, it has a greater quantitative effect than if added to a tetraploid. 



Specific kinds of aneuploids have names descriptive of the condition: 

 polysomic (dupHcation of one or more chromosomes of a set); mono- 

 somic (having only one of a pair of chromosomes); millosomic (lacking 

 a pair of chromosomes) . Regardless of the exact condition, the principles 

 remain the same and are illustrated by the following example: 



If at the first division of a diploid zygote one chromosome fails to di- 

 vide properly, we may have two cells, one of which has an extra chromo- 

 some (2n plus 1) and the other lacking a chromosome (2n minus 1). 

 In fact, neither cell is very likely to survive, but the chances are better 

 for the 2n plus 1 than for its deficient mate. Should the 2n plus 1 cell 

 survive and give rise to the two kinds of gametes, namely, n and n plus 1, 



CYTOGENETICS / 179 



