fertilization with itself or with one of its kind should produce three kinds 

 of zygotes: 2n, 2n plus 1, and 2n plus 2, in the ratio of 1:2:1. However, 

 because of the differential viability of the three types, this is seldom if 

 ever realized. The two classes of gametes are the result of random distri- 

 bution of the extra chromosome at first anaphase of meiosis. The best 

 known series of polysemies is in Datura, where strains involving dupli- 

 cation of each one of the twelve chromosomes have been found and; or 

 induced (Blakeslee, 1930). 



Some distinction should be made between true aneuploids and false 

 ones. In the former case there are one or more complete genomes plus a 

 fraction of a genome, in the latter, at least two kinds are found: ( 1 ) those 

 which are apparent aneuploids by chromosome count because, in addi- 

 tion to one or more complete complements, they contain a certain num- 

 ber of supernumerary chromosomes; and (2) those which are aneuploid 

 by count because they contain two or more complements derived from 

 parents of different chromosome numbers. 



As already noted, true aneuploids, by the definition used here, are 

 much less viable than corresponding euploids, as well as being sterile to 

 a considerable degree. So far as the evidence goes aneuploid animals 

 seem to be less viable than plants. For example, the Datura trisomies 

 already mentioned vary from moderate to good viability, especially in 

 cultivation. On the other hand, trisomic Drosophila are rather inviable 

 except for those involving the small IV chromosome. Before one can rely 

 on such generalities, however, it is necessary to know whether or not one 

 is dealing with true aneuploids, a designation which cannot always be 

 made solely on chromosome counts. 



Variations Within an Organism 

 It has long been assumed that, apart from accidental exceptions, all 

 somatic cells of an organism are alike in chromosome constitution. 

 Recent work, however, has laid this tenet open to suspicion. It has been 

 known for a long time that occasional polyploid cells may be found in 

 an otherwise diploid organism, and it now appears that varying degrees 

 of polyploidy or polyteny may be the rule rather than the exception, 

 especially in differentiated tissue (Geider, 1937, 1941 ; Huskins, 1947). 

 Such variations appear to result from the process of endomitosis, the 

 degree of which may differ between chromosomes and e^"Jn within chro- 

 mosomes to provide considerable quantitative variation from cell to cell. 

 In some tissues such divisions appear to be sporadic and in others the 

 rule. Endomitosis appears to be common in specialized tissues such as 

 corn endosperm (Duncan and Ross, 1950) (Figure 8-3), and in the 



180 / CHAPTER 8 



