160 



CHAPTF.R I I 



figure 11-10. Chromosomal constitution found in a female showing Down's syndrome. 

 (By permission of M. A. Ferguson-Smith and A. W. Johnston, and The Annals of 

 Internal Medicine, vol. 53, p. 361, I960.) After photographing a squash preparation 

 like that in Figure 11-10, the chromosomes are cut out and "paired" as shown here. 



although this does not seem to contribute 

 very significantly to the total observed fre- 

 quency. In man, almost all nondisjunction 

 is associated with the aging of oocytes. 

 The reverse is true in the mouse, however, 

 even though mouse females — like human fe- 

 males — are born with all their germ cells 

 in the oocyte stage. Thus, in the mouse, 

 marked chromosomes show that spontaneous 

 aneusomy almost always has a paternal 

 origin. It should be noted that, in the 

 mouse, viable aneusomy also occurs for the 

 sex chromosomes and for certain small auto- 

 somes when trisomic.' 

 4 See A. B. Griffen and M- C Bunker (1964). 



The incidence of nondisjunction can be 

 increased by high energy radiations. Carbon 

 dioxide, other chemical substances, and cer- 

 tain diploid genotypes can increase the non- 

 disjunction rate in Drosophila. In human 

 beings, the evidence that older women are 

 more apt to have trisomic children suggests 

 that some metabolic defect associated with 

 increased age increases the chance for non- 

 disjunction. 



Although chromosome loss may result 

 from spontaneous mciotic and nonmeiotic 

 nondisjunction in diploids as well as from 

 the meiotic process normally taking place 

 in polyploids, it should not be inferred that 



