Changes Involving Genomes and Chromosomes 



147 



FIGURE 18-10. Chromosoiual constitution found in a fcnuile showing Mongolism. {By per- 

 mission of M. A. Ferguson-Smith and A. W. Johnston, and The Annals of Internal Medicine, 

 vol. 53, p. 361, 1960.) 



that is monosomic and one that is trisomic, 

 each giving rise to patches of tissue of these 

 respective types when this "misdivision" or 

 "mitotic dislocation" is followed by normal 

 mitosis. 



While chromosome addition and subtrac- 

 tion may occur as a result of spontaneous 

 nondisjunction and of the meiotic processes 

 that normally take place in triploids, penta- 

 ploids, etc., we should not conclude that these 

 are the only mechanisms for their origin. 

 Moreover, it is known that the incidence of 

 nondisjunction can be increased by means 

 of high energy radiations, such as are released 

 by radium or involved in X rays. Carbon 

 dioxide, other chemical substances, and 



certain diploid genotypes can increase the 

 nondisjunction rate in Drosophila. In human 

 beings, the fact that older women are more 

 apt to have Mongoloid children suggests 

 that some metabolic defect occurs with age 

 which increases the chance for nondisjunc- 

 tion (cf. p. 106), and, therefore, the frequency 

 of eggs carrying the appropriate chromosome 

 in addition to the 23 chromosomes of a hap- 

 loid set. 



Because of the large genetic unbalance 

 produced, we would expect that addition and 

 subtraction of whole chromosomes is a class 

 of mutation which involves too drastic a 

 phenotypic change to be very useful in evolu- 

 tion. 



