HEREDITY IN SOMATIC CELLS 377 



cells. Variation and selection in the development of drug resistance in 

 tumors may take different paths; just as in bacteria, high-level resistance 

 mav appear in one step or after several (Figure 12.11). But the one- 

 step resistant cells are sometimes readily reversible in the absence of the 

 drug, pointing up once again the possibilitv that mechanisms other than 

 gene mutation are responsible. 



One alternative to mutation in chromosomal genes is mutation or loss 

 of some cytoplasmic factor. Not much is known about such processes in 

 somatic cells, but there is precedent to suppose that they do take place 

 (see Chapter 9). In the corn plant, for example, plastid mutations occur 

 in the presence of the chromosomal gene iojap, and sometimes mutations 

 seem to occur spontaneously. In higher plants and in the cells of 

 microorganisms, plastids may be modified or lost in the presence of 

 chemicals such as streptomycin, or as a consequence of other environ- 



TABLE 12.2 



Variance Analysis of the Mean Weight of Leukemic Tumors 



Developed in the Presence of Antitumor Agent A-Methopterin, from Sublines 



Maintained Independently for 7 Transfers and for Subdivisions 



Made from a Single Line at the Last Transfer 



(from Law, 1952, Nature, 169:628) 



Independent Sublines Single Line 



