The Source of Variability 



81 



ment have increased the mutation rate. Unusual dosages of ultra- 

 violet rays, gamma rays, and other wave-lengths, and various chemi- 

 cals such as mustard gas and streptomycin have produced increased 

 mutation rates in many organisms. The mutations induced by 

 streptomycin are particularly interesting. Sager (1960) found that 

 one particular streptomycin-resistant, cytoplasmic mutation of 

 Chlainydomonas appeared only in certain strains of Chlamy- 

 domonas exposed to hyper-toxic dosages of streptomycin. Thus the 

 drug apparently induced a mutation in some way linked with the 

 drug itself. Although only this one instance of such action is known, 

 it nevertheless indicates the possibility of new discoveries and 

 concepts in this intriguing relationship between environment and 

 mutation. 



3. The Mixing Process 



In addition to a process of change, the sexual processes of the 

 genetic system have an inherent behavior which effects a mixing 

 and recombination of characters. The component parts of this 

 mixing process include crossing-over of homologous chromosomes 

 at meiosis and independent segregation of chromosomes at meiosis. 



CROSSING-OVER 



In typical crossing-over two strands or chromatids of homologous 

 chromosome pairs exchange pieces ( Fig. 31 ) . This effects a changing 



Fig. 31. Diagram of a simple pattern of crossing-over occurring between 

 homologous chromatids during meiosis. Letters indicate the position of hypo- 

 thetical genes. 



