316 Cell as Basis of Organic Activity 



BIOCHEMICAL GENETICS 



Another recent study throws some Hght on the methods by which 

 genes operate. For these studies, one of the bread molds, Neurospora, 

 has been extensively used. Neurospora makes a very useful experi- 

 mental material for eight spores are produced in a single ascus or sac. 

 These spores have a haploid number of chromosomes, all of which have 

 precisely the same genetic composition. The mold is normally able 

 to synthesize, by means of its various enzymes, many of the chemical 

 substances which it needs for growth. It is thus possible to grow this 

 mold on a minimal medium composed of agar, biotin, sugar or fat, and 

 some inorganic salts. 



Fig. 104. — An ascus of Neurospora, showing the eight spores. 



In the experiment, a single ascus is exposed to x-rays, and then 

 the individual spores are separated out and grown on an artificial 

 medium. When the mature mold is formed, one spore from each culture 

 is placed in a minimal medium. If mutations have occurred the spores 

 will be unable to grow properly due to the absence of certain enzymes. 

 Those spores which do grow in this minimal medium are discarded as no 

 biochemical enzyme changes have occurred. Those which do not grow 

 are next transferred to a new medium which is deficient in only one 

 substance. If growth occurs, it is clear that this enzyme needed for the 

 manufacture of the food substances is present, but the transfers con- 

 tinue until a medium is found in which growth cannot occur. Thus the 

 mutant strain is lacking in one gene which controls the synthesis of 

 growth substances. Thus it is demonstrated that the genes control the 

 enzyme systems. 



THE APPLICATION OF GENETIC PRINCIPLES 



So far the discussion has been based upon the theoretical principles 

 upon which the science of genetics is based. These same principles have 

 wide application in the practical aspects of plant and animal breeding. 

 The two general methods which are utilized are known as inbreeding 

 and outbreeding. 



