58 CELL HEREDITY 



life processes. The pre-eminent position that genes occupy in the 

 economy of the cell justifies the presumption that they are universally 

 similar. 



A more serious limitation remains, however, in our attempts to study 

 mutation in any cell or any organism. In experiments on the mutable 

 unit what is observed is the mutant organism. The new genetic con- 

 stitution is seen through the filter of all the proces.ses which inter- 

 vene between it and the cell character it determines. This may cloud our 

 vision of the gene. For example, would genes that did not mutate, or 

 that were so unstable as to pass unobserved back to the original state, be 

 different in structure from others? Again, the mutant units studied are 

 those which influence cell characters to such an extent and in such a way 

 that they can be detected and analyzed. Is this selected class unrepre- 

 sentative? There are reparable and irreparable lethal mutations. The 

 former can be easily investigated, the latter with difficulty. Are genetic 

 units with irreparable functions different from others? Even if they 

 mutate in the same fashion because of similarities in structure, do they 

 function differently? May they, for example, have more than one func- 

 tion? These questions have been investigated, although insufficiently. 

 The probable answer to all of them is "no." Although we accept this 

 answer with reservation for the reasons enumerated, there seems to be 

 a basic similarity in all genes. The similarity in mutation may be 

 greater because the mutable unit is only a small part of the segment 

 of nucleic acid which acts as a functional genetic unit. These small 

 parts, the nucleotides, may be the common denominators, functional 

 differentiation occurring at higher levels of organization. 



What image of the gene can be created from the facts thus far pre- 

 sented? In the first chapter we saw that in chemical composition genes 

 are probably nucleic acids; in many cases, at least, DNA. They are the 

 residence of the information necessary for the metabolism and develop- 

 ment of the cell. Thus they form the thread of life extending from gen- 

 eration to generation. 



Considerations from mutation analysis lead us to conclude further that 

 genes are probably: 



1. Particulate, at least in behavior; one of many small units making up 

 the genotype; 



2. structures of considerable stability but subject to rare spontaneous 

 and induced change back and forth among a variety of possible states; 



3. subject to a mutation that is random in space and time, that usually 

 occurs independent of mutation in other genetic units, and that has no 

 adaptive relation to the environment in which it takes place; 



